Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging trusted digital certificates to sign malicious executables, a technique designed to bypass signature-based defenses and evade detection by impersonating legitimate software. SOC teams should proactively hunt for this behavior in Azure Sentinel because relying solely on certificate validity can create a false sense of security, necessitating deeper analysis of the certificate’s issuance context and usage patterns to uncover sophisticated supply chain or living-off-the-land attacks.

YARA Rule

rule cert_blocklist_40575df73eaa1b6140c7ef62c08bf216 {
    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 "Dali Feifang Tech Co.,LTD." and
            pe.signatures[i].serial == "40:57:5d:f7:3e:aa:1b:61:40:c7:ef:62:c0:8b:f2:16" and
            1394063999 <= 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 rule “Certificate used for digitally signing malware,” including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Patching Tools Signing Updates

  • Context: Enterprise patch management tools like Microsoft System Center Configuration Manager (SCCM) or Ivanti Neurons frequently download and install updates. These tools often use a specific internal or vendor certificate to sign the update packages, which may be flagged as “malware-signing” if that same certificate has been observed in other contexts or if the signature hash overlaps with known malware families.
  • Suggested Filter/Exclusion: Exclude processes where the Parent Process Name is ccmsetup.exe, wsman.exe, or IvantiService.exe. Additionally, filter out events where the Certificate Issuer contains “Microsoft Corporation” and the File Path resides within the standard patching directory (e.g., C:\Windows\CCM\Cache).

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Endpoint Protection platforms such as CrowdStrike Falcon, SentinelOne, or Symantec Endpoint Protection run scheduled jobs to download and install new definition databases. These updates are digitally signed by the vendor’s root certificate, which is also commonly used to sign their own detection engines that might be flagged as “malware” in a broader context if the rule logic is too broad.
  • Suggested Filter/Exclusion: Create an exclusion for Process Name matching FalconService.exe, s1agent.exe, or Rtvscan64.exe. Add a condition to exclude events occurring during defined maintenance windows (e.g., 02:00–04:00 UTC) where the Event ID corresponds to definition update completion.

• **Scenario: CI