Certificate used for digitally signing malware.

Hunt Hypothesis

This detection identifies adversaries leveraging valid digital certificates to sign malicious executables, a technique often employed to bypass signature-based defenses and establish trust within the environment. Proactive hunting in Azure Sentinel is essential because attackers frequently abuse legitimate or compromised certificates to evade initial security controls, necessitating deeper analysis of certificate provenance and usage patterns beyond standard rule triggers.

YARA Rule

rule cert_blocklist_57d7153a89bbf4729be87f3c927043aa {
    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 "Open Source Developer, zhenganjun" and
            pe.signatures[i].serial == "57:d7:15:3a:89:bb:f4:72:9b:e8:7f:3c:92:70:43:aa" and
            1469059200 <= 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 targeted filters and exclusions:

• Scenario: Legitimate enterprise antivirus or EDR agents (e.g., Microsoft Defender, CrowdStrike Falcon) performing real-time scanning or scheduled updates.

  • Context: These security tools frequently download new definition packs or update their own binaries using certificates that are shared across the industry and may occasionally be flagged if a specific hash is seen in a “malware” context within the detection logic.
  • Filter/Exclusion: Exclude processes where ImageName contains *DefenderSvc.exe, *FalconSensor.exe, or *ccSvcHst.exe. Additionally, filter out events where the ParentImageName is a known security management console (e.g., *Console.exe).

• Scenario: Automated software deployment and patching via Configuration Management tools.

  • Context: Tools like Microsoft System Center Configuration Manager (SCCM/MECM) or Ansible Tower push signed application updates to endpoints. The signing certificate used for the payload might be identical to one previously associated with a malware signature in the threat intelligence feed.
  • Filter/Exclusion: Create an exclusion based on ParentImageName matching known deployment agents (e.g., *ccmsetup.exe, *AnsibleTower.exe) and restrict the rule trigger to only execute during non-business hours or when the CommandLine contains specific deployment flags like /install or /update.

• Scenario: Scheduled administrative tasks running as a local system account.

  • Context: Windows Task Scheduler often executes maintenance scripts (PowerShell, Python) that sign and verify internal tools using a corporate root CA certificate. If this same certificate was recently implicated in a malware campaign, legitimate scheduled jobs will trigger the rule.
  • **Filter