Detect Asynchronous Procedure Call in Elastic Security
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 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 LoadLibraryA pointing to a malicious DLL). A variation called Early Bird injection involves creating a suspended process in which malicious code is written and executed before the process' entry point via an APC. AtomBombing is another variation that utilizes APCs to invoke malicious code previously written to the global atom table.
MITRE ATT&CK
- Technique
- T1055 Process Injection
- Sub-technique
- T1055.004 Asynchronous Procedure Call
- Canonical reference
- https://attack.mitre.org/techniques/T1055/004/
Elastic Detection Query
process where event.action == "process_accessed" and
winlog.event_id == 10 and
winlog.event_data.GrantedAccess in ("0x1FFFFF", "0x001F0FFF", "0x1F3FFF", "0x1410", "0x143A", "0x1F1FFF") and
(
winlog.event_data.SourceImage like~ ("*\\powershell.exe", "*\\cmd.exe", "*\\wscript.exe",
"*\\cscript.exe", "*\\mshta.exe", "*\\rundll32.exe", "*\\regsvr32.exe") or
not winlog.event_data.SourceImage like "C:\\Windows\\*"
) and
winlog.event_data.TargetImage like~ ("*\\svchost.exe", "*\\rundll32.exe", "*\\EhStorAuthn.exe",
"*\\ctfmon.exe", "*\\conhost.exe", "*\\dllhost.exe")Detects T1055.004 APC Injection via Sysmon Event ID 10 (Process Access) by identifying cross-process handles opened with injection-capable access rights (PROCESS_ALL_ACCESS 0x1FFFFF, PROCESS_VM_WRITE+PROCESS_VM_OPERATION+PROCESS_CREATE_THREAD combinations including 0x001F0FFF, 0x1F3FFF, 0x1410, 0x143A, 0x1F1FFF) from known LOLBin parents or non-Windows source paths targeting common Early Bird APC victim processes. Covers QueueUserAPC, NtQueueApcThread, and AtomBombing delivery patterns.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate EDR and AV agents (CrowdStrike Falcon, SentinelOne, Carbon Black) that open PROCESS_ALL_ACCESS handles to svchost.exe, conhost.exe, and dllhost.exe for in-memory scanning and behavioral monitoring
- Sysinternals tools (Process Explorer, ProcMon) and administrative debuggers (WinDbg, x64dbg) opened by administrators that request high-privilege cross-process access for legitimate system inspection
- Software deployment tools (SCCM, Tanium, Ansible) invoked via PowerShell or cmd.exe that perform in-memory operations against services hosted within svchost.exe during patch and policy enforcement cycles
Other platforms for T1055.004
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Early Bird Injection - Suspended Process Creation
Expected signal: Sysmon Event ID 1: rundll32.exe spawned by PowerShell with empty/minimal CommandLine. This alone is suspicious — rundll32.exe should always have arguments specifying the DLL and function to execute.
- Test 2QueueUserAPC API Call via PowerShell P/Invoke
Expected signal: Sysmon Event ID 1: PowerShell execution with the command line. When using the actual API chain: ETW NtQueueApcThread event, Sysmon Event ID 10 (ProcessAccess) with PROCESS_ALL_ACCESS.
- Test 3AtomBombing Variant - Global Atom Table Write
Expected signal: Sysmon Event ID 1: PowerShell execution. ETW: GlobalAddAtomW API call logged. In a full AtomBombing attack, this would be followed by NtQueueApcThread to copy atom data into the target process.
References (6)
- https://attack.mitre.org/techniques/T1055/004/
- https://msdn.microsoft.com/library/windows/desktop/ms681951.aspx
- https://www.cyberbit.com/blog/endpoint-security/new-early-bird-code-injection-technique-discovered/
- https://blog.ensilo.com/atombombing-brand-new-code-injection-for-windows
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1055.004/T1055.004.md
- https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-process
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