AverCryptor10os1r1s

Hunt Hypothesis

This detection identifies the execution of the AverCryptor10os1r1s malware signature, which indicates potential file encryption or ransomware activity within the environment. Proactively hunting for this behavior in Azure Sentinel is essential to validate false positives and confirm early-stage infection before adversaries escalate their impact on critical data assets.

YARA Rule

rule AverCryptor10os1r1s
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED 75 17 40 00 8B BD 9C 18 40 00 8B 8D A4 18 40 00 B8 BC 18 40 00 03 C5 80 30 05 83 F9 00 74 71 81 7F 1C AB 00 00 00 75 62 8B 57 0C 03 95 A0 18 40 00 33 C0 51 33 C9 66 B9 FA 00 66 83 F9 00 74 49 8B 57 0C 03 95 A0 18 40 00 8B 85 A8 18 40 00 83 F8 02 75 06 81 C2 00 02 00 00 51 8B 4F 10 83 F8 02 75 06 81 E9 00 02 00 00 57 BF C8 00 00 00 8B F1 E8 27 00 00 00 8B C8 5F B8 BC 18 40 00 03 C5 E8 24 00 00 00 59 49 EB B1 59 83 C7 28 49 EB 8A 8B 85 98 18 40 00 89 44 24 1C 61 FF E0 56 57 4F F7 D7 23 F7 8B C6 5F 5E C3 }

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 AverCryptor10os1r1s detection rule in an enterprise environment, along with suggested filters and exclusions:

• Scenario: Microsoft Office Document Repair via PowerShell

  • Context: An automated remediation script runs nightly using PowerShell.exe to repair corrupted .docx or .xlsx files stored on a shared network drive. The YARA rule flags the memory activity of PowerShell as suspicious encryption behavior because it reads and rewrites binary document structures, mimicking ransomware file modification patterns.
  • Suggested Filter: Create an exclusion for PowerShell.exe when executed from the specific directory path C:\Program Files\Microsoft Office Server Scripts\RepairTools\. Additionally, filter based on the parent process being a known scheduled task (Task Scheduler) with the name “Office_Daily_Repair”.

• Scenario: Antivirus Real-Time Scanning of Large Archives

  • Context: The enterprise endpoint protection suite (e.g., CrowdStrike Falcon or Microsoft Defender for Endpoint) performs a real-time scan on a large .7z archive being downloaded by the IT Helpdesk team. As the antivirus engine decompresses and scans the internal files, it triggers high I/O and memory mapping activities that resemble the encryption phase of ransomware.
  • Suggested Filter: Exclude processes belonging to C:\ProgramData\CrowdStrike\fs\ (or equivalent Defender paths) from this rule. Implement a logic check where if the parent process is an AV service (e.g., ccSvcHst.exe, MsMpEng.exe) and the target file extension is .zip, .7z, or .rar, suppress the alert.

• Scenario: Database Backup Job (SQL Server)

  • Context: A scheduled SQL Server maintenance job