Detect Asynchronous Procedure Call in Microsoft Sentinel
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/
KQL Detection Query
// Detect APC injection: suspended process creation followed by remote thread/APC execution
// Early Bird pattern: CreateProcess(SUSPENDED) -> WriteProcessMemory -> QueueUserAPC -> ResumeThread
let SuspiciousParents = dynamic(["powershell.exe", "cmd.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe"]);
DeviceProcessEvents
| where Timestamp > ago(24h)
| where ProcessCommandLine == "" or ProcessCommandLine == FileName
| where InitiatingProcessFileName in~ (SuspiciousParents)
| where FileName in~ ("svchost.exe", "rundll32.exe", "EhStorAuthn.exe", "ctfmon.exe", "conhost.exe", "dllhost.exe")
| join kind=leftouter (
DeviceEvents
| where Timestamp > ago(24h)
| where ActionType == "CreateRemoteThreadApiCall"
| project ThreadTime=Timestamp, DeviceName, TargetProcessId=ProcessId, InjectorProcess=InitiatingProcessFileName
) on DeviceName, $left.ProcessId == $right.TargetProcessId
| project Timestamp, DeviceName, AccountName, InitiatingProcessFileName, InitiatingProcessCommandLine, FileName, ProcessId, InjectorProcess
| sort by Timestamp descDetects the Early Bird APC injection pattern by identifying processes spawned with empty or minimal command lines by suspicious parent processes — a telltale sign of suspended process creation for injection. Correlates with subsequent CreateRemoteThread events targeting the spawned process. Common Early Bird targets include svchost.exe, rundll32.exe, and EhStorAuthn.exe.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate COM object activation spawning dllhost.exe with minimal command lines
- Service Control Manager spawning svchost.exe instances
- Windows Update creating suspended processes for staged updates
- Application installers spawning helper processes in suspended state for configuration
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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