CrunchPEv40

Hunt Hypothesis

This detection identifies potential malicious or suspicious PE executables that match the specific structural patterns defined by the CrunchPEv40 YARA signature, which often indicate obfuscated code or known malware families. SOC teams should proactively hunt for these artifacts in Azure Sentinel to uncover low-severity indicators of compromise that may evade standard signature-based defenses and require deeper behavioral analysis before escalating into active incidents.

YARA Rule

rule CrunchPEv40
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 10 [16] 55 E8 [4] 5D 81 ED 18 [3] 8B C5 55 60 9C 2B 85 E9 06 [2] 89 85 E1 06 [2] FF 74 24 2C E8 BB 01 00 00 0F 82 92 05 00 00 E8 F1 03 00 00 49 0F 88 86 05 00 00 68 6C D9 B2 96 33 C0 50 E8 24 }

condition:
		$a0
}

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 CrunchPEv40 detection rule in an enterprise environment, along with suggested filters or exclusions:

• Scenario: Microsoft Office Click-to-Run Update Service

  • Context: The OfficeClickToRun.exe process frequently spawns child processes to download and install updates. These executables often contain embedded resources that match the PE header signatures targeted by CrunchPEv40, particularly when updating the “Common” or “Shared” components of Office 365.
  • Filter/Exclusion: Exclude processes where ImageFileName matches OfficeClickToRun.exe and the parent process is Microsoft Office Click-to-Run Service. Alternatively, add a hash exclusion for the specific update binaries located in %ProgramFiles%\Common Files\microsoft shared\ClickToRun.

• Scenario: CrowdStrike Falcon Sensor Self-Update

  • Context: The CrowdStrike Falcon sensor (C-Sensor.exe) periodically updates its own definition files and internal modules. During these self-modification events, the sensor creates temporary PE executables that trigger the rule due to their specific compilation timestamps and embedded signatures.
  • Filter/Exclusion: Create an exclusion for any process where ImageFileName is C-Sensor.exe or csfalcon.exe running under the SYSTEM account. Additionally, exclude file paths containing \Program Files\CrowdStrike\.

• Scenario: Scheduled Antivirus Definition Sync (Symantec Endpoint Protection)

  • Context: A nightly scheduled job runs the Symantec Endpoint Protection client (Rtvscan64.exe) to pull new virus definitions. This process extracts compressed definition archives into temporary PE files before merging them, which often mimics the “suspicious PE structure” logic of the CrunchPEv40 rule.

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