IS THAT ALLOWED? AUTHENTICATION AND AUTHORIZATION IN MODEL CONTEXT PROTOCOL

Summary from the TLDR InfoSec newsletter

Remote MCP servers should implement the OAuth 2.1 Authorization Code grant with PKCE for client authentication, leveraging protected resource metadata (RFC 9728) and authorization server discovery (RFC 8414) for the handshake. The 2025-11-25 MCP specification introduces step-up authorization for scope negotiation and recommends Token Exchange for downstream service authentication, while explicitly prohibiting token passthrough. Organizations deploying MCP servers should design granular scopes with clear read/write/delete separation, ensure short-lived access tokens, and implement proper validation, including checks for issuer, audience, and expiration.