Mumblehard i386 assembly code responsible for decrypting Perl code

Hunt Hypothesis

This hypothesis targets the execution of Mumblehard i386 assembly routines used to decrypt and execute Perl payloads, a technique often employed by commodity malware to evade static analysis. Proactively hunting for this behavior in Azure Sentinel allows the SOC to identify low-severity, potentially stealthy infections that may serve as initial access vectors or persistence mechanisms before they escalate in severity.

YARA Rule

rule mumblehard_packer
{
    meta:
        description = "Mumblehard i386 assembly code responsible for decrypting Perl code"
        author = "Marc-Etienne M.Leveille"
        date = "2015-04-07"
        reference = "http://www.welivesecurity.com"
        version = "1"

    strings:

	$decrypt = { 31 db [1-10] ba ?? 00 00 00 [0-6] (56 5f | 89 F7) 39 d3 75 13 81 fa ?? 00 00 00 75 02 31 d2 81 c2 ?? 00 00 00 31 db 43 ac 30 d8 aa 43 e2 e2 }

    condition:
        $decrypt
}

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.

References

• http://www.welivesecurity.com
• Source Rule

False Positive Guidance

• Legacy Perl Script Maintenance by DevOps Team: A senior DevOps engineer manually edits a legacy Catalyst or Mojolicious application script to fix a bug, using a custom i386 assembly stub (compiled via nasm or gas) to decrypt a hardcoded configuration blob for backward compatibility with older 32-bit Perl interpreters.

  • Filter/Exclusion: Exclude alerts where the parent process is perl.exe (version < 5.20) or nasm.exe/as.exe (assembler) and the user account belongs to the DevOps-Admins security group, provided the script path resides in a known application directory (e.g., C:\Apps\LegacyPerl\).

• Scheduled Backup Encryption Utility: A nightly scheduled task (Task Scheduler job named Backup-Encrypt-Perl) runs a custom i386 assembly routine (linked into a C++ wrapper) to decrypt and re-encrypt Perl-based backup manifests stored in a 32-bit compatibility folder. The assembly code is part of a signed internal tool perl_backup_tool.exe.

  • Filter/Exclusion: Exclude alerts where the process name is perl_backup_tool.exe, the file hash matches the known-good internal build, and the execution path is under C:\Program Files (x86)\InternalTools\Backup\.

• Legacy 32-bit Perl Interpreter on x64 Host: An application server running a 32-bit Perl 5.8 interpreter (common in older LAMP stacks) executes a module that uses inline assembly (via Inline::ASM or perlasm) to perform fast decryption of encrypted config files. The i386 assembly runs within the 32-bit process space on a 64-bit OS.

  • Filter/Exclusion: Exclude