fix(core): validate MCP OAuth resources from metadata URL

[pragnyanramtha]

Fixes #20017.

Summary

• derive the expected protected resource from the metadata URL that actually returned OAuth resource metadata
• keep path-based RFC 9728 validation intact when the path metadata endpoint succeeds
• allow root .well-known/oauth-protected-resource fallback metadata to validate against the root resource for path-based MCP server URLs
• apply the same metadata-URL-based validation to WWW-Authenticate discovery

Root cause

For path-based MCP URLs like https://mcp.slack.com/mcp, discovery first tries the path-based metadata URL and then falls back to the root metadata URL when the path endpoint is unavailable. After that fallback succeeded, validation still compared the returned root resource against the original MCP server path, producing errors such as Protected resource https://mcp.slack.com does not match expected https://mcp.slack.com/mcp.

Relation to prior OAuth fixes: #19038 fixed the earlier 401/resource-metadata routing path. This PR is a follow-up for the later #20017 root-metadata fallback/resource mismatch.

Validation

• npm exec -- vitest run packages/core/src/mcp/oauth-utils.test.ts
• npm run test:ci --workspace @google/gemini-cli-core -- src/mcp/oauth-utils.test.ts
• npm exec -- prettier --check packages/core/src/mcp/oauth-utils.ts packages/core/src/mcp/oauth-utils.test.ts
• npm exec -- eslint packages/core/src/mcp/oauth-utils.ts packages/core/src/mcp/oauth-utils.test.ts --max-warnings 0
• npm run typecheck --workspace @google/gemini-cli-core
• npm run build --workspace @google/gemini-cli-core
• git diff --check

[gemini-code-assist]

Summary of Changes

Hello, 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 addresses an issue where path-based MCP server URLs failed validation during OAuth metadata discovery when falling back to root-based endpoints. By shifting the validation logic to rely on the metadata URL itself rather than the original server path, the system now correctly handles resource identification in both standard and fallback scenarios, ensuring better adherence to RFC 9728.

Highlights

• RFC 9728 Validation Logic: Updated the OAuth resource validation to derive the expected protected resource directly from the metadata URL, ensuring compliance with RFC 9728 even when falling back to root-based discovery.
• Metadata URL Fallback Handling: Improved the fallback mechanism for path-based MCP URLs, allowing successful root-based metadata discovery to correctly validate against the root resource.
• WWW-Authenticate Discovery: Applied consistent metadata-URL-based validation to WWW-Authenticate discovery, resolving potential resource mismatch errors.

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[gemini-code-assist]

Code Review

This pull request introduces the buildResourceParameterFromMetadataUrl utility to OAuthUtils to correctly extract resource identifiers from metadata URLs according to RFC 9728. The OAuth discovery logic has been updated to use this method for improved validation, supported by new test cases for various discovery scenarios. Feedback was provided regarding a high-severity security vulnerability in the new utility method, where improper handling of protocol-relative paths could allow for URL host injection and bypass origin-binding security requirements.

[gemini-cli]

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This PR will be closed in 7 days if it remains without that designation. We encourage you to find and contribute to existing 'help wanted' issues in our backlog! Thank you for your understanding.

[gemini-cli]

This pull request is being closed as it has been open for 14 days without a 'help wanted' designation. We encourage you to find and contribute to existing 'help wanted' issues in our backlog! Thank you for your understanding.