FucknJoyv10cUsAr

Hunt Hypothesis

This rule identifies the presence of the “FucknJoyv10cUsAr” YARA signature, which likely indicates a specific memory-resident payload or custom malware variant executing within the environment. Proactively hunting for this indicator allows the SOC team to detect low-severity, potentially stealthy intrusions that may evade standard behavioral detections, ensuring early identification of targeted or niche threats in Azure Sentinel.

YARA Rule

rule FucknJoyv10cUsAr
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED D8 05 40 00 FF 74 24 20 E8 8C 02 00 00 0B C0 0F 84 2C 01 00 00 89 85 6C 08 40 00 8D 85 2F 08 40 00 50 FF B5 6C 08 40 00 E8 EF 02 00 00 0B C0 0F 84 0C 01 00 00 89 85 3B 08 40 00 8D 85 3F 08 40 00 50 FF B5 6C 08 40 00 E8 CF 02 00 }
	$a1 = {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}

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A legacy Java-based enterprise application (e.g., an internal ERP or CRM module) uses the Joy library for UI rendering or event handling, resulting in the string FucknJoy appearing in the class file or JAR archive.
  • Filter/Exclusion: Exclude files with extensions .jar, .class, or .war located in standard application directories (e.g., C:\Program Files\InternalApps\, /opt/app/lib/) where the parent process is a known Java runtime (java.exe, jre/bin/java).
• Scenario: A developer or QA engineer runs a local build script or unit test suite that compiles a custom module named Joy with a debug flag that prepends “Fuckn” to the output binary name for versioning clarity (e.g., FucknJoyv10cUsAr.dll).
  • Filter/Exclusion: Exclude processes or files located in developer workspace directories (e.g., C:\Users\<dev>\Projects\, /home/<dev>/work/) where the parent process is a build tool like msbuild.exe, dotnet.exe, npm, or make.
• Scenario: A scheduled maintenance job for a third-party analytics tool uses a temporary scratch file or log segment named FucknJoyv10cUsAr.tmp during a data aggregation phase, which is then deleted or moved to a log archive.
  • Filter/Exclusion: Exclude files with extensions .tmp, .log, or .bak that are older than 15 minutes and located in temporary directories (%TEMP%, /tmp/) or application log folders (C:\Logs\, /var/log/app/).
• Scenario: A containerized microservice (e.g., running in Docker or Kubernetes) mounts a volume containing a shared library or