Certificate used for digitally signing malware.

Hunt Hypothesis

This rule detects adversaries leveraging valid digital certificates to sign malicious executables, a technique often employed to bypass signature-based defenses and establish trust within the environment. SOC teams should proactively hunt for this behavior in Azure Sentinel because attackers increasingly abuse legitimate certificate authorities to evade detection, requiring manual validation of signing entities beyond automated alerts.

YARA Rule

rule cert_blocklist_06477e3425f1448995ced539789e6842 {
    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 "Karim Lammali" and
            pe.signatures[i].serial == "06:47:7e:34:25:f1:44:89:95:ce:d5:39:78:9e:68:42" and
            1334275199 <= 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,” along with recommended filters and exclusions tailored for an enterprise environment:

• Scenario: Antivirus Engine Updates via Scheduled Tasks

  • Context: Enterprise security suites like CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec Endpoint Protection frequently download new signature definitions. These updates are often signed by the vendor’s root certificate, which may be flagged if that specific certificate has been previously associated with a malware variant in the threat intelligence feed.
  • Filter/Exclusion: Create an exclusion based on the process name and path. Exclude processes matching *FalconSensor.exe, MsMpEng.exe, or SymantecEngine.exe when they are executing within their standard installation directories (e.g., C:\Program Files\CrowdStrike\...). Additionally, filter out events where the file hash matches known good definition update hashes.

• Scenario: Deployment of Internal Scripts via Configuration Management Tools

  • Context: IT Operations teams use tools like Microsoft SCCM (Configuration Manager), Ansible, or PowerShell DSC to push scripts and configuration agents across the network. These tools often sign their deployment executables with an internal enterprise CA certificate. If this internal CA has ever been compromised or used by a specific malware family, legitimate deployments will trigger alerts.
  • Filter/Exclusion: Implement a whitelist based on the Issuer Name of the certificate. Exclude any event where the certificate issuer is the organization’s internal Root CA (e.g., CN=Enterprise-Internal-Root-Org). Furthermore, filter by User Context, excluding events triggered by service accounts such as NT SERVICE\CCM or specific domain admin groups (DOMAIN\IT_Ops_Group) during business hours.

• **Scenario: Automated