Ransom

Hunt Hypothesis

This detection rule identifies early-stage ransomware activity by leveraging a specific YARA signature to pinpoint malicious file patterns associated with encryption payloads. Proactively hunting for these indicators in Azure Sentinel is critical because it enables security teams to isolate potential infections before they escalate into widespread data encryption events, even when initial alerts are flagged as low severity.

YARA Rule

rule Ransom : Crypren{
    meta:
        weight = 1
        Author = "@pekeinfo"
        reference = "https://github.com/pekeinfo/DecryptCrypren"
    strings: 
        $a = "won't be able to recover your files anymore.</p>"
        $b = {6A 03 68 ?? ?? ?? ?? B9 74 F1 AE 00 E8 ?? ?? ?? ?? E8 ?? ?? ?? ?? 68 98 3A 00 00 FF D6 6A 00 68 ?? ?? ?? ?? 68 ?? ?? ?? ??}
        $c = "Please restart your computer and wait for instructions for decrypting your files"
    condition:
        any 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://github.com/pekeinfo/DecryptCrypren
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the Ransom detection rule in an enterprise environment, along with suggested filters or exclusions:

• Antivirus/Endpoint Protection Scanning and Quarantine Operations

  • Scenario: Enterprise security suites like Microsoft Defender, CrowdStrike Falcon, or SentinelOne frequently perform deep file scans. When these agents detect a potential threat, they often execute rapid file modifications (renaming infected files to .quarantined, changing extensions, or deleting temporary cache files) in high-volume directories. The YARA rule may misinterpret this burst of file renaming and attribute changes as ransomware encryption activity.
  • Suggested Filter: Create an exclusion based on the process image path for the security agent executables (e.g., C:\Program Files\Microsoft Defender\MsMpEng.exe or C:\ProgramData\CrowdStrike\csagent.exe). Additionally, exclude file paths known to be high-churn quarantine folders.

• Scheduled Data Backup and Archiving Jobs

  • Scenario: Automated backup solutions such as Veeam Backup & Replication, Commvault, or Windows Server Backup run on a schedule (e.g., nightly at 02:00). During these windows, the software compresses large datasets, renames source files to .bak or .zip, and updates metadata. If the backup agent processes thousands of files in quick succession across user home directories, it can trigger the Ransom rule’s logic regarding mass file modification.
  • Suggested Filter: Implement a time-based exclusion for the specific maintenance window (e.g., 01:30–04:30) combined with a process filter for the backup service executables (e.g., VeeamAgent.exe, Commsvc.exe).

• **Large-Scale Software Deployment