Yayih code features

Hunt Hypothesis

This detection rule identifies anomalous code execution patterns associated with the Yayih malware family to uncover potential early-stage infections within the environment. Proactive hunting for these specific code features in Azure Sentinel is essential to validate false negatives and ensure rapid containment before the adversary establishes persistence or initiates lateral movement.

YARA Rule

rule YayihCode : Yayih Family 
{
    meta:
        description = "Yayih code features"
        author = "Seth Hardy"
        last_modified = "2014-07-11"
    
    strings:
        //  encryption
        $ = { 80 04 08 7A 03 C1 8B 45 FC 80 34 08 19 03 C1 41 3B 0A 7C E9 }
    
    condition:
        any of them
}

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
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Yayih code features detection rule in an enterprise environment, including suggested filters and exclusions:

• Automated CI/CD Pipeline Artifact Generation

  • Scenario: The build server (e.g., Jenkins, Azure DevOps, or GitLab CI) executes a scheduled nightly job that compiles source code and generates temporary feature flags or configuration manifests containing “Yayih” identifiers. These artifacts are written to the file system by the build-agent service account, triggering the rule as if it were a manual developer action.
  • Suggested Filter/Exclusion: Exclude events where the SourceUser is a known service account (e.g., svc-jenkins-build, azdo-pipeline-runner) AND the ProcessName matches build tools like dotnet.exe, mvn, or npm.

• Enterprise Antivirus Real-Time Scanning

  • Scenario: The endpoint protection suite (CrowdStrike, SentinelOne, or Microsoft Defender for Endpoint) performs a real-time scan on the development repository directory. During this scan, the antivirus engine extracts and analyzes code files containing Yayih feature signatures, creating file access logs that mimic code execution events.
  • Suggested Filter/Exclusion: Create an exclusion for processes with ImageName matching known security agents (e.g., C:\Program Files\CrowdStrike\csfalcon.exe) accessing paths under the standard source control directories (e.g., D:\SourceControl\*).

• Scheduled Database Backup and Indexing Jobs

  • Scenario: A nightly SQL Server maintenance job (SQL Agent) runs a backup or index rebuild on a database containing application configuration tables. If these tables store serialized code feature metadata labeled “Yayih,” the database engine (`sqlserv