APC INJECTION

Asynchronous Procedure Call (APC)process injection is a technique attackers use to execute malicious code within a legitimate process. This technique involves creating a new thread within a target process and then queuing an asynchronous procedure call (APC) to that thread. The APC can be used to execute arbitrary code within the context of the target process, allowing the attacker to bypass security measures that would

Otherwise, prevent the execution of unauthorized code.

This stealthy snippet uses the GetModuleFileNameA function to retrieve the executable path of the running process. After lowering the case, it confirms that the classic anti-analysis trick “explorer.exe ” is contained in the file. The code proceeds accordingly, whether it is found or not. If found, the code trusts the environment.

r.exe or not.

The code uses `CreateToolhelp32Snapshot` to capture a snapshot of all running processes and iterates through them with `Process32Next` and `Process32First`. It identifies the portion, parses the executable name, and compares it to `explorer.exe` for each process. If a match is found, the process ID of `explorer.exe ‘  is returned and extracted. This method is mainly used in enumeration scenarios or process injection where a specific target process, like Explorer, is needed.

The code targets the explorer.exe process and conducts DLL injection. It stores the DLL path using `writeProcessMemory` by allocating memory in the address space. It opens and retrieves thread IDs with `openThread`. In the context of those threads, a call to `LoadLibraryA` using `QueueUserAPC` is scheduled. The DLL is loaded into the target process for arbitrary code execution without direct execution. This method ensures stealth by indirect injection of code through queued procedure calls.

This PowerShell code allocates memory in its own process and performs APC-based code injection using virtualAllocEx and copying malicious code. After this, it targets the current thread, which is opened via OpenThread, and queues an asynchronous procedure call (APC) to QueueUserAPC. After placing it in an alertable state using SleepEx, the thread allows the malicious code to execute. This technique of stealthy injection abuses the Windows API without creating new processes for in-memory execution.

Example: ISFB APC Process Injection

M. Ahmad

M. Ahmad is a cybersecurity expert with over four years of experience in threat research and intelligence. He has done master’s from Staffordshire University London in Cyber Security and Forensics. He specializes in cloud security, threat hunting and incident response having worked at FireEye, Blue Hexagon, and Trustwave. He has certifications in Azure Security, Microsoft Defender, and MITRE ATT and CK Defender. Ahmad is a proficient writer and a speaker with his research focusing on vulnerability management, threat detection and malware analysis. He has a passion for sharing his experience and knowledge to keep everyone aware of emerging cybersecurity threats. He has received various awards and certifications.