MCP Authentication and Authorization: A Comprehensive Guide

Authentication and Authorization in MCP

Introduction

The Model Context Protocol (MCP) provides a framework for integrating LLM applications with external tools and data sources. This guide focuses on implementing authentication and authorization within MCP, which is essential for secure and controlled access to various system capabilities.

Understanding MCP's Architecture

MCP is built on a flexible architecture that connects client-server applications with LLMs. Key components include:

  • Hosts: LLM applications initiating connections.
  • Clients: Maintain 1:1 connections with servers.
  • Servers: Provide context, tools, and prompts to clients.

Server-Side Components

  1. Resources: Expose data context to clients.
  2. Tools: Provide executable functionalities.
  3. Prompts: Offer interaction templates for user guidance.

Client-Side Features

  1. Sampling: Enables LLM-driven actions.
  2. Roots: Define operational boundaries.

Protocol Specification

Base Protocol

MCP uses JSON-RPC 2.0 for communication, with support for requests, responses, and notifications. Messages are encoded in UTF-8 and follow a structured format ensuring interoperability.

Lifecycle Management

The MCP lifecycle consists of:

  1. Initialization: Protocol version negotiation, capability declaration.
  2. Operation: Normal message exchanges.
  3. Shutdown: Graceful termination.

Capability Negotiation

Clients and servers negotiate capabilities during initialization. Key capabilities include:

  • Resources: Contextual data exposure.
  • Tools: Executable functions.
  • Prompts: Structured interactions.
  • Sampling: Agentic interactions.

Servers must declare capabilities like resource subscriptions and tool support, while clients declare features like sampling.

Authentication and Authorization

Overview

MCP supports an optional authorization framework for HTTP-based transports, allowing client requests to be restricted based on resource owner permissions.

OAuth 2.1 Framework

  • Implementations MUST support OAuth 2.1 with security measures.
  • OAuth 2.0 Dynamic Client Registration Protocol is RECOMMENDED.
  • Authorization Server Metadata should be implemented for discovery.

Authorization Flow

When authorization is required and not yet proven:

  1. Client sends a request.
  2. Server responds with HTTP 401 Unauthorized.
  3. Client initiates OAuth 2.1 authorization.

This process uses code challenges (PKCE) for enhanced security.

Server Metadata Discovery

  • Clients MUST discover server capabilities via OAuth 2.0 Authorization Server Metadata.
  • Metadata provides the endpoints for authorization.
  • Fallbacks are used if discovery is unsupported.

Access Tokens

Access tokens are used to authenticate requests:

  • Tokens are included via the Authorization header.
  • Servers validate tokens per OAuth standards.

Security Considerations

Implementations MUST enforce these security measures:

  • Secure token storage.
  • Token expiration and rotation.
  • HTTPS for all authorization endpoints.
  • Validation of redirect URIs.

Implementing Secure MCP Servers

Using OAuth in Server Implementation

Integrate OAuth within your server as follows:

// Example OAuth flow implementation in a Node.js MCP server
import { Server } from "@modelcontextprotocol/sdk/server/index.js";

const server = new Server({
  name: "secure-server",
  version: "1.0.0"
});

server.useOAuth({
  authorizationEndpoint: "/authorize",
  tokenEndpoint: "/token",
  clientRegistration: "/register"
});

server.onAuthorize(async (req) => {
  // Handle authorization request
});

Handlers

  1. Authorization Handler: Manages OAuth flow.
  2. Token Handler: Issues and validates access tokens.

Error Responses

Ensure clear error handling for authorization issues:

{
  "jsonrpc": "2.0",
  "error": {
    "code": 401,
    "message": "Unauthorized access",
    "data": {
      "reason": "Invalid token"
    }
  }
}

Security Best Practices

  1. Validate all inputs against schemas.
  2. Implement robust access controls.
  3. Monitor token usage for anomalies.
  4. Regularly audit server permissions.

Conclusion

By implementing a robust authentication and authorization strategy within MCP, we ensure secure and compliant integration between LLM applications and various external systems. While the protocol specifies high-level requirements, the specific implementation may vary based on application needs and security considerations.