fix(storage): Implement path containment to prevent traversal attacks

[mahendra-google]

Please see vulnerability b/449635520

[gemini-code-assist]

Summary of Changes

Hello @mahendra-google, 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 introduces a critical security enhancement to the Google Cloud Storage client library by implementing robust path containment checks during object download operations. The primary goal is to prevent directory traversal vulnerabilities, ensuring that downloaded files are securely written only within the application's designated base directory. This mitigation protects against potential malicious file overwrites or unauthorized file system access.

Highlights

• Security Enhancement: Implemented path containment logic to prevent directory traversal attacks when downloading objects, ensuring files are written only within the application's base directory.
• Validation Integration: The new path validation is integrated into all DownloadObject and DownloadObjectAsync methods within the StorageClientImpl.
• New Tests: Added new integration tests to verify that attempts to download objects to relative or absolute paths outside the base directory correctly throw an ArgumentException.

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[mahendra-google]

/gemini review

[gemini-code-assist]

Code Review

This pull request aims to prevent path traversal attacks by adding validation for the destination stream in DownloadObject methods. While the intention is good, the current implementation is overly restrictive. It limits file downloads to the application's base directory, which is a significant breaking change that will prevent legitimate use cases like downloading files to a user's 'Downloads' folder. The core validation logic needs to be reconsidered to avoid breaking existing functionality. I have also left a minor suggestion to improve the robustness of the new integration tests.

[gemini-code-assist]

Code Review

This pull request introduces a security check to prevent path traversal attacks when downloading objects to a file stream. The approach is to validate that the destination file path is within the application's base directory. While this is a valuable security enhancement, the current implementation of the validation logic has critical vulnerabilities that could allow an attacker to bypass the check. Additionally, the new integration tests are brittle and could be improved for robustness. My review includes detailed feedback on how to address these security flaws and improve the tests.

[mahendra-google]

Holding off this change for now as this could be a breaking change for customers who are already using this feature.