Detect AppCert DLLs in CrowdStrike LogScale
Adversaries may establish persistence and/or elevate privileges by executing malicious content triggered by AppCert DLLs loaded into processes. Dynamic-link libraries (DLLs) that are specified in the AppCertDLLs Registry key under HKLM\System\CurrentControlSet\Control\Session Manager are loaded into every process that calls the commonly used application programming interface (API) functions CreateProcess, CreateProcessAsUser, CreateProcessWithLoginW, CreateProcessWithTokenW, or WinExec. This provides adversaries a way to have code execute in the security context of every process on the system, including processes with high privilege levels.
MITRE ATT&CK
- Tactic
- Privilege Escalation Persistence
- Technique
- T1546 Event Triggered Execution
- Sub-technique
- T1546.009 AppCert DLLs
- Canonical reference
- https://attack.mitre.org/techniques/T1546/009/
LogScale Detection Query
// Phase 1: Registry write to AppCertDlls (persistence establishment)
(#event_simpleName=RegOperationValue OR #event_simpleName=AsepValueUpdate)
| TargetObjectName = /(?i)\\System\\CurrentControlSet\\Control\\Session Manager\\AppCertDlls/
| eval SuspiciousPath = if(
RegStringValue != /(?i)(C:\\Windows\\(system32|SysWOW64|WinSxS)\\)/,
"SUSPICIOUS_NON_SYSTEM_DLL",
"SYSTEM_PATH_OK"
)
| table([@timestamp, ComputerName, UserName, TargetObjectName, RegStringValue, SuspiciousPath, ProcessImageFileName, CommandLine])
// Phase 2: Suspicious DLL module load (runtime execution) — run as separate query
// Uncomment and run independently or union via Falcon Fusion
// #event_simpleName=ModuleLoadV3
// | FilePath = /(?i)appcert/
// | FilePath != /(?i)(\\Windows\\system32\\|\\Windows\\SysWOW64\\|\\Windows\\WinSxS\\|\\Program Files\\)/
// | groupBy([ComputerName, UserName, FilePath, FileName, ProcessImageFileName, TargetProcessId], function=count(as=LoadCount))
// | sort(LoadCount, order=desc)Detects AppCert DLL persistence in CrowdStrike Falcon LogScale by querying RegOperationValue and AsepValueUpdate events for writes to the AppCertDlls registry key where the registered DLL path is outside Windows system directories. A companion query (Phase 2) identifies runtime DLL loads matching appcert patterns from non-system paths via ModuleLoadV3 events. AsepValueUpdate events indicate Falcon's auto-start extensibility point detection for AppCert DLL changes.
Data Sources
Required Tables
False Positives & Tuning
- CrowdStrike Falcon sensor itself or competing EDR products that register monitoring DLLs in AppCertDlls from their agent install directory rather than system32
- Application compatibility or virtual desktop infrastructure (VDI) agents that use AppCertDlls to intercept CreateProcess calls for session management from custom install paths
- Security assessment or red team tooling running in authorized penetration testing engagements where AppCert DLL persistence is being tested against production-equivalent environments
Other platforms for T1546.009
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 1Register DLL in AppCertDlls Registry Key
Expected signal: Sysmon Event ID 13: TargetObject=HKLM\System\CurrentControlSet\Control\Session Manager\AppCertDlls\ArgusTestDll, Details=C:\Users\Public\argus_appcert_test.dll. Security Event ID 4657 if registry auditing enabled.
- Test 2Query Existing AppCertDlls Registrations
Expected signal: Process creation for reg.exe with query arguments for the AppCertDlls key. Registry access events if object access auditing is enabled. The output reveals all currently registered DLLs.
- Test 3Create and Register AppCert DLL for Process Injection
Expected signal: File creation event for argus_appcert.dll in Public folder. Sysmon Event ID 13 for AppCertDlls registry key modification. Subsequently, Sysmon Event ID 7 records showing the DLL loaded by various processes that call CreateProcess (e.g., cmd.exe spawning children).
References (4)
- https://attack.mitre.org/techniques/T1546/009/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1546.009/T1546.009.md
- https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-windows
- https://pentestlab.blog/2019/12/16/persistence-appcert-dlls/
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