A new stealthy Linux malware known as Shikitega has been discovered infecting computers and IoT devices with additional payloads.
The malware exploits vulnerabilities to elevate its privileges, adds persistence on the host via crontab, and eventually launches a cryptocurrency miner on infected devices.
Shikitega is quite stealthy, managing to evade anti-virus detection using a polymorphic encoder that makes static, signature-based detection impossible.
An intricate infection chain
While the initial infection method is not known at this time, researchers at AT&T who discovered Shikitega say the malware uses a multi-step infection chain where each layer delivers only a few hundred bytes, activating a simple module and then moving to the next one.
«Shiketega malware is delivered in a sophisticated way, it uses a polymorphic encoder, and it gradually delivers its payload where each step reveals only part of the total payload.,» explains AT&T’s report.
The infection begins with a 370 bytes ELF file, which is the dropper containing encoded shellcode.
The encoding is performed using the polymorphic XOS additive feedback encoder ‘Shikata Ga Nai,’ previously analyzed by Mandiant.
“Using the encoder, the malware runs through several decode loops, where one loop decodes the next layer until the final shellcode payload is decoded and executed,” continues the report.
“The encoder stud is generated based on dynamic instruction substitution and dynamic block ordering. In addition, registers are selected dynamically.”
After the decryption is completed, the shellcode is executed to contact the malware’s command and control servers (C2) and receive additional shellcode (commands) stored and run directly from memory.
One of these commands downloads and executes ‘Mettle,’ a small and portable Metasploit Meterpreter payload that gives the attackers further remote control and code execution options on the host.
Mettle fetches yet a smaller ELF file, which exploits CVE-2021-4034 (aka PwnKit) and CVE-2021-3493 to elevate privileges and download the final stage payload, a cryptocurrency miner, as root.
Persistence for the crypto miner is achieved by downloading five shell scripts that add four cronjobs, two for the root user and two for the current user.
The crontabs are an effective persistence mechanism, so all downloaded files are wiped to reduce the likelihood of the malware being discovered.
The crypto miner is XMRig version 6.17.0, focusing on mining the anonymity-focused and hard-to-trace Monero.
To further reduce the chances of raising alarms on network security products, the threat actors behind Shikitega use legitimate cloud hosting services to host their command and control infrastructure.
This choice costs more money and puts the operators at risk of being traced and identified by law enforcement but offers better stealthiness in the compromised systems.
The AT&T team reports a sharp rise in Linux malware this year, advising system admins to apply the available security updates, use EDR on all endpoints, and take regular backups of most important data.
For now, Shikitega appears focused on Monero mining, but the threat actors may decide that other, more potent payloads can be more profitable in the long run.
About two years ago I quit being a full-time red team operator. However, it still is a field of expertise that stays very close to my heart. A few weeks ago, I was looking for a new side project and decided to pick up an old red teaming hobby of mine: bypassing/evading endpoint protection solutions.
In this post, I’d like to lay out a collection of techniques that together can be used to bypassed industry leading enterprise endpoint protection solutions. This is purely for educational purposes for (ethical) red teamers and alike, so I’ve decided not to publicly release the source code. The aim for this post is to be accessible to a wide audience in the security industry, but not to drill down to the nitty gritty details of every technique. Instead, I will refer to writeups of others that deep dive better than I can.
In adversary simulations, a key challenge in the “initial access” phase is bypassing the detection and response capabilities (EDR) on enterprise endpoints. Commercial command and control frameworks provide unmodifiable shellcode and binaries to the red team operator that are heavily signatured by the endpoint protection industry and in order to execute that implant, the signatures (both static and behavioural) of that shellcode need to be obfuscated.
In this post, I will cover the following techniques, with the ultimate goal of executing malicious shellcode, also known as a (shellcode) loader:
1. Shellcode encryption
Let’s start with a basic but important topic, static shellcode obfuscation. In my loader, I leverage a XOR or RC4 encryption algorithm, because it is easy to implement and doesn’t leave a lot of external indicators of encryption activities performed by the loader. AES encryption to obfuscate static signatures of the shellcode leaves traces in the import address table of the binary, which increase suspicion. I’ve had Windows Defender specifically trigger on AES decryption functions (e.g.
etc.) in earlier versions of this loader.
2. Reducing entropy
Many AV/EDR solutions consider binary entropy in their assessment of an unknown binary. Since we’re encrypting the shellcode, the entropy of our binary is rather high, which is a clear indicator of obfuscated parts of code in the binary.
