Detect Regsvcs/Regasm in CrowdStrike LogScale
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/
LogScale Detection Query
// T1218.009 - Regsvcs/Regasm Proxy Execution Detection
// Detect regsvcs.exe or regasm.exe as process or parent, with suspicious indicators
#event_simpleName = ProcessRollup2
| FileName = /(?i)(regsvcs|regasm)\.exe/ OR ParentBaseFileName = /(?i)(regsvcs|regasm)\.exe/
| eval SuspiciousPath = if(match(field=CommandLine, /(?i)(Temp|AppData|Downloads|Public|Desktop|ProgramData)/), 1, 0)
| eval UnregisterFlag = if(match(field=CommandLine, /(?i)(/u|/unregister|/silent|/s)\b/), 1, 0)
| eval SuspiciousParent = if(match(field=ParentBaseFileName, /(?i)(cmd|powershell|wscript|cscript|mshta|winword|excel)\.exe/), 1, 0)
| eval SuspiciousChild = if(
match(field=ParentBaseFileName, /(?i)(regsvcs|regasm)\.exe/) AND
match(field=FileName, /(?i)(cmd|powershell|wscript|cscript|net|rundll32)\.exe/),
1, 0)
| eval RemotePath = if(match(field=CommandLine, /(?i)(http:\/\/|https:\/\/|\\\\)/), 1, 0)
| eval RiskScore = SuspiciousPath + UnregisterFlag + SuspiciousParent + SuspiciousChild + RemotePath
| where RiskScore > 0 OR SuspiciousChild = 1
| select timestamp, ComputerName, UserName, FileName, CommandLine, ParentBaseFileName, ParentCommandLine,
SuspiciousPath, UnregisterFlag, SuspiciousParent, SuspiciousChild, RemotePath, RiskScore
| sort(field=RiskScore, order=desc)
| sort(field=timestamp, order=desc)
| limit 500CrowdStrike LogScale (Falcon) query detecting Regsvcs/Regasm proxy execution via ProcessRollup2 telemetry. Evaluates multiple suspicious indicators and produces a risk score, flagging events where regsvcs or regasm runs from anomalous paths, with suspicious flags, from scripting engine parents, or spawns suspicious child processes.
Data Sources
Required Tables
False Positives & Tuning
- Software packaging solutions (e.g., InstallShield, Advanced Installer) that invoke regasm.exe as part of MSI custom actions, potentially from temp extraction paths
- Security tools or EDR agents that themselves inspect or register .NET assemblies during their own installation or update cycles
- Developers using PowerShell scripts in CI pipelines on developer workstations to register and test COM-visible .NET assemblies before packaging
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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