mkfpackllydd

Hunt Hypothesis

This YARA rule targets specific file patterns or memory artifacts associated with the mkfpackllydd signature, potentially indicating the presence of a known malware family or obfuscated payload on endpoints. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify low-severity indicators of compromise that may have been missed by standard behavioral detections, ensuring early containment of targeted threats.

YARA Rule

rule mkfpackllydd
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 00 00 5B 81 EB 05 00 00 00 8B 93 9F 08 00 00 53 6A 40 68 00 10 00 00 52 6A 00 FF 93 32 08 00 00 5B 8B F0 8B BB 9B 08 00 00 03 FB 56 57 E8 86 08 00 00 83 C4 08 8D 93 BB 08 00 00 52 53 FF E6 }

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 is compiling a large C++ or C# project using a build system that invokes the Microsoft Visual C++ linker (link.exe) or a similar native compiler toolchain. The resulting executable or intermediate object files may contain specific byte patterns or structural artifacts that match the mkfpackllydd YARA signature, especially if the code is packed or contains specific library headers.

  • Filter/Exclusion: Exclude processes where the parent process is msbuild.exe, dotnet.exe, cl.exe, or link.exe. Additionally, filter out events where the file path resides in standard build directories such as C:\Users\<User>\AppData\Local\Temp\, C:\src\, or C:\build\.

• Scenario: An IT administrator is performing a routine software update or installation for a third-party application (e.g., Adobe Creative Cloud, Oracle Java, or a proprietary ERP system) that uses a custom installer or updater. These installers often extract or write temporary executables to disk that may be digitally signed but still match the YARA rule due to their internal structure or packing method.

  • Filter/Exclusion: Exclude events where the file has a valid digital signature from a known vendor (e.g., Adobe Inc., Oracle Corporation). You can also exclude processes where the parent process is msiexec.exe, setup.exe, or uninstall.exe and the file location is within the C:\Program Files\ or C:\Program Files (x86)\ directories.

• Scenario: A security team or QA engineer is running a penetration test or vulnerability scan using tools like Nessus, Qualys, or Nmap that deploy temporary agents or probes on the target host. These agents are often small, packed executables designed to run briefly and collect