There are several ways of reducing the entropy of our binary, two simple ones that work are:
Adding low entropy resources to the binary, such as (low entropy) images.
Adding strings, such as the English dictionary or some of
A more elegant solution would be to design and implement an algorithm that would obfuscate (encode/encrypt) the shellcode into English words (low entropy). That would kill two birds with one stone.
3. Escaping the (local) AV sandbox
Many EDR solutions will run the binary in a local sandbox for a few seconds to inspect its behaviour. To avoid compromising on the end user experience, they cannot afford to inspect the binary for longer than a few seconds (I’ve seen Avast taking up to 30 seconds in the past, but that was an exception). We can abuse this limitation by delaying the execution of our shellcode. Simply calculating a large prime number is my personal favourite. You can go a bit further and deterministically calculate a prime number and use that number as (a part of) the key to your encrypted shellcode.
4. Import table obfuscation
You want to avoid suspicious Windows API (WINAPI) from ending up in our IAT (import address table). This table consists of an overview of all the Windows APIs that your binary imports from other system libraries. A list of suspicious (oftentimes therefore inspected by EDR solutions) APIs can be found here. Typically, these are
dumpbin /exports <binary.exe>
will list all the imports. For the most part, we’ll use Direct System calls to bypass both EDR hooks (refer to section 7) of suspicious WINAPI calls, but for less suspicious API calls this method works just fine.
We add the function signature of the WINAPI call, get the address of the WINAPI in
and then create a function pointer to that address:
Obfuscating strings using a character array cuts the string up in smaller pieces making them more difficult to extract from a binary.
The call will still be to an
WINAPI, and will not bypass any hooks in WINAPIs in
, but is purely to remove suspicious functions from the IAT.
5. Disabling Event Tracing for Windows (ETW)
Many EDR solutions leverage Event Tracing for Windows (ETW) extensively, in particular Microsoft Defender for Endpoint (formerly known as Microsoft ATP). ETW allows for extensive instrumentation and tracing of a process’ functionality and WINAPI calls. ETW has components in the kernel, mainly to register callbacks for system calls and other kernel operations, but also consists of a userland component that is part of
is a DLL loaded into the process of our binary, we have full control over this DLL and therefore the ETW functionality. There are quite a fewdifferent bypasses for ETW in userspace, but the most common one is patching the function
which is called to write/log ETW events. We fetch its address in
, and replace its first instructions with instructions to return 0 (
I’ve found the above method to still work on the two tested EDRs, but this is a noisy ETW patch.
6. Evading common malicious API call patterns
Most behavioural detection is ultimately based on detecting malicious patterns. One of these patters is the order of specific WINAPI calls in a short timeframe. The suspicious WINAPI calls briefly mentioned in section 4 are typically used to execute shellcode and therefore heavily monitored. However, these calls are also used for benign activity (the
pattern in combination with a memory allocation and write of ~250KB of shellcode) and so the challenge for EDR solutions is to distinguish benign from malicious calls. Filip Olszak wrote a great blog post leveraging delays and smaller chunks of allocating and writing memory to blend in with benign WINAPI call behaviour. In short, his method adjusts the following behaviour of a typical shellcode loader:
Instead of allocating one large chuck of memory and directly write the ~250KB implant shellcode into that memory, allocate small contiguous chunks of e.g. <64KB memory and mark them as
. Then write the shellcode in a similar chunk size to the allocated memory pages.
Introduce delays between every of the above mentioned operations. This will increase the time required to execute the shellcode, but will also make the consecutive execution pattern stand out much less.
One catch with this technique is to make sure you find a memory location that can fit your entire shellcode in consecutive memory pages. Filip’s DripLoader implements this concept.
The loader I’ve built does not inject the shellcode into another process but instead starts the shellcode in a thread in its own process space using
. An unknown process (our binary will de facto have low prevalence) into other processes (typically a Windows native ones) is suspicious activity that stands out (recommended read “Fork&Run – you’re history”). It is much easier to blend into the noise of benign thread executions and memory operations within a process when we run the shellcode within a thread in the loader’s process space. The downside however is that any crashing post-exploitation modules will also crash the process of the loader and therefore the implant. Persistence techniques as well as running stable and reliable BOFs can help to overcome this downside.
7. Direct system calls and evading “mark of the syscall”
The loader leverages direct system calls for bypassing any hooks put in
by the EDRs. I want to avoid going into too much detail on how direct syscalls work, since it’s not the purpose of this post and a lot of great posts have been written about it (e.g. Outflank).
