EXECryptor239DLLcompressedresources

Hunt Hypothesis

This rule identifies executable files containing compressed resources, a common obfuscation technique used by adversaries to hide malicious payloads or reduce file size while evading static analysis. Proactively hunting for these artifacts helps the SOC team uncover stealthy implants or trojans that may be leveraging resource compression to bypass initial detection layers within the Azure Sentinel environment.

YARA Rule

rule EXECryptor239DLLcompressedresources
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 50 68 [4] 58 C1 C0 0F E9 [3] 00 87 04 24 58 89 45 FC E9 [3] FF FF 05 [4] E9 [3] 00 C1 C3 18 E9 [4] 8B 55 08 09 42 F8 E9 [3] FF 83 7D F0 01 0F 85 [4] E9 [3] 00 87 34 24 5E 8B 45 FC 33 D2 56 8B F2 E9 [3] 00 BA [4] E8 [3] 00 A3 [4] C3 E9 [3] 00 C3 83 C4 04 C3 E9 [3] FF 64 FF 35 00 00 00 00 64 89 25 00 00 00 00 E8 [3] 00 E9 [3] FF C1 C2 03 81 CA [4] 81 C2 [4] 03 C2 5A E9 [3] FF 81 E7 [4] 81 EF [4] 81 C7 [4] 89 07 E9 [4] 0F 89 [4] 87 14 24 5A 50 C1 C8 10 }

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 build engineer uses 7-Zip or WinRAR to compress a large .dll file (e.g., libcrypto.dll or a proprietary SDK component) for distribution or backup, resulting in a compressed archive that retains the .dll extension or is extracted in a way that the YARA rule misidentifies the compressed resource structure.

  • Filter/Exclusion: Exclude files located in known build artifact directories (e.g., C:\Builds\, C:\Artifacts\) or exclude processes like 7z.exe, WinRAR.exe, or tar.exe when the file size is significantly larger than the threshold for typical compressed DLLs, or add an exclusion for file paths matching *\dist\*.dll and *\backup\*.dll.

• Scenario: An enterprise application using Qt or WPF frameworks stores its resources (icons, strings, layouts) in a compressed section within the DLL, which is a standard part of the build process. The YARA rule may trigger on the specific byte pattern of the compressed resource header in these legitimate, signed binaries.

  • Filter/Exclusion: Exclude DLLs that are digitally signed by trusted internal CA or known vendor CAs (e.g., Microsoft Corporation, Oracle Corporation, IBM Corporation) and are located in standard application installation directories (e.g., C:\Program Files\, C:\Program Files (x86)\).

• Scenario: A scheduled maintenance job runs NtBackup or Windows Server Backup to create a compressed backup of application binaries, including DLLs, to a network share. If the backup process temporarily writes compressed chunks to a local temp folder with a .dll extension or if the YARA rule scans intermediate backup files, it may trigger.

  • Filter/Exclusion: