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  of a process's thread. Queued APC functions are executed when the thread enters an alterable state. 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.  AtomBombing  is another variation that utilizes APCs to invoke malicious code previously written to the global 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.
|M1040||Behavior Prevention on Endpoint||
Some endpoint security solutions can be configured to block some types of process injection based on common sequences of behavior that occur during the injection process.
|ID||Data Source||Data Component|
|DS0009||Process||OS API Execution|
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
NtQueueApcThread, and those that can be used to modify memory within another process, such as
WriteProcessMemory, may be used for this technique.
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.