René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Question: MCP Python SDK Redirect URI Validation - Implementation Granularity?","articleBody":"### Question\n\n## Question\n\nDoes the current `validate_redirect_uri()` implementation in `OAuthClientMetadata` fully comply with OAuth 2.1 Section 7.12.2 requirement that \"Authorization servers **MUST** take precautions to prevent redirecting user agents to untrusted URI's\"?\n\n## Current Implementation Analysis\n\n**File**: `src/mcp/shared/auth.py`\n\n```python\n# Lines 82-91: validate_redirect_uri()\ndef validate_redirect_uri(self, redirect_uri: AnyUrl | None) -\u003e AnyUrl:\n    if redirect_uri is not None:\n        # Validate redirect_uri against client's registered redirect URIs\n        if redirect_uri not in self.redirect_uris:\n            raise InvalidRedirectUriError(f\"Redirect URI '{redirect_uri}' not registered for client\")\n        return redirect_uri\n    elif len(self.redirect_uris) == 1:\n        return self.redirect_uris[0]\n    else:\n        raise InvalidRedirectUriError(\"redirect_uri must be specified when client has multiple registered URIs\")\n```\n\n## Current Security Measures\n- ✅ Validates redirect URI against client's registered URIs\n- ✅ Uses Pydantic's AnyUrl for basic URL format validation\n- ✅ Handles single vs. multiple registered URIs correctly\n\n## Key Security Concern\n**What if the registered URIs themselves are untrusted?**\n\nThe current implementation only checks if a redirect URI is in the client's registered list, but **does not validate whether those registered URIs are themselves trustworthy**. For example:\n\n- A client could register `http://evil.com/callback` as a valid redirect URI\n- The current validation would **allow** this redirect\n- This could lead to open redirect attacks or phishing\n\n## Potential Security Gaps\n- ❌ No validation of URI protocol (HTTP vs HTTPS)\n- ❌ No domain whitelist validation\n- ❌ No STRONG restrictions protection against open redirect attacks?\n- ❌ No validation of URI path components\n- ❌ No check for internal/local network redirects\n- ❌ **No validation that registered URIs themselves are trustworthy**\n\n## Question\nIs the current implementation sufficient for OAuth 2.1 Section 7.12.2 compliance, or should additional security measures be added to prevent redirection to untrusted URIs beyond just checking against registered URIs?\n\n**P.S.** I am not very confident that it is a real issue, which seems just a \"implementation choice\". I will be very appreciative if you maintainers, have any time to review it and tell me your opinion.\n\n### Additional Context\n\n_No response_","author":{"url":"https://github.com/younaman","@type":"Person","name":"younaman"},"datePublished":"2025-10-07T22:18:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/1446/python-sdk/issues/1446"}