Hunt Hypothesis
This rule detects the execution of CrypWrap, a lightweight cross-platform file encryption tool often used by adversaries to encrypt data for ransom or exfiltration. Proactively hunting for this indicator in Azure Sentinel allows the SOC to identify potential ransomware staging or data encryption activities before they escalate into a full-blown incident.
YARA Rule
rule CrypWrapvxx
{
meta:
author="malware-lu"
strings:
$a0 = { E8 B8 [3] E8 90 02 [2] 83 F8 ?? 75 07 6A ?? E8 [4] FF 15 49 8F 40 ?? A9 [3] 80 74 0E }
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.
False Positive Guidance
- Legitimate Backup Encryption (Veeam/Commvault): Enterprise backup solutions often encrypt backup files using AES-256 or similar algorithms, which may match YARA patterns looking for encrypted data structures or specific wrapper headers.
- Filter/Exclusion: Exclude processes named
vbr.exe (Veeam Backup & Replication) or cvpp.exe (Commvault) and file paths containing \Backups\ or \Vault\.
- Disk Encryption Agents (BitLocker/Veracrypt): When BitLocker or Veracrypt performs volume encryption or decryption operations, the underlying driver or user-mode service may create temporary encrypted containers or memory mappings that trigger generic “wrapped” or “encrypted” YARA signatures.
- Filter/Exclusion: Exclude processes
lsass.exe (if related to credential protection), bitlocker.exe, or veracrypt.exe, and monitor for file extensions like .vhdx or .vhd being modified by these specific services.
- Secure File Transfer Clients (WinSCP/Cyberduck): Administrators frequently use SFTP/FTPS clients to transfer sensitive configuration files or scripts. These tools may use local temporary files or in-memory buffers that match patterns for encrypted payloads, especially if the client uses GPG or OpenSSL wrappers for pre-transfer encryption.
- Filter/Exclusion: Exclude processes
winscp.exe or cyberduck.exe and file paths in %TEMP% or %APPDATA%\WinSCP that are created and deleted within a short timeframe (e.g., < 30 seconds).
- Antivirus/EDR Quarantine Scans: During deep scans or on-access protection, EDR agents (like CrowdStrike, SentinelOne, or Microsoft Defender) may temporarily wrap or isolate suspicious files in encrypted or compressed containers for analysis, which can mimic the