PEIntrov10

Hunt Hypothesis

This hypothesis targets the presence of PE files containing specific introductory or header anomalies that may indicate obfuscated or modified executables, potentially used by adversaries to hide malicious payloads or evade static analysis. Proactively hunting for these patterns in Azure Sentinel allows the SOC team to identify suspicious binaries early in the kill chain, reducing the risk of undetected execution or persistence mechanisms within the environment.

YARA Rule

rule PEIntrov10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B 04 24 9C 60 E8 [4] 5D 81 ED 0A 45 40 ?? 80 BD 67 44 40 [2] 0F 85 48 }

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: A developer or DevOps engineer runs a local build script that uses objdump or readpe to inspect the header of a newly compiled C++ application or Python extension module. The YARA rule may match specific byte patterns in the PE header or import table that resemble known intrusion signatures (e.g., specific section names or checksum anomalies) during the analysis phase.

  • Filter/Exclusion: Exclude processes where the parent process is a build tool (e.g., msbuild.exe, dotnet.exe, python.exe running a script) or where the file path matches *.tmp, *.obj, or build\ directories. Additionally, exclude if the process command line contains arguments like --check, --dump, or --info.

• Scenario: An endpoint DLP (Data Loss Prevention) agent or backup software (e.g., Veeam, Commvault, or Microsoft Defender for Endpoint’s DLP component) opens an executable file in read-only mode to scan for sensitive data or verify integrity. The YARA rule might trigger if the scanner’s internal buffer or temporary copy of the file exhibits a specific PE structure quirk (such as an unusual section alignment or a zeroed-out checksum) that matches the PEIntrov10 pattern.

  • Filter/Exclusion: Exclude processes belonging to known DLP/backup vendors (e.g., dliagent.exe, vscan.exe, msmpeng.exe if it’s the AV engine itself, or commvault\cv.exe). Filter by checking if the file handle is opened with FILE_SHARE_READ only and the process is running from a vendor-specific service directory.

• Scenario: A scheduled maintenance job runs a custom PowerShell script that uses System.Reflection.Assembly to load and inspect a .NET assembly (which is a PE file) for versioning