Vx Caz1204

Hunt Hypothesis

This detection identifies potential variants of the Caz1204 virus, a legacy file infector known for appending malicious code to executable files, indicating possible legacy malware persistence or dormant threats within the environment. Proactively hunting for this signature in Azure Sentinel helps identify overlooked or reactivated legacy infections that may have evaded modern behavioral detections, ensuring comprehensive coverage against known file-based threats.

YARA Rule

rule Vx_Caz1204: PEiD
{
    strings:
        $a = { E8 ?? ?? 5E 83 EE 03 1E 06 B8 FF FF CD 2F 3C 10 }
    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., a custom internal ERP or middleware) is deployed or updated via a package manager like Chocolatey or Ansible. The JAR file or embedded native library contains the specific byte sequence matched by the Vx_Caz1204 YARA rule, which often targets specific obfuscated or packed Java bytecode patterns.

  • Filter/Exclusion: Exclude files located in standard Java application directories (e.g., C:\Program Files\Java\, C:\opt\java\, or specific application install paths) if the file extension is .jar, .war, or .ear. Additionally, exclude processes running from known Java runtime paths (e.g., java.exe, javaw.exe) if the detection is triggered on the process image rather than the file itself.

• Scenario: A development team uses a specific version of a C++ compiler or linker (e.g., older versions of MSVC or GCC) that generates object files or executables with a header structure or padding pattern matching the rule. This can occur during local builds in developer workstations or CI/CD build agents before the final artifact is stripped or optimized.

  • Filter/Exclusion: Exclude files in developer workspace directories (e.g., C:\Users\<user>\source\, C:\builds\, or /home/<user>/projects/) if the file extension is .obj, .o, .lib, or .a. For CI/CD agents, exclude paths under the build tool’s temporary workspace (e.g., C:\jenkins\workspace\, /var/lib/jenkins/workspace/).

• Scenario: A third-party vendor-provided utility or agent (e.g., a specific version of a backup client, antivirus scanner, or monitoring agent) is installed or updated. The vendor’s binary is compiled in a way