UnnamedScrambler251Beta2252p0ke

Hunt Hypothesis

This hypothesis targets the execution of obfuscated or scrambled scripts, potentially indicating an adversary using custom tooling or low-fidelity malware to evade signature-based detection. Proactively hunting for these patterns in Azure Sentinel helps identify stealthy initial access or lateral movement activities that may otherwise go unnoticed due to their low severity and lack of standard MITRE mapping.

YARA Rule

rule UnnamedScrambler251Beta2252p0ke
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC B9 ?? 00 00 00 6A 00 6A 00 49 75 F9 53 56 57 B8 [2] 40 00 E8 ?? EA FF FF 33 C0 55 68 [2] 40 00 64 FF 30 64 89 20 BA [2] 40 00 B8 [2] 40 00 E8 63 F3 FF FF 8B D8 85 DB 75 07 6A 00 E8 [2] FF FF BA [2] 40 00 8B C3 8B 0D [2] 40 00 E8 [2] FF FF C7 05 [2] 40 00 0A 00 00 00 BB [2] 40 00 BE [2] 40 00 BF [2] 40 00 B8 [2] 40 00 BA 04 00 00 00 E8 ?? EB FF FF 83 3B 00 74 04 33 C0 89 03 8B D7 8B C6 E8 0A F3 FF FF 89 03 83 3B 00 0F 84 F7 04 00 00 B8 [2] 40 00 8B 16 E8 ?? E1 FF FF B8 [2] 40 00 E8 ?? E0 FF FF 8B D0 8B 03 8B 0E E8 [2] FF FF 8B C7 A3 [2] 40 00 8D 55 EC 33 C0 E8 ?? D3 FF FF 8B 45 EC B9 [2] 40 00 BA [2] 40 00 E8 8B ED FF FF 3C 01 75 2B A1 }

condition:
		$a0
}

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

• Scenario: A developer or DevOps engineer runs a local build script that uses python -m py_compile or a similar tool to generate .pyc bytecode files in a temporary directory (e.g., C:\Users\dev\AppData\Local\Temp\build_cache) for performance optimization. The YARA rule may match the specific byte patterns of the compiled output or the scrambling header used by the build tool.

  • Filter/Exclusion: Exclude file paths containing \AppData\Local\Temp\ or \build_cache\ where the file extension is .pyc or .pyo, and the parent process is python.exe or py.exe.

• Scenario: An IT administrator uses a proprietary internal utility (e.g., CorpEncryptor.exe) to obfuscate configuration files (.ini or .xml) before deploying them to edge servers. The tool uses a custom scrambling algorithm that produces binary output with a header signature matching the YARA rule’s pattern.

  • Filter/Exclusion: Exclude files with extensions .enc, .obf, or .scrambled located in specific deployment directories like C:\Corp\Deploy\ or D:\EdgeConfig\, where the writing process is CorpEncryptor.exe or deploy_tool.exe.

• Scenario: A scheduled PowerShell job (BackupConfig.ps1) runs nightly to compress and encrypt backup logs using Compress-Archive followed by a custom encryption wrapper. The intermediate file created during the process may temporarily reside in C:\Windows\Temp\ with a random name and exhibit byte patterns that trigger the YARA rule due to the specific compression/encryption header.

  • Filter/Exclusion: Exclude files in C:\Windows\Temp\ or C:\Users\Public\Temp\ that are created by powershell.exe