Add ransomware extension hijacking detection signature

[kevross33]

Lockbit

BASTA (also showing other recent signatures detecting too for ransomware helping to ID it)

[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 system's ability to detect ransomware by introducing a new signature that identifies file extension and icon hijacking attempts. This proactive detection helps in recognizing malicious behavior, such as that exhibited by Lockbit, by monitoring specific registry modifications.

Highlights

• New Ransomware Detection Signature: Introduced a new Cuckoo signature, RansomwareSetsIcon, to identify potential ransomware activity by monitoring registry key modifications associated with file extension and icon hijacking.
• Registry Key Monitoring: The signature specifically targets RegSetValueExA and RegSetValueExW API calls, checking for modifications to \defaulticon, \applications, or \userchoice registry paths, especially when an .ico file is involved.

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Changelog

• modules/signatures/windows/ransomware_extensionhijack.py
  • Added a new Cuckoo signature to detect ransomware-related file extension and icon hijacking.

Activity

• No specific activity (comments, reviews, etc.) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request introduces a new signature to detect ransomware-like behavior, specifically file extension hijacking through registry modifications. The signature monitors for changes to DefaultIcon and UserChoice registry keys. My review focuses on improving the accuracy and maintainability of this new signature. I've suggested renaming the class and signature for better clarity and tightening the detection logic for UserChoice hijacking to reduce potential false positives.

[gemini-code-assist]

The condition for is_userchoice_hijack is very broad. It flags any registry write to a key path containing \userchoice as long as the data being written is a string. This is likely to cause false positives, as legitimate actions like changing a default program also modify these keys. To improve accuracy, this check should be more specific. For example, you could verify that the registry value being modified is ProgId and potentially inspect its new value for suspicious patterns.

[gemini-code-assist]

The class name RansomwareSetsIcon and signature name ransomware_sets_icon are misleading given the signature's functionality. It detects both icon changes and broader file-extension hijacking via UserChoice keys. To improve clarity and align with the filename and PR's intent, I suggest renaming them to RansomwareExtensionHijack and ransomware_extension_hijack respectively.

Suggested change

	class RansomwareSetsIcon(Signature):
	name = "ransomware_sets_icon"
	class RansomwareExtensionHijack(Signature):
	name = "ransomware_extension_hijack"