Vx Trivial25

Hunt Hypothesis

This rule targets specific memory patterns associated with the Vx_Trivial25 YARA signature, indicating the presence of a low-severity malicious or suspicious code artifact in process memory. Proactively hunting for this indicator allows the SOC to identify early-stage infections or benign false positives that may have been missed by real-time detections, ensuring comprehensive coverage of the Azure Sentinel environment.

YARA Rule

rule Vx_Trivial25: PEiD
{
    strings:
        $a = { B4 4E FE C6 CD 21 B8 ?? 3D BA ?? 00 CD 21 93 B4 40 CD }
    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-based enterprise application (e.g., an internal HR portal or legacy ERP system) uses a custom bytecode obfuscation or packing routine that embeds a specific, static header string or magic number matching the Vx_Trivial25 signature. This is common in older, unpatched JAR files or WAR archives deployed via Jenkins or Artifactory.

  • Filter/Exclusion: Exclude files located in standard Java artifact directories (e.g., C:\Program Files\Java\, C:\Artifacts\, or *.jar/*.war extensions) if the parent process is a known JVM launcher (java.exe, jre.exe) or a CI/CD agent (jenkins.exe, maven.exe).

• Scenario: A widely used, legitimate third-party installer or updater (e.g., Adobe Creative Cloud, Oracle Java Updater, or a specific vendor’s agent like CrowdStrike Falcon) performs a self-extraction or temporary file creation that contains a benign binary pattern matching the rule’s byte sequence. This often happens during scheduled maintenance windows or post-reboot cleanup tasks.

  • Filter/Exclusion: Exclude processes with known legitimate parent hashes (e.g., AdobeUpdateService.exe, javaw.exe from Oracle, or FalconService.exe) and limit exclusions to specific temporary directories (%TEMP%, %LOCALAPPDATA%\Temp) where the file age is less than 1 hour.

• Scenario: A development team uses a proprietary build tool or a custom C++/C# compiler wrapper that generates intermediate object files or debug symbols with a unique, non-standard header. These files are frequently written to the build output directory during continuous integration pipelines or local developer workstations.

  • Filter/Exclusion: Exclude files in common build/output directories (e.g., bin\, `obj