Detect Regsvcs/Regasm in Splunk
Adversaries may abuse Regsvcs and Regasm to proxy execution of code through trusted Windows utilities. Regsvcs and Regasm are Windows command-line utilities used to register .NET Component Object Model (COM) assemblies. Both are digitally signed by Microsoft. These utilities can bypass application control through use of attributes within the binary to specify code that should run before registration or unregistration: [ComRegisterFunction] or [ComUnregisterFunction] respectively. Critically, the code decorated with these attributes executes even if the process runs with insufficient privileges and fails. Agent Tesla is a notable malware family that uses Regasm.exe for proxy execution.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1218 System Binary Proxy Execution
- Sub-technique
- T1218.009 Regsvcs/Regasm
- Canonical reference
- https://attack.mitre.org/techniques/T1218/009/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
(Image="*\\regsvcs.exe" OR Image="*\\regasm.exe" OR ParentImage="*\\regsvcs.exe" OR ParentImage="*\\regasm.exe")
| eval SuspiciousPath=if(match(CommandLine, "(Temp|AppData|Downloads|Public|Desktop|ProgramData)"), 1, 0)
| eval Unregister=if(match(CommandLine, "(/u|/unregister)"), 1, 0)
| eval SilentFlag=if(match(CommandLine, "(/silent|/s)"), 1, 0)
| eval SuspiciousParent=if(match(ParentImage, "(cmd|powershell|wscript|cscript|mshta|winword|excel)\.exe"), 1, 0)
| eval SuspiciousChild=if((ParentImage="*\\regsvcs.exe" OR ParentImage="*\\regasm.exe") AND match(Image, "(cmd|powershell|wscript|cscript|net|rundll32)\.exe"), 1, 0)
| eval RiskScore=SuspiciousPath + Unregister + SuspiciousParent + SuspiciousChild
| where RiskScore > 0 OR SuspiciousChild=1
| table _time, host, User, Image, CommandLine, ParentImage, ParentCommandLine, SuspiciousPath, Unregister, SilentFlag, SuspiciousParent, SuspiciousChild, RiskScore
| sort - _timeDetects Regsvcs/Regasm abuse using Sysmon Event ID 1. Covers both process execution and child process spawning. The /unregister flag is a key indicator as adversaries use it to invoke ComUnregisterFunction-decorated code even when registration fails due to insufficient privileges.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate .NET software that registers COM interop assemblies via Regasm.exe during installation
- Software development activities where developers register and unregister .NET COM assemblies for testing
- Enterprise applications with .NET-based COM components registered during software deployment
- Windows SDK tools and Visual Studio that use Regasm.exe for .NET COM registration during build processes
Other platforms for T1218.009
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 1Regasm Execution from Temp Directory
Expected signal: Sysmon Event ID 1: regasm.exe with /s flag and Temp path. Security Event ID 4688. Sysmon Event ID 7 for the DLL being loaded.
- Test 2Regasm with Unregister Flag
Expected signal: Sysmon Event ID 1: regasm.exe with /u and /s flags, Temp path. Security Event ID 4688. The process will fail if the file does not exist but the process creation event fires.
- Test 3Regsvcs Launched from PowerShell
Expected signal: Sysmon Event ID 1: powershell.exe then regsvcs.exe with ParentImage=powershell.exe. SuspiciousParent and SuspiciousPath both fire. Security Event ID 4688.
References (7)
- https://attack.mitre.org/techniques/T1218/009/
- https://lolbas-project.github.io/lolbas/Binaries/Regsvcs/
- https://lolbas-project.github.io/lolbas/Binaries/Regasm/
- https://msdn.microsoft.com/en-us/library/04za0hca.aspx
- https://msdn.microsoft.com/en-us/library/tzat5yw6.aspx
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1218.009/T1218.009.md
- https://www.sentinelone.com/labs/agent-tesla-old-rat-uses-new-tricks-to-stay-on-top/
Unlock Pro Content
Get the full detection package for T1218.009 including response playbook, investigation guide, and atomic red team tests.