In short, a direct syscall is a WINAPI call directly to the kernel system call equivalent. Instead of calling the
we call its kernel equivalent
defined in the Windows kernel. This is great because we’re bypassing any EDR hooks used to monitor calls to (in this example)
In order to call a system call directly, we fetch the syscall ID of the system call we want to call from
, use the function signature to push the correct order and types of function arguments to the stack, and call the
instruction. There are several tools that arrange all this for us, SysWhispers2 and SysWhisper3 are two great examples. From an evasion perspective, there are two issues with calling direct system calls:
that is loaded by default (and hooked by the EDR) with a fresh copy from
is the first DLL that gets loaded by any Windows process. EDR solutions make sure their DLL is loaded shortly after, which puts all the hooks in place in the loaded
before our own code will execute. If our code loads a fresh copy of
in memory afterwards, those EDR hooks will be overwritten. RefleXXion is a C++ library that implements the research done for this technique by MDSec. RelfeXXion uses direct system calls
to get a handle to a clean
(registry path with previously loaded DLLs). It then overwrites the
section of the loaded
, which flushes out the EDR hooks.
I recommend to use adjust the RefleXXion library to use the same trick as described above in section 7.
9. Spoofing the thread call stack
The next two sections cover two techniques that provide evasions against detecting our shellcode in memory. Due to the beaconing behaviour of an implant, for a majority of the time the implant is sleeping, waiting for incoming tasks from its operator. During this time the implant is vulnerable for memory scanning techniques from the EDR. The first of the two evasions described in this post is spoofing the thread call stack.
When the implant is sleeping, its thread return address is pointing to our shellcode residing in memory. By examining the return addresses of threads in a suspicious process, our implant shellcode can be easily identified. In order to avoid this, want to break this connection between the return address and shellcode. We can do so by hooking the
function. When that hook is called (by the implant/beacon shellcode), we overwrite the return address with
and call the original
returns, we put the original return address back in place so the thread returns to the correct address to continue execution. Mariusz Banach has implemented this technique in his ThreadStackSpoofer project. This repo provides much more detail on the technique and also outlines some caveats.
We can observe the result of spoofing the thread call stack in the two screenshots below, where the non-spoofed call stack points to non-backed memory locations and a spoofed thread call stack points to our hooked Sleep (
) function and “cuts off” the rest of the call stack.
10. In-memory encryption of beacon
The other evasion for in-memory detection is to encrypt the implant’s executable memory regions while sleeping. Using the same sleep hook as described in the section above, we can obtain the shellcode memory segment by examining the caller address (the beacon code that calls
and therefore our
hook). If the caller memory region is
and roughly the size of our shellcode, then the memory segment is encrypted with a XOR function and
is called. Then
returns, it decrypts the memory segment and returns to it.
Another technique is to register a Vectored Exception Handler (VEH) that handles
violation exceptions, decrypts the memory segments and changes the permissions to
. Then just before sleeping, mark the memory segments as
, so that when
returns, it throws a memory access violation exception. Because we registered a VEH, the exception is handled within that thread context and can be resumed at the exact same location the exception was thrown. The VEH can simply decrypt and change the permissions back to RX and the implant can continue execution. This technique prevents a detectible
The beacon shellcode that we execute in this loader ultimately is a DLL that needs to be executed in memory. Many C2 frameworks leverage Stephen Fewer’s ReflectiveLoader. There are many well written explanations of how exactly a relfective DLL loader works, and Stephen Fewer’s code is also well documented, but in short a Reflective Loader does the following:
Resolve addresses to necessary
WINAPIs required for loading the DLL (e.g.
Write the DLL and its sections to memory
Build up the DLL import table, so the DLL can call
Load any additional library’s and resolve their respective imported function addresses
Call the DLL entrypoint
Cobalt Strike added support for a custom way for reflectively loading a DLL in memory that allows a red team operator to customize the way a beacon DLL gets loaded and add evasion techniques. Bobby Cooke and Santiago P built a stealthy loader (BokuLoader) using Cobalt Strike’s UDRL which I’ve used in my loader. BokuLoader implements several evasion techniques:
Limit calls to
(commonly EDR hooked WINAPI call to resolve a function address, as we do in section 4)
Make sure to uncomment the two defines to leverage direct system calls via HellsGate & HalosGate and bypass ETW and AMSI (not really necessary, as we’ve already disabled ETW and are not injecting the loader into another process).
