Certificate used for digitally signing malware.

Hunt Hypothesis

This hypothesis detects adversaries leveraging trusted digital certificates to sign malicious executables, thereby evading signature-based detection and establishing a foothold within the environment. A proactive hunt is essential in Azure Sentinel to identify these legitimate-looking yet compromised artifacts before they facilitate lateral movement or data exfiltration under the guise of trust.

YARA Rule

rule cert_blocklist_6ed2450ceac0f72e73fda1727e66e654 {
    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 "Hohhot Handing Trade and Business Co., Ltd." and
            pe.signatures[i].serial == "6e:d2:45:0c:ea:c0:f7:2e:73:fd:a1:72:7e:66:e6:54" and
            1376092799 <= 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 4 specific false positive scenarios where legitimate enterprise activities trigger the “Certificate used for digitally signing malware” rule, along with suggested filters:

• Antivirus Engine Updates

  • Scenario: Major endpoint protection suites (e.g., Microsoft Defender Antivirus, CrowdStrike Falcon, or Symantec Endpoint Protection) frequently update their definition databases and engine binaries. These updates are signed by the vendor’s root certificate, which may also be associated with known malware signatures in threat intelligence feeds due to previous compromises of that specific signing key or shared infrastructure.
  • Filter/Exclusion: Create a whitelist for the specific process names (e.g., MpCmdRun.exe, CsaService.exe) and their parent processes (MsMpSvc, FalconSensor). Exclude alerts where the file path contains standard update directories like C:\ProgramData\Microsoft\Windows Defender\Platform or C:\Program Files\CrowdStrike\Falcon.

• Scheduled PowerShell Maintenance Scripts

  • Scenario: IT Operations teams often deploy scheduled tasks (via Task Scheduler) that run PowerShell scripts to perform patch management, log rotation, or configuration backups. These scripts are frequently signed by an internal enterprise CA certificate. If this specific internal CA has been flagged in a global threat feed due to a past compromise of the issuing authority, legitimate script executions will trigger the rule.
  • Filter/Exclusion: Exclude events where the ProcessName is powershell.exe or pwsh.exe and the CommandLine contains specific internal keywords (e.g., “PatchManagement”, “LogRotation”). Additionally, filter by the Publisher field to exclude known internal CAs (e.g., “Contoso Root CA”) that are not currently under investigation.

• CI/CD Pipeline Artifact Deployment

  • Scenario: DevOps teams utilize tools like Jenkins, GitLab Runner,