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 <code>OpenThread</code>. At this point <code>QueueUserAPC</code> can be used to invoke a function (such as <code>LoadLibrayA</code> 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. 

Platforms: Windows
Tactics:
phase_name: defense-evasion
phase_name: privilege-escalation

Detection:
Monitoring Windows API calls indicative of the various types of code injection may generate a significant amount of data and may not be directly useful for defense unless collected under specific circumstances for known bad sequences of calls, since benign use of API functions may be common and difficult to distinguish from malicious behavior. Windows API calls such as <code>SuspendThread</code>/<code>SetThreadContext</code>/<code>ResumeThread</code>, <code>QueueUserAPC</code>/<code>NtQueueApcThread</code>, and those that can be used to modify memory within another process, such as <code>VirtualAllocEx</code>/<code>WriteProcessMemory</code>, may be used for this technique.(Citation: Elastic Process Injection July 2017)

Analyze process behavior to determine if a process is performing actions it usually does not, such as opening network connections, reading files, or other suspicious actions that could relate to post-compromise behavior. 

References:
source_name: mitre-attack
description: Microsoft. (n.d.). Asynchronous Procedure Calls. Retrieved December 8, 2017.
source_name: Microsoft APC
description: Gavriel, H. & Erbesfeld, B. (2018, April 11). New ‘Early Bird’ Code Injection Technique Discovered. Retrieved May 24, 2018.
source_name: CyberBit Early Bird Apr 2018
description: Liberman, T. (2016, October 27). ATOMBOMBING: BRAND NEW CODE INJECTION FOR WINDOWS. Retrieved December 8, 2017.
source_name: ENSIL AtomBombing Oct 2016
description: Microsoft. (n.d.). About Atom Tables. Retrieved December 8, 2017.
source_name: Microsoft Atom Table
description: Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.
source_name: Elastic Process Injection July 2017