AnslymFUDCrypter

Hunt Hypothesis

This rule detects the presence of the AnslymFUDCrypter, a tool used by adversaries to obfuscate malicious payloads and evade static analysis during the initial stages of an attack. Proactively hunting for this indicator allows the SOC to identify compromised hosts where attackers are leveraging custom crypters to hide their intent before executing further post-exploitation activities.

YARA Rule

rule AnslymFUDCrypter
{
      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 }

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

• Scenario: Development teams using Visual Studio Code or JetBrains IDEs with specific extensions that compile or obfuscate C/C++ code in-memory for testing purposes. The YARA rule may match the specific byte patterns of the crypter’s header or initialization routine if the compiler output is cached or loaded into memory during a build process.
  • Filter/Exclusion: Exclude processes where the parent process is code.exe, idea64.exe, or devenv.exe, and the file path contains \node_modules\ or \bin\Debug\.
• Scenario: CI/CD Pipeline Agents (e.g., Azure DevOps Agent, Jenkins, or GitLab Runner) executing build tasks that involve custom encryption/obfuscation steps for release binaries. The agent process may load the crypter library or execute a small stub that matches the YARA signature during the packaging phase.
  • Filter/Exclusion: Exclude processes running under user accounts named svc-ci, build-agent, or jenkins, or where the command line arguments contain --build, --package, or --obfuscate.
• Scenario: Endpoint Detection and Response (EDR) Agents (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or Sentinel One) performing real-time memory scanning or hash computation on loaded modules. The EDR service process may temporarily load or inspect the crypter binary, causing a self-match or a false positive if the rule is not aware of the EDR’s internal scanning context.
  • Filter/Exclusion: Exclude processes where the executable name is FalconSensor.exe, MsMpEng.exe, or SentinelOneAgent.exe, or where the process path resides in C:\Program Files\CrowdStrike\ or `C:\ProgramData