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high advisory

Regsvr32 Execution with Masqueraded DLL Files

Adversaries utilize regsvr32.exe to execute malicious dynamic link libraries (DLLs) by masquerading them as benign file types, such as images or text files, to bypass basic execution controls.

Triage facts

Revision 1
Applicability
Windows
Evidence
Unknown
Severity
High
Action
Elevated
Sources
3 public sources
Freshness
Revision 1Nov 10, 03:03 UTC
Detection
1 rule
Source quality
B2Likely

Adversaries frequently use the Windows utility regsvr32.exe to execute arbitrary code. By renaming a malicious DLL with a deceptive extension (e.g., .jpg, .txt, .log), attackers can bypass security filters that rely on simple file extension allowlisting. This technique leverages the fact that regsvr32.exe will attempt to load and execute the file as a COM server regardless of the extension provided in the command line.

Recommendation

  • Deploy the Sigma rule below to monitor for execution patterns where regsvr32.exe is invoked with arguments ending in non-standard extensions.
  • Audit command-line execution logs for regsvr32.exe activity and baseline legitimate usage to identify deviations.
  • Implement endpoint controls to restrict the execution of regsvr32.exe by non-administrative users or unauthorized processes.

Immediate actions

Deploy the Sigma rule to detect suspicious regsvr32 activity

Detection Engineering 48h

Threat Hunt

Hunt for regsvr32.exe process executions where the command line argument does not end in .dll or .ocx

T1218.010 medium high confidence convert to detection

Data: Process command line logging

Detection coverage 1

Detect Regsvr32 Execution With Suspicious File Extension

high

Detects the execution of regsvr32.exe with file arguments masquerading as images, logs, or other non-DLL file types

sigma tactics: stealth techniques: T1218.010 sources: process_creation, windows

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