UnnamedScrambler25Ap0ke

Hunt Hypothesis

This rule detects the execution of a known obfuscated or scrambled binary, often used by adversaries to hide malicious logic and evade static analysis during initial access or payload delivery. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify compromised endpoints or suspicious processes before they can establish persistence or execute further post-compromise actions.

YARA Rule

rule UnnamedScrambler25Ap0ke
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC B9 0B 00 00 00 6A 00 6A 00 49 75 F9 51 53 56 57 B8 6C 3E 40 00 E8 F7 EA FF FF 33 C0 55 68 60 44 40 00 64 FF 30 64 89 20 BA 70 44 40 00 B8 B8 6C 40 00 E8 62 F3 FF FF 8B D8 85 DB 75 07 6A 00 E8 A1 EB FF FF BA E8 64 40 00 8B C3 8B 0D B8 6C 40 00 E8 37 D3 FF FF C7 05 BC 6C 40 00 0A 00 00 00 BB 68 6C 40 00 BE 90 6C 40 00 BF E8 64 40 00 B8 C0 6C 40 00 BA 04 00 00 00 E8 07 EC FF FF 83 3B 00 74 04 33 C0 89 03 8B D7 8B C6 E8 09 F3 FF FF 89 03 83 3B 00 0F 84 BB 04 00 00 B8 C0 6C 40 00 8B 16 E8 06 E2 FF FF B8 C0 6C 40 00 E8 24 E1 FF FF 8B D0 8B 03 8B 0E E8 D1 D2 FF FF 8B C7 A3 20 6E 40 00 8D 55 EC 33 C0 E8 0C D4 FF FF 8B 45 EC B9 1C 6E 40 00 BA 18 6E 40 00 }

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 legacy .NET application or custom internal tool uses a specific string obfuscation technique (e.g., XOR with key 0x25 or base64 encoding of specific API names) that matches the byte pattern of UnnamedScrambler25Ap0ke. This is common in older codebases that haven’t been refactored to use standard P/Invoke wrappers.
  • Filter/Exclusion: Exclude processes where the parent is msbuild.exe, dotnet.exe, or vbc.exe (Visual Basic Compiler), or exclude specific known internal binaries by SHA256 hash if the application is signed by the internal CA.
• Scenario: An enterprise antivirus or EDR agent (e.g., CrowdStrike Falcon, Carbon Black) performs a real-time scan or memory inspection on a large, complex document (like a .pptx or .xlsx file) that contains embedded macros or complex XML structures. The scanner may temporarily load the file content into memory in a way that triggers the YARA signature due to the specific arrangement of strings or byte sequences in the compressed data.
  • Filter/Exclusion: Exclude alerts where the process name is a known EDR/AV sensor (e.g., FalconSensor.exe, cb.exe, defender.exe) and the action is “Scan” or “Inspect”. Alternatively, exclude if the file extension is .docx, .xlsx, or .pptx and the process is a known office suite component.
• Scenario: A scheduled PowerShell job or a CI/CD pipeline step (e.g., Jenkins, Azure DevOps) executes a script that generates a temporary configuration file or log entry using a custom encoding function (e.g., Convert::ToBase64String with a specific padding or character set) that coincidentally matches the YARA rule’s string pattern. This often happens during