T1055.004No distinct observable

Asynchronous Procedure Call

Stealth · Privilege EscalationWindows

Where this stands

A reviewer read the telemetry and concluded there is nothing here to detect that a sibling technique does not already cover.

AN0277's APC-specific steps (OpenThread, QueueUserAPC, altered thread context) produce no event on any of DET0100's three log sources, and the Early Bird variant produces no EventID 10 at all since the injector creates its own victim process and never calls OpenProcess. The only observable this brief actually carries on Sysmon EventID 10 — a process_access handle with memory-write rights and the create-thread bit clear — is not APC-specific: it is the identical discriminator already published for T1055.003 (b927cdb6-59b1-4468-bd9f-922b8c55b074), whose six-value mask list is a subset of this rule's ten and whose target list already carries winlogon.exe. […]

Covered instead by T1055.003.

What the technique is

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.

Read it on attack.mitre.org

What MITRE says you would watch

AN0277
Detects malicious injection behavior involving memory allocation, remote thread queuing via APC (e.g., QueueUserAPC), and altered thread context within another live process to execute unauthorized code under legitimate context.