PEMangle

Hunt Hypothesis

This detection identifies potential anomalies or tampering within PEM-formatted certificate files that may indicate an adversary attempting to manipulate trust chains or inject malicious code. A proactive hunt is essential in Azure Sentinel to uncover subtle supply chain compromises or credential theft scenarios that standard signature-based alerts might overlook, ensuring the integrity of the organization’s PKI infrastructure remains uncompromised.

YARA Rule

rule PEMangle
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 9C BE [4] 8B FE B9 [4] BB 44 52 4F 4C AD 33 C3 }

condition:
		$a0 at pe.entry_point
}

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

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the PEMangle detection rule, along with suggested filters or exclusions tailored for an enterprise environment:

• Scenario: Automated Certificate Renewal by Certbot/Let’s Encrypt

  • Context: The automated renewal process often involves downloading new certificate bundles and temporarily modifying existing .pem files (or creating new ones) in the /etc/letsencrypt/live/ directory. This file manipulation can trigger the PEMangle rule if the YARA signature detects structural anomalies during the write operation.
  • Filter/Exclusion: Exclude processes matching certbot, acme.sh, or openssl running from the standard installation path (e.g., /usr/bin/certbot) when accessing files within the letsencrypt directory tree.

• Scenario: CI/CD Pipeline Artifact Generation

  • Context: During nightly build jobs, tools like Jenkins, GitLab Runners, or GitHub Actions frequently generate temporary .pem keys for containerization and secure artifact signing. These builds often run in ephemeral containers where file I/O patterns differ from standard production workloads, causing the YARA rule to flag transient files as anomalies.
  • Filter/Exclusion: Create a process exclusion based on the parent process name (e.g., java, node, or specific runner binaries like gitlab-runner) and limit detection scope to exclude paths containing /tmp/builds or .github/workflows.

• Scenario: Scheduled Database Backup Encryption

  • Context: Enterprise database backup solutions such as Veeam, Rubrik, or native PostgreSQL/MySQL backup scripts often generate temporary encryption keys in .pem format before compressing data. The rapid creation and deletion of these files during the scheduled maintenance window can mimic the behavior of a malicious certificate injection attack.