Asynchronous Procedure Call (APC) Injection via NtTestAlert is a malware technique that forces a target thread to execute malicious code using Asynchronous Procedure Calls (APCs). The attacker first allocates memory in the target process and writes the payload. Then, they queue an APC to the target thread. To ensure execution, they use NtTestAlert, which forces the thread into an alertable state, making it process the queued APC immediately. This technique is stealthy because it avoids direct thread creation and can bypass some security mechanisms.