Certificate used for digitally signing malware.

Hunt Hypothesis

This detection identifies adversaries leveraging legitimate digital certificates to sign malicious executables, thereby evading signature-based security controls and establishing trust with endpoints. Proactively hunting for this behavior in Azure Sentinel is critical because attackers increasingly abuse valid certificates to bypass initial threat scans, allowing malware to execute undetected before triggering broader incident response workflows.

YARA Rule

rule cert_blocklist_0c0f {
    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 "Dmitry Vasilev" and
            pe.signatures[i].serial == "0c:0f" and
            1386719999 <= 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,” along with suggested filters or exclusions:

• Scenario: Antivirus/EDR Self-Signing and Update Mechanisms

  • Context: Major security vendors (e.g., CrowdStrike, Microsoft Defender, SentinelOne) frequently use their own internal code-signing certificates to sign update agents, definition files, and real-time scanning engines. If a specific certificate is flagged because it was used to sign a file that the rule’s threat intelligence feed has previously identified as “malware” (perhaps due to a false positive in the feed itself), legitimate security updates may be blocked or alerted on unnecessarily.
  • Suggested Filter/Exclusion: Create an exclusion for the Publisher field matching known security vendor names (e.g., Microsoft Corporation, CrowdStrike Holdings, Inc.) and restrict the rule to only trigger when the file path is outside standard security installation directories (e.g., exclude paths starting with C:\Program Files\Microsoft Defender Antivirus\).

• Scenario: Scheduled Administrative Scripts and Patch Management

  • Context: Enterprise IT teams often utilize scheduled tasks or tools like SCCM, Ansible, or PowerShell scripts to deploy patches and configurations. These scripts are frequently signed by the organization’s internal Root CA certificate (e.g., “Enterprise-Root-2023”). If this specific internal certificate was previously associated with a malware family in the threat intelligence database due to a past incident, legitimate nightly patch deployment jobs will trigger false alerts every time they run.
  • Suggested Filter/Exclusion: Implement an exclusion based on Process Name and User Context. Exclude events where the process is Task Scheduler (SchTasks.exe) or PowerShell.exe running under a specific service account (e.g., `DOMAIN\svc-patch-de