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. 
Asynchronous Procedure Call
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1055/004  external_id: T1055.004     source_name: Microsoft APC  description: Microsoft. (n.d.). Asynchronous Procedure Calls. Retrieved December 8, 2017.  url: https://msdn.microsoft.com/library/windows/desktop/ms681951.aspx     source_name: CyberBit Early Bird Apr 2018  description: Gavriel, H. & Erbesfeld, B. (2018, April 11). New ‘Early Bird’ Code Injection Technique Discovered. Retrieved May 24, 2018.  url: https://www.cyberbit.com/blog/endpoint-security/new-early-bird-code-injection-technique-discovered/     source_name: ENSIL AtomBombing Oct 2016  description: Liberman, T. (2016, October 27). ATOMBOMBING: BRAND NEW CODE INJECTION FOR WINDOWS. Retrieved December 8, 2017.  url: https://blog.ensilo.com/atombombing-brand-new-code-injection-for-windows     source_name: Microsoft Atom Table  description: Microsoft. (n.d.). About Atom Tables. Retrieved December 8, 2017.  url: https://msdn.microsoft.com/library/windows/desktop/ms649053.aspx     source_name: Elastic Process Injection July 2017  description: Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.  url: https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-process     