12. OpSec configurations in your Malleable profile
In your Malleable C2 profile, make sure the following options are configured, which limit the use of
marked memory (suspicious and easily detected) and clean up the shellcode after beacon has started.
set startrwx "false";
set userwx "false";
set cleanup "true";
set stomppe "true";
set obfuscate "true";
set sleep_mask "true";
set smartinject "true";
Combining these techniques allow you to bypass (among others) Microsoft Defender for Endpoint and CrowdStrike Falcon with 0 detections (tested mid April 2022), which together with SentinelOne lead the endpoint protection industry.
Of course this is just one and the first step in fully compromising an endpoint, and this doesn’t mean “game over” for the EDR solution. Depending on what post-exploitation activity/modules the red team operator choses next, it can still be “game over” for the implant. In general, either run BOFs, or tunnel post-ex tools through the implant’s SOCKS proxy feature. Also consider putting the EDR hooks patches back in place in our
hook to avoid detection of unhooking, as well as removing the ETW/AMSI patches.
It’s a cat and mouse game, and the cat is undoubtedly getting better.
A cybersecurity researcher bought the same item from AliExpress and took it apart. Walmart removed the hard drive after Motherboard contacted the company.
Until Monday, Walmart shoppers could grab an incredible deal: a massive solid state hard drive (SSD) for the incredible price of $17.99. Often, similar drives go for much more, depending on their size. Generally, a TB of storage on an SSD can cost between $50 to $100.
Here’s the Walmart ad:
Ray, a cybersecurity researcher, who saw a similar item on online retailer AliExpress, knew the offer was too good to be true. He bought the drive, suspecting it was a scam, and took it apart to find out what exactly was happening here. Sure enough, he found what amounted to a different item cosplaying as a big SSD. Inside were two small memory cards and the item had been programmed in such a way so as to appear it had 30TB of storage when plugged into a computer.
“I knew going in it would be a scam but I thought we might use it as an educational opportunity,” the cybersecurity researcher who goes by the name Ray, who tweeted his findings last week, told Motherboard in an online chat. “My son and I worked on it together.”
After Motherboard contacted Walmart about the listing on its own site on Friday, the company removed the item from its digital shelves on Monday.
Ray’s finding highlights something that many consumers may be unaware of: that Walmart, much like Amazon, runs an online marketplace that sells many, many products from third party sellers and not just those found on Walmart’s physical shelves, opening itself up to reputation, quality, and content moderation issues.
“Thanks for reaching out and bringing this to our attention,” Robyn Babbitt, director of corporate communications at Walmart, told Motherboard in an email. “Walmart has a robust trust and safety program, which actively works to protect our customers and help ensure items are authentic. After reviewing this item, it has been removed from our site.”
After using a razor blade to open up the item’s casing, instead of an SSD Ray found a board with two glued down SD cards. But when plugged in to a Windows machine, the computer reported detecting two 15TB drives. That is unrealistic, to say the least: although SD cards can reach into the terabytes, they are typically much more expensive than what this drive costs. Ray believes the scammers modified the firmware that makes the device misreport its storage size.
Ray wasn’t the only person who found the harddrive suspicious, judging by reviews left on the item on Walmart’s website.
“DO NOT BUY THIS—it is a scam,” the title of one review, left by someone calling themselves William, reads. “Walmart should get smarter than to sell products like this. I thought I was buying a 8 terabyte SSD drive, for $28, and this piece of garbage does not work, in any way, shape or form. This product is a scam, and Walmart should be ashamed of itself to sell them.”
“Don’t buy it,” another review reads. “The product is not what there [sic] saying it is.”
Other reviewers have given the item a 5 star rating, although it’s not clear if that is actually the manufacturers dressing up as someone else, much like the hard drive they are selling.
I have been getting a lot of messages from people asking about AV evasion. I won’t give away the source code for a fully undetectable payload, but I thought I’d share a basic implementation of a shell code runner that will take AES encrypted shell code and decrypt it and inject into a process such as explorer! Before we proceed, the technique used to inject shell code is well known to AVs and you will get flagged, the purpose of this writeup is to show how AES can be implemented, and you can use same/similar techniques to bypass EDR solution with more sophisticated techniques
What do you need to follow along?
• Visual Studio
• Windows Machine for testing payload
The first thing to do is to create a shell code using msfvenom: