Add DecompressEXE signature for memory analysis

[kevross33]

This module defines a signature for detecting decompression of executables in memory, which may indicate the use of packers or manual loaders.

Pikabot:

[gemini-code-assist]

Summary of Changes

Hello @kevross33, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the malware analysis capabilities by introducing a new signature designed to identify executables that decompress themselves in memory. This is a crucial step in detecting packed or manually loaded malware, providing deeper insight into their runtime behavior and improving the overall efficacy of threat detection.

Highlights

• New Cuckoo Signature: A new Cuckoo Sandbox signature, DecompressEXE, has been added to detect in-memory decompression of executables.
• Detection Logic: The signature specifically monitors for calls to the RtlDecompressBuffer API and checks if the resulting uncompressed buffer begins with the 'MZ' magic bytes, indicating a Portable Executable (PE) file.
• Malware Analysis Focus: This detection aims to identify the use of packers or manual loaders, which are common techniques used by malicious software to evade detection and obfuscate their true nature.

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Changelog

• modules/signatures/windows/packer_decompress_buffer.py
  • Added a new file defining the DecompressEXE signature.
  • Implemented logic to detect RtlDecompressBuffer API calls.
  • Included a check for the 'MZ' header in the uncompressed buffer to confirm it's an executable.
  • Configured signature metadata including severity, confidence, categories (packer), authors, minimum Cuckoo version, and relevant MITRE ATT&CK (TTPs) and Malware Behavior Catalog (MBCs) identifiers.

Activity

• No human activity has occurred on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request introduces a new Cuckoo signature to detect in-memory decompression of executables by monitoring RtlDecompressBuffer API calls. The overall logic is sound, but I've identified a critical issue that could lead to a crash, as well as several opportunities for simplification and adherence to the framework's patterns. Additionally, some of the MITRE ATT&CK and MBC references appear to be invalid.

[gemini-code-assist]

The ttps and mbcs lists contain references that are not defined in the corresponding data files (data/mitre/TTPs.json and data/mbc.json).

• T1620 is not present in TTPs.json.
• F0001 and OB0006 are not present in mbc.json. The mbc.json file uses a B prefix for behaviors (e.g., B0002). OB0006 might be a typo for B0006 or one of its sub-behaviors.

Using invalid references can lead to errors or incomplete information in the analysis reports. Please verify these references and update them to valid ones.

[gemini-code-assist]

The self.ret flag is redundant. The Signature base class already provides a mechanism for tracking matched calls via self.mark_call() and self.has_marks(). Using the base class functionality simplifies the code and removes unnecessary state management.

I recommend the following refactoring:

1. Remove the __init__ method entirely.
2. In on_call, remove the line self.ret = True. The self.mark_call() is sufficient.
3. In on_complete, change the implementation to return self.has_marks().

This will make the signature cleaner and more aligned with the framework's intended patterns.

[gemini-code-assist]

This check for call["api"] is redundant because you've already specified filter_apinames. The on_call handler will only be triggered for RtlDecompressBuffer. You can remove this if statement and un-indent the nested code block.