EXECryptor

Hunt Hypothesis

This rule detects the presence of EXECryptor, a tool commonly used by adversaries to obfuscate malicious executables and evade static analysis during post-exploitation phases. Proactively hunting for this signature allows the SOC team to identify hidden payloads that may be staged on endpoints or in memory, reducing the risk of undetected execution in the Azure Sentinel environment.

YARA Rule

rule execryptor : Protector
{
	meta:
		author="Kevin Falcoz"
		date_create="25/02/2013"
		description="EXECryptor"

	strings:
		$str1={E8 24 00 00 00 8B 4C 24 0C C7 01 17 00 01 00 C7 81 B8 00 00 00 00 00 00 00 31 C0 89 41 14 89 41 18 80 A1 C1 00 00 00 FE C3 31 C0 64 FF 30 64 89 20 64 8F 05 00 00 00 00} /*EntryPoint*/

	condition:
		$str1 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

• Antivirus Self-Update or Definition Download: When enterprise endpoint protection agents (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or SentinelOne) download new definition packs or update their core engine binaries, they often write new, signed executables to disk or replace existing ones. If the YARA rule matches on generic PE header characteristics or specific import tables common in modern C++/Rust-based security tools, this can trigger false positives.

  • Filter/Exclusion: Exclude processes where the parent process is the known AV agent service (e.g., FalconSensor.exe, MsMpEng.exe, SentinelOneAgent.exe) and the file path resides within the vendor’s specific installation directory (e.g., C:\ProgramData\CrowdStrike\, C:\Program Files\Microsoft Security Client\).

• Java/Go Runtime Compilation or AOT (Ahead-of-Time) Compilation: In environments using Go-based microservices or Java applications with GraalVM native image compilation, the build process generates standalone native executables. These binaries often have minimal import tables or unusual section alignments that may match broad YARA patterns intended to catch packed or obfuscated executables.

  • Filter/Exclusion: Exclude files located in build output directories (e.g., target/, build/, dist/) or where the parent process is a known build tool (e.g., go.exe, javac.exe, graalvm-native-image.exe). Additionally, check for the presence of a go or java string in the binary’s metadata if the YARA rule allows for such refinement.

• CI/CD Pipeline Artifact Generation: In DevOps pipelines (e.g., Azure DevOps, Jenkins, GitHub Actions), build agents frequently compile source code into executable artifacts (.exe, .dll) and store them in temporary workspace directories. These artifacts are