Detect Regsvcs/Regasm in Microsoft Sentinel
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/
KQL Detection Query
DeviceProcessEvents
| where Timestamp > ago(24h)
| where FileName in~ ("regsvcs.exe", "regasm.exe")
| extend SuspiciousPath = ProcessCommandLine has_any ("Temp", "AppData", "Downloads", "Public", "Desktop", "ProgramData")
| extend UnregisterFlag = ProcessCommandLine has_any ("/u", "/unregister")
| extend SilentFlag = ProcessCommandLine has_any ("/silent", "/s")
| extend SuspiciousParent = InitiatingProcessFileName has_any ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "winword.exe", "excel.exe")
| extend RemotePath = ProcessCommandLine has_any ("http://", "https://", "\\\\")
| project Timestamp, DeviceName, AccountName, FileName, ProcessCommandLine, InitiatingProcessFileName,
InitiatingProcessCommandLine, SuspiciousPath, UnregisterFlag, SilentFlag, SuspiciousParent, RemotePath
| sort by Timestamp desc
union (
DeviceProcessEvents
| where Timestamp > ago(24h)
| where InitiatingProcessFileName in~ ("regsvcs.exe", "regasm.exe")
| project Timestamp, DeviceName, AccountName, FileName, ProcessCommandLine,
InitiatingProcessFileName, InitiatingProcessCommandLine
| sort by Timestamp desc
)Detects Regsvcs/Regasm abuse by monitoring for .NET assemblies loaded from user-writable paths, /unregister flag usage (triggers ComUnregisterFunction decorated code), suspicious parent processes, and child process spawning. Also flags any execution from scripting engine parents which indicates multi-stage malware.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate .NET software that registers COM interop assemblies via Regasm.exe during installation (common with Office interop assemblies)
- 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/
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