Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging valid digital certificates to sign malicious executables, a technique often employed to bypass signature-based defenses and establish trust within the environment. SOC teams should proactively hunt for this behavior in Azure Sentinel because attackers increasingly abuse legitimate certificate authorities to evade detection, requiring correlation with user activity and file reputation data to distinguish between benign updates and sophisticated supply chain compromises.

YARA Rule

rule cert_blocklist_0450a7c1c36951da09c8ad0e7f716ff2 {
    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 "PS Partnership" and
            pe.signatures[i].serial == "04:50:a7:c1:c3:69:51:da:09:c8:ad:0e:7f:71:6f:f2" and
            1362182399 <= 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,” along with suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Enterprise antivirus solutions (e.g., Microsoft Defender, CrowdStrike Falcon) frequently download new definition packs or update their core engine binaries. These updates are signed by the vendor’s root certificate (e.g., Microsoft Windows or CrowdStrike Holdings). If a specific malware signature is detected within the same time window as an update deployment, the rule may flag the updater process itself as suspicious because it shares the signing certificate with known malicious payloads.
  • Filter/Exclusion: Create an exclusion based on the Process Name and Parent Process. Exclude processes named MpCmdRun.exe (Microsoft Defender) or Cfsensservice.exe (CrowdStrike) where the parent process is a known service host (svchost.exe) running under the SYSTEM account during standard maintenance windows.

• Scenario: Deployment of Internal Patching Tools

  • Context: IT operations teams often use internal patch management tools like Microsoft SCCM (Configuration Manager) or Ansible Tower to push updates across endpoints. These tools utilize a specific enterprise CA certificate to sign the deployment agents and scripts they execute. If this CA certificate has been compromised in a past incident or is widely used, any new script execution signed by it might trigger a false positive if the rule assumes that specific certificate is exclusively associated with external malware.
  • Filter/Exclusion: Implement an exclusion based on the Publisher Name and Source IP. Filter out events where the Publisher matches Contoso Internal CA (or your specific enterprise root) and the source IP belongs to the internal patching server subnet (e.g., 10.20.30.x).