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Attack: Process Injection: Asynchronous Procedure Call

Adversaries may inject malicious code into processes via the asynchronous procedure call (APC) queue in order to evade process-based defenses as well as possibly elevate privileges. APC injection is a method of executing arbitrary code in the address space of a separate live process.

APC injection is commonly performed by attaching malicious code to the APC Queue (Citation: Microsoft APC) of a process’s thread. Queued APC functions are executed when the thread enters an alterable state.(Citation: Microsoft APC) A handle to an existing victim process is first created with native Windows API calls such as OpenThread. At this point QueueUserAPC can be used to invoke a function (such as LoadLibrayA pointing to a malicious DLL).

A variation of APC injection, dubbed “Early Bird injection”, involves creating a suspended process in which malicious code can be written and executed before the process’ entry point (and potentially subsequent anti-malware hooks) via an APC. (Citation: CyberBit Early Bird Apr 2018) AtomBombing (Citation: ENSIL AtomBombing Oct 2016) is another variation that utilizes APCs to invoke malicious code previously written to the global atom table.(Citation: Microsoft Atom Table)

Running code in the context of another process may allow access to the process’s memory, system/network resources, and possibly elevated privileges. Execution via APC injection may also evade detection from security products since the execution is masked under a legitimate process.

MITRE

Tactic

• defense-evasion
• privilege-escalation

technique

• T1055.004

Test : Remote Process Injection with Go using NtQueueApcThreadEx WinAPI

OS

• windows

Description:

Uses the undocumented NtQueueAPCThreadEx WinAPI to create a “Special User APC” in the current thread of the current process to execute shellcode. Since the shellcode is loaded and executed in the current process it is considered local shellcode execution.

Steps taken with this technique

1. Allocate memory for the shellcode with VirtualAlloc setting the page permissions to Read/Write
2. Use the RtlCopyMemory macro to copy the shellcode to the allocated memory space
3. Change the memory page permissions to Execute/Read with VirtualProtect
4. Get a handle to the current thread
5. Execute the shellcode in the current thread by creating a Special User APC through the NtQueueApcThreadEx function

• PoC Credit: (https://github.com/Ne0nd0g/go-shellcode/tree/master#rtlcreateuserthread)
• References:
• https://repnz.github.io/posts/apc/user-apc/
• https://docs.rs/ntapi/0.3.1/ntapi/ntpsapi/fn.NtQueueApcThreadEx.html
• https://0x00sec.org/t/process-injection-apc-injection/24608
• https://twitter.com/aionescu/status/992264290924032005
• http://www.opening-windows.com/techart_windows_vista_apc_internals2.htm#_Toc229652505

Executor

powershell

Sigma Rule

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