What is the Mutator Kit?

The Mutator Kit leverages an LLVM (Low Level Virtual Machine) mutator to introduce subtle yet key changes to the byte patterns and characteristics of the Beacon payload while it uses sleep masks to reside dormant in memory. If pre-defined YARA signatures were previously capable of identifying a known sleep mask variant, adding a mutator adds an additional layer of obfuscation, rendering these signatures ineffective.

Benefits of Mutator Kit

The Mutator Kit decreases the chances of a simulated adversary’s presence being detected, enabling red teams to accurately emulate advanced persistent threats (APTs) that continuously adapt their tooling to bypass evolving defensive measures.

By introducing variations to the sleep mask, the kit ensures that static, signature-based detections are consistently thwarted, compelling blue teams to develop more advanced behavioral analytics. 

How Does Mutator Kit Work?

The Mutator Kit uses four obfuscation passes which are based on eShard’s obfuscator-llvm plugin.  This in turn is based on mutations introduced in the research by Pascal J., et al.  These passes include: 


Substitution

Replace binary operators with functionally equivalent ones.


Bogus

Insert fake control flow blocks that are not intended for execution but are instead meant to confuse static analysis.


Code Flattening

 Aim to break higher level code/control flow structure by placing basic blocks at the same level, removing hierarchical structure.


Basic-Block Splitting

Aim to break higher level code/control flow structure by dividing basic blocks into smaller fragments and adding unconditional jumps between fragments.

   Demo of Mutator Kit

This short video provides a high level overview on how to install and use the Cobalt Strike Mutator Kit, which uses an LLVM obfuscator to break in-memory YARA scanning of the sleep mask.

There are two methods to install the Mutator Kit: directly (“native”) and through Docker. An overview of these installation methods can be seen in the video above. More information  on installation and how to use the Mutator Kit can be found in this introductory blog.

The Mutator Kit and User Community Contributions

The obfuscator-llvm repository is publicly available, so Cobalt Strike users can create their own obfuscation passes. This enables the community to develop and share custom mutations, further expanding the tool’s capabilities.

Enhance your red team engagements with the Mutator Kit

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