Look for CRC32c (Castagnoli) [poly]

Hunt Hypothesis

Adversaries may use CRC32c (Castagnoli) hashes to obfuscate or manipulate data during exfiltration or persistence. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential data tampering or covert communication channels.

YARA Rule

rule CRC32c_poly_Constant {
	meta:
		author = "_pusher_"
		description = "Look for CRC32c (Castagnoli) [poly]"
		date = "2016-08"
	strings:
		$c0 = { 783BF682 }
	condition:
		$c0
}

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

• Scenario: A system update or patching tool (e.g., yum, apt, Chocolatey) generates temporary files with CRC32c hashes during package verification.
  Filter/Exclusion: Exclude processes related to package managers using process.name or process.args containing yum, apt, choco, etc.

• Scenario: A backup or sync tool (e.g., rsync, Veeam, SyncBack) computes CRC32c hashes for data integrity checks during file transfer.
  Filter/Exclusion: Exclude processes associated with backup tools using process.name or process.args containing rsync, veeam, syncback, etc.

• Scenario: A scheduled system cleanup or disk defragmentation task (e.g., cleanmgr.exe, defrag.exe) uses CRC32c for file verification.
  Filter/Exclusion: Exclude processes with process.name matching cleanmgr.exe, defrag.exe, or similar system maintenance tools.

• Scenario: A database or log management tool (e.g., MySQL, Logstash, Elasticsearch) computes CRC32c hashes for data consistency checks.
  Filter/Exclusion: Exclude processes related to database or log tools using process.name or process.args containing mysql, logstash, elasticsearch, etc.

• Scenario: A software development or build tool (e.g., Gradle, Maven, npm) uses CRC32c for artifact verification during dependency resolution.
  Filter/Exclusion: Exclude processes associated with build tools using process.name or process.args containing gradle, maven, npm, etc.