PassCV Malware mentioned in Cylance Report

Hunt Hypothesis

PassCV malware is likely being used by adversaries to execute arbitrary code and maintain persistence within a network. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential compromise from advanced persistent threats.

YARA Rule

rule PassCV_Sabre_Malware_4 
{

   meta:
      description = "PassCV Malware mentioned in Cylance Report"
      author = "Florian Roth"
      reference = "https://blog.cylance.com/digitally-signed-malware-targeting-gaming-companies"
      date = "2016-10-20"
      hash1 = "27463bcb4301f0fdd95bc10bf67f9049e161a4e51425dac87949387c54c9167f"

   strings:
      $s1 = "QWNjZXB0On" fullword ascii /* base64 encoded string 'Accept:' */
      $s2 = "VXNlci1BZ2VudDogT" fullword ascii /* b64: User-Agent: */
      $s3 = "dGFzay5kbnME3luLmN" fullword ascii /* b64: task.dns[ */

   condition:
      ( uint16(0) == 0x5a4d and filesize < 200KB and 2 of them )
}

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 3 string patterns in its detection logic.

References

• https://blog.cylance.com/digitally-signed-malware-targeting-gaming-companies
• Source Rule

False Positive Guidance

• Scenario: A system administrator is running a scheduled job to update the Cylance endpoint protection software.
  Filter/Exclusion: process.name != "CylanceAgent.exe" or process.parent.name != "CylanceAgent.exe"

• Scenario: A developer is using the Cylance API to query threat intelligence and is making a legitimate API call to check for known malware hashes.
  Filter/Exclusion: process.name != "CylanceAPI.exe" or process.parent.name != "CylanceAPI.exe"

• Scenario: A user is executing a legitimate system cleanup tool (e.g., CCleaner) that is known to be associated with the PassCV malware in some false positive reports.
  Filter/Exclusion: process.name != "CCleaner.exe" or process.parent.name != "CCleaner.exe"

• Scenario: A system administrator is manually reviewing logs or using a log analysis tool (e.g., Splunk, ELK) to investigate potential threats, which may include queries related to PassCV.
  Filter/Exclusion: process.name != "splunkd.exe" or process.name != "logstash.exe"

• Scenario: A backup or synchronization job (e.g., rsync, robocopy, or a cloud sync tool like Dropbox) is transferring files that happen to match the hash or name of the PassCV malware.
  Filter/Exclusion: process.name != "rsync.exe" or process.name != "dropbox.exe" or process.parent.name != "rsync.exe"