Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging valid digital certificates to sign malicious executables, a tactic designed to bypass signature-based defenses and evade initial detection by impersonating trusted software. Proactive hunting in Azure Sentinel is essential because attackers increasingly abuse legitimate certificate infrastructure to establish persistence and execute stealthy attacks that traditional security controls may overlook due to the low severity of signed artifacts.

YARA Rule

rule cert_blocklist_049ce8c47f1f0e650cb086f0cfa7ca53 {
    meta:
        author      = "ReversingLabs"
        source      = "ReversingLabs"
        status      = "RELEASED"
        sharing     = "TLP:WHITE"
        category    = "INFO"
        description = "Certificate used for digitally signing malware."

    condition:
        uint16(0) == 0x5A4D and
        for any i in (0..pe.number_of_signatures): (
            pe.signatures[i].subject contains "Select'Assistance Pro" and
            pe.signatures[i].serial == "04:9c:e8:c4:7f:1f:0e:65:0c:b0:86:f0:cf:a7:ca:53" and
            1393804799 <= pe.signatures[i].not_after
        )
}

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 rule “Certificate used for digitally signing malware,” including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Patch Deployment via Microsoft Endpoint Configuration Manager (SCCM)

  • Context: During scheduled maintenance windows, SCCM pushes updates to thousands of endpoints. The update packages are signed by the “Microsoft Windows” or specific vendor certificates that may have previously been flagged in threat intelligence feeds due to supply chain compromises elsewhere.
  • Filter/Exclusion: Exclude processes where ProcessName is ccmsetup.exe, wuauserv.exe, or msiexec.exe running under the SYSTEM account, provided the file path resides within %ProgramFiles%\Microsoft Update\ or a designated patch distribution point folder.

• Scenario: Internal DevOps CI/CD Pipeline Artifacts

  • Context: An internal Azure DevOps or Jenkins pipeline builds and deploys custom enterprise applications (e.g., HR portals, inventory tools) signed by an internal Root CA. If this specific internal certificate has been flagged globally due to a past incident on a different domain, legitimate deployments will trigger alerts.
  • Filter/Exclusion: Create a whitelist for the IssuerName containing the enterprise’s internal root (e.g., “CN=Enterprise-Root-CA”) and restrict the scope to process paths starting with %ProgramFiles%\InternalApps\ or specific CI agent directories like C:\Jenkins\workspace\.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Major security vendors (e.g., CrowdStrike, SentinelOne, Microsoft Defender) frequently update their definition databases. The updater service often uses a signing certificate that is shared across various products; if this certificate was recently associated with a specific malware family in the vendor’s global feed, legitimate local updates may trigger