Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging valid digital certificates to sign malicious executables, a technique designed to bypass trust-based security controls and evade signature-based detection. Proactive hunting in Azure Sentinel is essential because attackers increasingly abuse legitimate certificate authorities to establish persistence and execute code that appears benign to standard endpoint protection mechanisms.

YARA Rule

rule cert_blocklist_01a90094c83412c00cf98dd2eb0d7042 {
    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 "FreeVox SA" and
            pe.signatures[i].serial == "01:a9:00:94:c8:34:12:c0:0c:f9:8d:d2:eb:0d:70:42" and
            1376956799 <= 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 3-5 specific false positive scenarios for the “Certificate used for digitally signing malware” detection rule, including suggested filters and exclusions:

• Scenario: Automated Security Tool Updates

  • Context: Enterprise Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon, Microsoft Defender for Endpoint, or SentinelOne frequently update their own signature databases and engine binaries. These updates are often signed by the vendor’s root certificate, which is also used to sign known malware samples in public threat intelligence feeds.
  • Suggested Filter: Exclude processes where the Process Name matches specific EDR service executables (e.g., FalconSensor.exe, MsMpEng.exe) and the Parent Process is the EDR updater service or Windows Update (usoc.dll).

• Scenario: Scheduled Antivirus Definition Syncs

  • Context: During off-hours, scheduled tasks trigger antivirus definition updates (e.g., via Symantec Endpoint Protection, Kaspersky, or Sophos). The update installer downloads new signature files and executes them using the vendor’s signing certificate. If this certificate has been flagged in a malware database due to a past compromise of that specific vendor, legitimate updates may trigger an alert.
  • Suggested Exclusion: Create a whitelist for the File Hash (SHA-256) of known update installer executables associated with these vendors, or filter alerts where the Event Type is “Scheduled Task Execution” occurring between 01:00 and 04:00 local time.

• Scenario: Internal DevOps CI/CD Pipeline Artifacts

  • Context: In a hybrid environment, internal DevOps pipelines (e.g., using Azure DevOps, Jenkins, or GitHub Actions) build and deploy custom applications. These builds