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ATT&CK Technique

Asynchronous Procedure Call

T1055.004 · stealth, privilege-escalation

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.

Windows
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Atomic Tests

3
Executable Atomic Red Team test cases for exercising this technique in a lab. Copy a command, run it on the listed platform, confirm your detections fire.
command_promptwindowsProcess Injection via C#
Process Injection using C# reference: https://github.com/pwndizzle/c-sharp-memory-injection Excercises Five Techniques 1. Process injection 2. ApcInjectionAnyProcess 3. ApcInjectionNewProcess 4. IatInjection 5. ThreadHijack Upon successful execution, cmd.exe will execute T1055.exe, which exercises 5 techniques. Output will be via stdout. 
"#{exe_binary}"
powershellwindowsEarlyBird APC Queue Injection in Go
Creates a process in a suspended state and calls QueueUserAPC WinAPI to add a UserAPC to the child process that points to allocated shellcode. ResumeThread is called which then calls NtTestAlert to execute the created UserAPC which then executes the shellcode. This technique allows for the early execution of shellcode and potentially before AV/EDR can hook functions to support detection. - PoC Credit: (https://github.com/Ne0nd0g/go-shellcode#createprocesswithpipe) - References: - https://www.bleepingcomputer.com/news/security/early-bird-code-injection-technique-helps-malware-stay-undetected/ - https://www.ired.team/offensive-security/code-injection-process-injection/early-bird-apc-queue-code-injection 
$PathToAtomicsFolder\T1055.004\bin\x64\EarlyBird.exe -program "#{spawn_process_path}" -debug
powershellwindowsRemote Process Injection with Go using NtQueueApcThreadEx WinAPI
Uses the undocumented NtQueueAPCThreadEx WinAPI to create a "Special User APC" in the current thread of the current process to execute shellcode. Since the shellcode is loaded and executed in the current process it is considered local shellcode execution. Steps taken with this technique 1. Allocate memory for the shellcode with VirtualAlloc setting the page permissions to Read/Write 2. Use the RtlCopyMemory macro to copy the shellcode to the allocated memory space 3. Change the memory page permissions to Execute/Read with VirtualProtect 4. Get a handle to the current thread 5. Execute the shellcode in the current thread by creating a Special User APC through the NtQueueApcThreadEx function - PoC Credit: (https://github.com/Ne0nd0g/go-shellcode/tree/master#rtlcreateuserthread) - References: - https://repnz.github.io/posts/apc/user-apc/ - https://docs.rs/ntapi/0.3.1/ntapi/ntpsapi/fn.NtQueueApcThreadEx.html - https://0x00sec.org/t/process-injection-apc-injection/24608 - https://twitter.com/aionescu/status/992264290924032005 - http://www.opening-windows.com/techart_windows_vista_apc_internals2.htm#_Toc229652505 
$PathToAtomicsFolder\T1055.004\bin\x64\NtQueueApcThreadEx.exe -debug
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Mitigations

1
MITRE ATT&CK mitigations - vendor-agnostic guidance for reducing exposure to this technique.
M1040Behavior Prevention on Endpoint

Behavior Prevention on Endpoint refers to the use of technologies and strategies to detect and block potentially malicious activities by analyzing the behavior of processes, files, API calls, and other endpoint events. Rather than relying solely on known signatures, this approach leverages heuristics, machine learning, and real-time monitoring to identify anomalous patterns indicative of an attack.

Suspicious Process Behavior
  • Implementation: Use Endpoint Detection and Response (EDR) tools to monitor and block processes exhibiting unusual behavior, such as privilege escalation attempts.
  • Use Case: An attacker uses a known vulnerability to spawn a privileged process from a user-level application. The endpoint tool detects the abnormal parent-child process relationship and blocks the action.
Unauthorized File Access
  • Implementation: Leverage Data Loss Prevention (DLP) or endpoint tools to block processes attempting to access sensitive files without proper authorization.
  • Use Case: A process tries to read or modify a sensitive file located in a restricted directory, such as /etc/shadow on Linux or the SAM registry hive on Windows. The endpoint tool identifies this anomalous behavior and prevents it.
Abnormal API Calls
  • Implementation: Implement runtime analysis tools to monitor API calls and block those associated with malicious activities.
  • Use Case: A process dynamically injects itself into another process to hijack its execution. The endpoint detects the abnormal use of APIs like OpenProcess and WriteProcessMemory and terminates the offending process.
Exploit Prevention
  • Implementation: Use behavioral exploit prevention tools to detect and block exploits attempting to gain unauthorized access.
  • Use Case: A buffer overflow exploit is launched against a vulnerable application. The endpoint detects the anomalous memory write operation and halts the process.
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Detection Coverage

0/6 layers
Coverage across standard detection surfaces. Rows marked none have no rule of that type mapped. Some are real blind spots worth closing; others are simply not applicable to this technique (e.g. YARA matches malware files, not network behaviour).
Behavioral / log (Sigma) none
Analytics (MITRE CAR) none
Runtime / container (Falco) none
File / malware (YARA) none
Network (Suricata/Snort) none
Vuln scan (Nuclei) none
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Comply & Defend

NIST 800-53AC-06, SC-07, SC-18, SI-02, SI-03, SI-04
D3FEND6 defensive techniques
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