Thoreador 12 Sergi

Hunt Hypothesis

This hypothesis targets the presence of the Thoreador 12 Sergi YARA signature, which indicates a specific malware variant or implant often associated with targeted intrusions or persistent backdoors. Proactively hunting for this indicator allows the SOC to identify compromised endpoints or memory regions before the adversary establishes full command-and-control or executes lateral movement within the Azure environment.

YARA Rule

rule Thoreador_12_Sergi: PEiD
{
    strings:
        $a = { 55 E8 00 00 00 00 5D 83 ED 06 81 ED E2 00 00 00 60 EB 6F 90 90 90 }
    condition:
        $a at pe.entry_point

}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A legacy Java application server (e.g., Apache Tomcat or WildFly) is performing a standard garbage collection cycle or loading a large, optimized class file during peak business hours. The specific byte sequence in the Sergi object header or method table may match the YARA pattern if the rule targets common JVM memory layout artifacts.
  • Filter/Exclusion: Exclude processes where the parent process is java.exe or jvm.dll and the file path contains \lib\ or \classes\. Alternatively, add a condition to ignore matches in memory regions tagged as heap or code_cache if the YARA rule allows metadata checks.
• Scenario: An enterprise endpoint protection agent (e.g., CrowdStrike Falcon, Carbon Black, or SentinelOne) is scanning a newly downloaded installer package (e.g., setup.exe for Adobe Creative Cloud or Microsoft Office) in its quarantine or staging directory. The YARA rule may match a specific code section or resource string within the installer that coincidentally aligns with the Thoreador_12_Sergi signature.
  • Filter/Exclusion: Exclude files located in standard staging directories such as C:\ProgramData\Microsoft\Windows Defender\Quarantine\, C:\Program Files\CrowdStrike\, or C:\temp\ if the file age is less than 1 hour. Add an exclusion for known installer hashes or file names like setup.exe, install.exe, or msiexec.exe child processes.
• Scenario: A developer is running a local build pipeline using MSBuild or Gradle on a build server, compiling a large C# or Java project. The YARA rule may match intermediate compiled assemblies (.dll or .jar) in the bin\ or build\ directories that contain specific constant strings or