AverCryptor102betaos1r1s

Hunt Hypothesis

This YARA rule targets the specific binary signature of the AverCryptor102betaos1r1s ransomware variant, which is known for encrypting files and appending a distinctive extension to facilitate ransom demands. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints early, particularly in environments where traditional behavioral detections may miss low-severity or novel ransomware strains before widespread encryption occurs.

YARA Rule

rule AverCryptor102betaos1r1s
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED 0C 17 40 00 8B BD 33 18 40 00 8B 8D 3B 18 40 00 B8 51 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 37 18 40 00 33 C0 51 33 C9 66 B9 F7 00 66 83 F9 00 74 49 8B 57 0C 03 95 37 18 40 00 8B 85 3F 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 51 18 40 00 03 C5 E8 24 00 00 00 59 49 EB B1 59 83 C7 28 49 EB 8A 8B 85 2F 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

• Legacy .NET Application Deployment: A custom internal line-of-business application built with an older version of the .NET Framework (e.g., 4.5.2) that uses specific obfuscation or packing techniques (like .NET Reactor or SmartAssembly) may exhibit byte patterns matching the AverCryptor signature. This is common in enterprise environments where legacy apps are not yet refactored.

  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\InternalTools\LegacyApp\) or exclude files with the .exe extension if the YARA rule is primarily targeting memory dumps or specific PE sections, provided the application is whitelisted by the software inventory team.

• Java/JavaScript Runtime Artifacts: Certain Java-based applications or Node.js runtimes that bundle native modules or use specific compression algorithms for their JAR/NPM packages can produce binary segments that mimic the entropy and structural patterns of the AverCryptor cryptor. This often occurs when developers use non-standard bundlers or minifiers.

  • Filter/Exclusion: Exclude files in standard runtime paths such as C:\Program Files\Java\ or C:\Program Files\nodejs\ and subdirectories containing node_modules, or filter based on the file extension .jar or .node if the YARA rule is overly broad on section headers.

• Scheduled Backup/Compression Jobs: Enterprise backup agents (e.g., Veeam, Commvault, or Windows Server Backup) that perform on-the-fly compression or encryption of backup blocks may create temporary files or memory mappings that match the cryptor’s signature, especially if they use proprietary compression algorithms similar to the ones detected by AverCryptor.

  • Filter/Exclusion: Exclude processes or files associated with known backup service accounts (e.g., `