AnslymCrypter

Hunt Hypothesis

This detection identifies the presence of the AnslymCrypter malware family by leveraging a specific YARA signature to pinpoint known encryption-based threats within the environment. Proactive hunting for this indicator in Azure Sentinel is essential to uncover early-stage cryptic activities that may precede ransomware deployment, allowing the SOC team to mitigate potential data loss before an active attack occurs.

YARA Rule

rule AnslymCrypter
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 F0 53 56 B8 38 17 05 10 E8 5A 45 FB FF 33 C0 55 68 21 1C 05 10 64 FF 30 64 89 20 EB 08 FC FC FC FC FC FC 27 54 E8 85 4C FB FF 6A 00 E8 0E 47 FB FF 6A 0A E8 27 49 FB FF E8 EA 47 FB FF 6A 0A 68 30 1C 05 10 A1 60 56 05 10 50 E8 68 47 FB FF 8B D8 85 DB 0F 84 B6 02 00 00 53 A1 60 56 05 10 50 E8 F2 48 FB FF 8B F0 85 F6 0F 84 A0 02 00 00 E8 F3 }

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 3-5 specific false positive scenarios for the AnslymCrypter detection rule in an enterprise environment, including suggested filters and exclusions:

• Scenario: Enterprise Antivirus Real-Time Scanning

  • Context: Legacy or modern EDR agents (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec Endpoint Protection) frequently spawn child processes that scan, decrypt, or re-encrypt files in real-time to check against malware signatures. The AnslymCrypter rule may misinterpret these internal cryptographic operations as suspicious encryption activity typical of ransomware.
  • Filter/Exclusion: Add a process exclusion for the specific EDR service executables (e.g., C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe) and their child processes when they are performing file I/O on known system directories like C:\Windows\System32 or C:\ProgramData.

• Scenario: Scheduled Backup Encryption Jobs

  • Context: Automated backup solutions (e.g., Veeam Backup & Replication, Commvault, or Rubrik) often run nightly scheduled tasks that encrypt large datasets before archiving them to the cloud or tape. These jobs involve high-volume file locking and encryption operations that mimic ransomware behavior, triggering the rule during maintenance windows.
  • Filter/Exclusion: Implement a time-based filter to suppress alerts for these specific backup service accounts (e.g., NT SERVICE\VeeamTransportService) between 02:00 and 06:00 local time on weekdays, or exclude paths dedicated to backup repositories (e.g., D:\VeeamBackup\).

• Scenario: Document Management & DLP Agents

  • Context: Data Loss Prevention (DLP) tools such as **Microsoft