MCP Python SDK Implementaion Gap 15- lack token isolation when calling upstream APIs

[younaman]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Summary

The MCP Python SDK's HttpResource class lacks token isolation mechanisms when making requests to external APIs, violating the MCP specification requirement that "MCP servers MUST NOT pass through the token it received from the MCP client to upstream APIs."

🎯 Core Problem

MCP specification

If the MCP server makes requests to upstream APIs, it may act as an OAuth client to them. The access token used at the upstream API is a separate token, issued by the upstream authorization server. The MCP server MUST NOT pass through the token it received from the MCP client.

HttpResource Lacks Token Isolation

File Location: src/mcp/server/fastmcp/resources/types.py:148-159

class HttpResource(Resource):
    """A resource that reads from an HTTP endpoint."""

    url: str = Field(description="URL to fetch content from")
    mime_type: str = Field(default="application/json", description="MIME type of the resource content")

    async def read(self) -> str | bytes:
        """Read the HTTP content."""
        async with httpx.AsyncClient() as client:
            response = await client.get(self.url)  # ⚠️ No authentication headers!
            response.raise_for_status()
            return response.text

Problem Analysis

1. Lacks Authentication Mechanism: HttpResource makes direct calls to external APIs without any authentication headers
2. Lacks Token Isolation Checks: No mechanism to prevent passing client tokens

🔍 Security Impact

• Lack of framework-level protection against token passthrough
• Developers may incorrectly pass client tokens

P.S. I am not entirely certain if this is the correct location where the upstream API calling issue occurs. If I have misunderstood the implementation or missed other relevant code paths, I would greatly welcome and appreciate discussion and clarification from the maintainers.

Example Code

Python & MCP Python SDK

latest