use issuer instead of authorization_endpoint for oauth discovery

[garrettsparks]

Summary

When retrieving authorization server metadata for OAuth dynamic client registration, use issuer from /.well-known/oauth-authorization-server instead of authorization_endpoint.

Details

OAuth authorization servers may have a different authorization_endpoint URL than the authorization server itself. Gemini CLI currently attempts to use authorization_endpoint as the discovery endpoint for registration which results in failed OAuth registration because the authorization_endpoint is not necessarily expected to be able to provide authorization server metadata.

For dynamic client registration, Gemini CLI should use the actual authorization server URL from issuer to discovery authorization server metadata.

📓 NOTE: This does not fix any existing SSRF vulnerability that may be present with gemini-cli's MCP integration (there was no validation of authorization_endpoint occurring before either). I would consider that out of scope for this change; it should be addressed separately.

Related Issues

Fixes #19458

How to Validate

Tested with cloudflare's audit log MCP server (https://auditlogs.mcp.cloudflare.com/mcp)

in settings.json

  "mcpServers": {
    "cfauditlogs": {
      "httpUrl": "https://auditlogs.mcp.cloudflare.com/mcp"
    }
  }

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker

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[gemini-code-assist]

Summary of Changes

Hello @garrettsparks, 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 refines the OAuth dynamic client registration process within the Gemini CLI by correcting the source used for authorization server metadata discovery. Previously, the system relied on the authorization_endpoint, which could lead to registration failures. The change now correctly utilizes the issuer URL, ensuring robust and successful client registration by pointing to the authoritative source of metadata.

Highlights

• OAuth Dynamic Client Registration: The mechanism for discovering OAuth authorization server metadata during dynamic client registration has been updated to use the issuer URL instead of the authorization_endpoint. This change addresses issues where the authorization_endpoint might not correctly provide the necessary metadata.
• Configuration and Validation: The MCPOAuthConfig interface now includes an optional issuer property. A new validation step has been introduced to ensure that the issuer is present when dynamic client registration is attempted, throwing an error if it's missing.
• Test Coverage: New test cases have been added to oauth-provider.test.ts and oauth-utils.test.ts to validate the correct usage and handling of the issuer property, including scenarios where it's missing during dynamic registration.

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[gemini-code-assist]

Code Review

This pull request refactors the OAuth discovery mechanism to use the issuer URL instead of authorization_endpoint for dynamic client registration, which improves alignment with OAuth specifications. The changes are consistently applied and well-tested. However, a critical Server-Side Request Forgery (SSRF) vulnerability has been identified. This pre-existing issue arises because the issuer URL from external metadata is used to make network requests without validation, allowing potential exploitation by a malicious server. Although noted as out of scope by the author, this high-severity risk requires strict URL validation for all discovered endpoints.

[gemini-code-assist]

The application performs OAuth discovery and dynamic client registration using URLs provided by external MCP servers without proper validation. Specifically, the issuer URL (and subsequent URLs derived from it or provided in metadata) is used in fetch calls. A malicious MCP server can provide URLs pointing to internal services (e.g., http://localhost, http://169.254.169.254) or other sensitive endpoints, leading to Server-Side Request Forgery (SSRF). This can be used to probe internal networks or access cloud metadata services from the user's machine.

Remediation: Implement strict validation for all URLs obtained from external metadata before using them in fetch calls. This should include:

1. Blocking internal IP ranges (e.g., 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
2. Blocking cloud metadata service IPs (e.g., 169.254.169.254).
3. Ensuring that the issuer in the metadata matches the expected issuer URL.
4. Consider using an allow-list of trusted domains if possible.

[garrettsparks]

This behavior is already present in the codebase and is not modified as a result of this change: authorization_endpoint (currently in use) is unvalidated.

[gemini-cli]

Hi there! Thank you for your contribution to Gemini CLI. We really appreciate the time and effort you've put into this pull request.

To keep our backlog manageable and ensure we're focusing on current priorities, we are closing pull requests that haven't seen maintainer activity for 30 days. Currently, the team is prioritizing work associated with 🔒 maintainer only or help wanted issues.

If you believe this change is still critical, please feel free to comment with updated details. Otherwise, we encourage contributors to focus on open issues labeled as help wanted. Thank you for your understanding!