PE Encrypt 10 Liwuyue additional

Hunt Hypothesis

This hypothesis targets adversaries who obfuscate malicious payloads using PE encryption techniques to evade static analysis, a tactic often employed by advanced threat actors to establish persistent footholds. Proactively hunting for these encrypted executables in Azure Sentinel allows the SOC to identify stealthy malware variants before they execute, reducing the risk of undetected lateral movement or data exfiltration within the environment.

YARA Rule

rule PE_Encrypt_10_Liwuyue_additional: PEiD
{
    strings:
        $a = { 55 8B EC 83 C4 D0 53 56 57 8D 75 FC 8B 44 24 30 25 00 00 FF FF 81 38 4D 5A 90 00 74 07 2D 00 10 00 00 EB F1 89 45 FC E8 C8 FF FF FF 2D 0F 05 00 00 89 45 F4 8B 06 8B 40 3C 03 06 8B 40 78 03 06 8B C8 8B 51 20 03 16 8B 59 24 03 1E 89 5D F0 8B 59 1C 03 1E 89 }
    condition:
        $a 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

• Legitimate Software Updates via MSI/EXE Installers: Many enterprise applications (e.g., Adobe Creative Cloud, Microsoft Office, or custom internal tools) use encrypted or packed installers to protect intellectual property or reduce file size. These often trigger PE encryption YARA rules.
  • Filter/Exclusion: Exclude files located in standard installation directories (e.g., C:\Program Files\, C:\Program Files (x86)\) and specific known-good hashes or publisher certificates (e.g., Adobe Inc., Microsoft Corporation). Additionally, whitelist processes like msiexec.exe or setup.exe when the file is being written by a trusted service.
• Antivirus/EDR Agent Self-Updates: Security agents (e.g., CrowdStrike Falcon, SentinelOne, Carbon Black) frequently download and stage encrypted or compressed update packages before deployment. These PE files may exhibit encryption characteristics that match the rule.
  • Filter/Exclusion: Exclude files in the vendor-specific update cache directories (e.g., C:\ProgramData\CrowdStrike\, C:\Program Files\SentinelOne\) and whitelist the update service executables (e.g., falconctl.exe, sentineloneagent.exe). Ensure the exclusion applies to the file path and the parent process.
• Development and Build Artifacts: Developers using tools like Go, Rust, or .NET may produce binaries with stripped symbols or specific compression/encryption for optimization. CI/CD pipelines (e.g., Jenkins, Azure DevOps) often generate these PE files in build agent workspaces.
  • Filter/Exclusion: Exclude files in build agent workspace directories (e.g., C:\jenkins\workspace\, C:\AzureDevOps\agent\_work\) and whitelist processes associated with build tools (e.g., dotnet.exe, `