Certificate used for digitally signing malware.

Hunt Hypothesis

This detection identifies adversaries leveraging valid digital certificates to sign malicious executables, a tactic designed to bypass signature-based defenses and establish trust with endpoint security tools. Proactively hunting for this behavior in Azure Sentinel is critical because attackers increasingly abuse legitimate certificate authorities to evade traditional antivirus solutions, necessitating deeper analysis of signing patterns to uncover stealthy malware campaigns.

YARA Rule

rule cert_blocklist_4effa8b216e24b16202940c1bc2fa8a5 {
    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 "Henan Maijiamai Technology Co., Ltd." and
            pe.signatures[i].serial == "4e:ff:a8:b2:16:e2:4b:16:20:29:40:c1:bc:2f:a8:a5" and
            1404691199 <= 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 rule “Certificate used for digitally signing malware,” including suggested filters and exclusions:

• Antivirus Engine Updates via Scheduled Tasks

  • Scenario: Enterprise endpoint protection suites (e.g., CrowdStrike Falcon, Microsoft Defender Antivirus, or Symantec Endpoint Protection) frequently download new definition updates. These updates are often signed by the vendor’s root certificate, which may also be used to sign known malware samples in threat intelligence feeds. When the update service executes a scheduled job (e.g., UpdateService.exe), it triggers the rule because the signing certificate matches one flagged for malicious use.
  • Filter/Exclusion: Exclude processes matching specific vendor names and paths, such as C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe or C:\ProgramData\McAfee\Agent\x86\mfeagent.exe, when the process name contains keywords like “Update”, “Scan”, or “Service”.

• Software Deployment via Configuration Management Tools

  • Scenario: IT administrators use tools like Microsoft Endpoint Configuration Manager (MECM/SCCM), Ansible, or Chocolatey to deploy applications across the fleet. These tools often utilize a central signing certificate to validate packages before installation. If this same certificate is used to sign a legitimate installer that happens to be flagged in a malware database, the deployment agent executing the install will trigger an alert.
  • Filter/Exclusion: Create an exclusion for processes running under the context of known management tools (e.g., ccmsetup.exe, choco.exe, or ansible-runner) and restrict the rule to only trigger if the parent process is a user-interactive shell (cmd.exe or powershell.exe) rather than a system service.

• **CI/CD Pipeline