Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging valid digital certificates to sign malicious executables, thereby evading signature-based defenses by masquerading as trusted software. Proactive hunting in Azure Sentinel is essential to identify these “living off the land” attacks early, preventing potential lateral movement and data exfiltration that might otherwise bypass standard security controls due to their legitimate appearance.

YARA Rule

rule cert_blocklist_4fe68d48634893d18de040d8f1c289d2 {
    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 "Xinghua Yile Network Tech Co.,Ltd." and
            pe.signatures[i].serial == "4f:e6:8d:48:63:48:93:d1:8d:e0:40:d8:f1:c2:89:d2" and
            1371081600 <= 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

• Scenario: Automated deployment of security updates via Microsoft System Center Configuration Manager (SCCM) or Windows Update for Business.

  • Context: When the enterprise deploys a new version of an endpoint protection agent (e.g., CrowdStrike, Symantec Endpoint Protection) or a critical OS patch, the installer executable is signed by the vendor’s certificate. If this specific certificate has been previously flagged in threat intelligence feeds as being associated with a different malware family (or if the rule logic flags “known signing certificates” broadly), legitimate installers may trigger an alert.
  • Filter/Exclusion: Create an exclusion for the Process Name matching known vendor installers (e.g., ccmsetup.exe, msiexec.exe) running under the System or a dedicated service account (NT SERVICE\SCCM). Additionally, exclude specific certificate thumbprints belonging to trusted vendors like Microsoft Corporation and CrowdStrike.

• Scenario: Execution of scheduled PowerShell scripts for administrative maintenance tasks.

  • Context: IT Operations teams often use scheduled jobs (Task Scheduler) to run PowerShell scripts that automate inventory collection or log rotation. These scripts are frequently signed with an internal enterprise Root CA certificate. If this internal CA certificate is also used to sign a specific piece of malware in the threat intelligence database, legitimate script executions will generate false positives.
  • Filter/Exclusion: Implement a filter based on Parent Process being Task Scheduler (svchost.exe) or PowerShell.exe, combined with an exclusion for the specific internal Root CA Subject Name (e.g., “Enterprise-Root-CA”). Ensure the script path matches a known secure directory, such as C:\ProgramData\CorpScripts.

• Scenario: Deployment of third-party software via Intune or Group Policy.

  • Context: During business hours, users may receive automatic updates for productivity suites like