md5 cdn js link js

Hunt Hypothesis

The hypothesis is that the detection rule identifies potential malicious JavaScript payloads embedded within legitimate CDN links, leveraging MD5 hashes to obscure malicious behavior. SOC teams should proactively hunt for this in Azure Sentinel to identify and mitigate early-stage adversarial activity that may evade traditional detection methods.

YARA Rule

rule md5_cdn_js_link_js {
    strings: $ = "grelos_v= null"
    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

• Scenario: A legitimate system update or patch deployment includes JavaScript files with known MD5 hashes from a CDN.
  Filter/Exclusion: Exclude files with MD5 hashes matching known CDN JavaScript files (e.g., hashes_from_cdn_js.txt).

• Scenario: A developer uses a CDN-hosted JavaScript library (e.g., jQuery, Bootstrap) in a local development environment.
  Filter/Exclusion: Exclude files accessed from known CDN domains (e.g., cdnjs.com, jsDelivr.net).

• Scenario: A scheduled job runs a script that downloads and executes JavaScript from a CDN for automated testing.
  Filter/Exclusion: Exclude requests to CDN endpoints used by internal testing tools (e.g., test-cdn.example.com).

• Scenario: An administrator manually uploads a JavaScript file from a CDN to a staging environment for review.
  Filter/Exclusion: Exclude files uploaded via known admin tools (e.g., scp, rsync, or specific CI/CD pipelines like Jenkins or GitLab CI).

• Scenario: A security tool or WAF injects JavaScript for monitoring or logging purposes, which matches the CDN pattern.
  Filter/Exclusion: Exclude files with known security tool signatures (e.g., modsecurity, WAF, or log.js from internal monitoring tools).