CRYPTVersion17cDismember

Hunt Hypothesis

This detection identifies adversary behavior where malicious processes utilize the Dismember technique to obfuscate cryptographic operations, potentially masking data exfiltration or ransomware staging activities within Azure Sentinel. A SOC team should proactively hunt for this pattern because low-severity signals often precede high-impact incidents, allowing analysts to uncover stealthy threats that might otherwise be overlooked by standard alerting thresholds.

YARA Rule

rule CRYPTVersion17cDismember
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 0E 17 9C 58 F6 [2] 74 ?? E9 }

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 5 specific false positive scenarios for the CRYPTVersion17cDismember detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe service frequently performs background updates that involve extracting and dismembering large cabinet files (.cab) containing new version manifests. This activity mimics the “dismember” behavior of a cryptominer or ransomware unpacking payload.
  • Filter/Exclusion: Add an exclusion for the process path C:\Program Files\Microsoft Office\root\Office16\OfficeClickToRun.exe and restrict the rule to trigger only if the parent process is not svchost.exe.

• Scenario: Windows Defender Antivirus Signature Updates

  • Context: The Windows Update service (usoc.dll) or MpCmdGp.exe downloads large definition signature packages. During installation, these tools often dismember compressed archives to extract new heuristic signatures, triggering the YARA rule’s file manipulation logic.
  • Filter/Exclusion: Exclude processes where the command line contains MPUpdate.exe or paths under C:\ProgramData\Microsoft\Windows Defender\. Additionally, filter out events occurring during standard maintenance windows (e.g., 02:00–04:00 AM local time).

• Scenario: System Center Configuration Manager (SCCM) Deployment

  • Context: When SCCM (ccmsetup.exe or ccmexec.exe) deploys new applications or patches to endpoints, it often utilizes the DISM tool to dismember and apply .wim or .msi packages. This involves heavy file I/O and archive manipulation that resembles the detection logic.
  • Filter/Exclusion: Create a whitelist for the