PackedwithPKLITEv150withCRCcheck1

Hunt Hypothesis

This detection identifies files packed with PKLITE v1.50 that include a CRC check, a technique often employed by adversaries to obfuscate malicious payloads and evade static analysis. Proactively hunting for these artifacts in Azure Sentinel allows the SOC team to uncover stealthy threats that may bypass traditional signature-based defenses due to their compressed structure.

YARA Rule

rule PackedwithPKLITEv150withCRCcheck1
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 1F B4 09 BA [2] CD 21 B8 [2] CD 21 }

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 4 specific false positive scenarios for the PackedwithPKLITEv150withCRCcheck1 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Update Standalone Installer Execution

  • Context: The Windows Update service or an automated patching tool (e.g., SCCM/Intune) frequently deploys standalone .exe installers for security updates. These installers are often compressed using the PkLite v1.50 packer to reduce download size and include a CRC check for integrity verification during installation.
  • Filter/Exclusion: Create an exclusion based on the file path C:\Windows\SoftwareDistribution\Download\* or exclude processes running under the Microsoft Windows Update service account (NT SERVICE\wuauserv). Additionally, filter by known Microsoft Publisher hashes for these specific update packages.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Endpoint protection suites like CrowdStrike Falcon, SentinelOne, or Carbon Black often download and unpack definition signature updates using PkLite v1.50 as their internal compression engine. The CRC check is triggered when the agent validates the integrity of the downloaded update package before applying it.
  • Filter/Exclusion: Exclude processes belonging to major EDR vendors (e.g., CrowdStrike.exe, SentinelOne.exe, CarbonBlack.exe) when they are accessing files within their specific installation directories (e.g., C:\ProgramData\CrowdStrike\* or C:\Program Files\SentinelOne\*).

• Scenario: Enterprise Backup and Archiving Jobs

  • Context: Scheduled backup jobs using tools like Veeam, Acronis Cyber Protect, or Commvault often create temporary compressed archives of configuration files or logs. These tools utilize PkLite v1.