PECryptv100v101

Hunt Hypothesis

This rule identifies Windows executables that have been packed or encrypted using the PECrypt v1.00 or v1.01 algorithms, a technique often employed by threat actors to obscure code structure and evade static analysis. Proactively hunting for these indicators in Azure Sentinel allows the SOC to detect potentially malicious or obfuscated binaries early in the kill chain, reducing the time required to investigate suspicious processes before they execute their intended payload.

YARA Rule

rule PECryptv100v101
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [4] 5B 83 EB 05 EB 04 52 4E 44 21 EB 02 CD 20 EB }

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

• Legacy Application Deployment via MSI/EXE Wrappers: Many older enterprise applications (e.g., specific versions of Adobe Creative Suite, Oracle Client, or legacy ERP modules) use custom installers that pack payloads with PE encryption to prevent tampering or simplify distribution. These installers often reside in shared network drives or deployment repositories.
  • Filter/Exclusion: Exclude files located in known deployment paths (e.g., \\corp\software\, C:\Deploy\) or specific known-good installer hashes. Additionally, exclude processes like msiexec.exe or setup.exe when the binary path matches the deployment repository.
• Antivirus/EDR Self-Update Binaries: Security tools like CrowdStrike Falcon, Carbon Black, or SentinelOne frequently download new agent versions or rule packs that are PE-packed or encrypted to protect the integrity of the update before it is decrypted in memory. These files are often temporarily stored in system temp folders or vendor-specific update directories.
  • Filter/Exclusion: Exclude binaries located in vendor-specific update directories (e.g., C:\Program Files\CrowdStrike\, C:\Program Files\Carbon Black\) or files with names matching update patterns (e.g., *.tmp, update_*.exe). Consider whitelisting specific SHA-256 hashes of known good update binaries.
• Scheduled Task for Backup/Archive Utilities: Tools like Veeam, Acronis, or custom PowerShell scripts that create compressed archives (e.g., .zip, .7z, or custom .pkg formats) may generate temporary PE-encrypted executables or use encryption layers for the archive header. If the detection rule scans temporary files or specific archive containers, these legitimate backups can trigger alerts.
  • Filter/Exclusion: Exclude files with extensions like .zip, .7z, .tar, or .pkg if