Certificate used for digitally signing malware.

Hunt Hypothesis

This detection identifies adversaries leveraging legitimate digital certificates to sign malicious executables, a technique often employed to evade signature-based security controls and establish trust within the environment. The SOC team should proactively hunt for this behavior in Azure Sentinel because attackers increasingly abuse valid certificates to bypass initial defenses, making early identification of suspicious signing patterns critical for preventing stealthy malware deployment.

YARA Rule

rule cert_blocklist_55efe24b9674855baf16e67716479c71 {
    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 "S2BVISIO BELGIQUE SA" and
            pe.signatures[i].serial == "55:ef:e2:4b:96:74:85:5b:af:16:e6:77:16:47:9c:71" and
            1374451199 <= 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 “Certificate used for digitally signing malware” detection rule, including targeted filters and exclusions:

• Antivirus Engine Update Execution

  • Scenario: The enterprise endpoint protection suite (e.g., Microsoft Defender Antivirus or CrowdStrike Falcon) automatically downloads and installs a new definition package. This process involves the execution of an updater service signed by the vendor’s certificate, which may have been flagged as “used for malware” in previous threat intelligence feeds due to its high volume of code signing across diverse environments.
  • Filter/Exclusion: Create a whitelist rule excluding processes where ImageName matches MsMpEng.exe or Csfalcon.exe and the Publisher is the specific vendor (e.g., “Microsoft Corporation” or “CrowdStrike, Inc.”).

• Scheduled PowerShell Script Deployment

  • Scenario: A nightly scheduled task runs a custom PowerShell script to patch application configurations. The script is digitally signed by an internal CA certificate that has been previously associated with a benign automation tool but flagged in the rule’s logic due to shared root trust or similar hash patterns found in malware campaigns.
  • Filter/Exclusion: Add an exclusion for PowerShell.exe processes where the command line contains specific keywords (e.g., “PatchDeployment.ps1”) and the CertificateIssuer matches the internal CA name (e.g., “Enterprise-Root-CA”).

• Third-Party Software Auto-Installer

  • Scenario: Users or IT admins trigger the silent installation of a widely used productivity tool (e.g., Zoom, Adobe Acrobat Reader DC) via an SCCM deployment. The installer executable is signed by a certificate that, while legitimate for this software, shares a common root or hash prefix with certificates observed in recent supply chain attacks involving similar vendors.
  • **Filter/Exclusion