ASPack

Hunt Hypothesis

Adversaries may employ the ASPack packer to compress and protect malicious executables, making static analysis more difficult and helping to evade signature-based detection tools. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to identify potentially obfuscated payloads that could be used for initial access or privilege escalation, ensuring that compressed binaries are inspected for hidden malicious logic before they execute.

YARA Rule

rule aspack : Packer
{
	meta:
		author="Kevin Falcoz"
		date_create="25/02/2013"
		description="ASPack"

	strings:
		$str1={60 E8 00 00 00 00 5D 81 ED 5D 3B 40 00 64 A1 30 00 00 00 0F B6 40 02 0A C0 74 04 33 C0 87 00 B9 [2] 00 00 8D BD B7 3B 40 00 8B F7 AC} /*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

• Scenario: A developer or build engineer uses the aspack.exe utility to compress a release build of a C++ application before packaging it for distribution.
  • Filter/Exclusion: Exclude processes where the parent process is a known build tool (e.g., msbuild.exe, dotnet.exe, ninja.exe) or where the working directory is within a standard source code repository path (e.g., C:\Src\, D:\Builds\).
• Scenario: An IT administrator manually compresses a large log archive or backup file using the ASPack command-line tool to reduce storage size before transferring it to a secondary storage server.
  • Filter/Exclusion: Exclude executions where the user account belongs to a specific administrative group (e.g., IT_Admins, Backup_Ops) and the target file extension is a common archive type (.zip, .tar, .7z) or the process runs from a standard administrative tools directory (e.g., C:\Program Files\ASPack\).
• Scenario: A scheduled maintenance job (Task Scheduler) runs a script that invokes ASPack to compress nightly database dump files before moving them to the archive share.
  • Filter/Exclusion: Exclude processes where the parent process is svchost.exe (indicating a scheduled task) or cmd.exe/powershell.exe initiated by a known scheduled task name, and the command line arguments contain specific keywords like dump, backup, or archive.
• Scenario: A software vendor’s installer or update mechanism uses ASPack internally to decompress or compress payload files during the installation of a legacy enterprise application.
  • Filter/Exclusion: Exclude processes where the parent process is an installer executable (e.g., msiexec.exe, setup.exe, install.exe) from