AI Engineering/Phase 13/Lesson 16

MCP Security II — OAuth 2.1, Resource Indicators, Incremental Scopes

Remote MCP servers need authorization, not just authentication. The 2025-11-25 spec aligns with OAuth 2.1 + PKCE + resource indicators (RFC 8707) + protected-resource metadata (RFC 9728). SEP-835 adds incremental scope consent with step-up authorization on 403 WWW-Authenticate. This lesson implements the step-up flow as a state machine so you can see every hop.

Build 1.3 hr Python (stdlib, OAuth state machine simulator)

Prerequisites: Phase 13 · 09 (transports), Phase 13 · 15 (security I)

#Learning Objectives

Distinguish resource server from authorization server responsibilities.
Walk the PKCE-protected OAuth 2.1 authorization code flow.
Use resource (RFC 8707) and protected-resource metadata (RFC 9728) to prevent confused-deputy attacks.
Implement step-up authorization: server responds 403 with WWW-Authenticate asking for a higher scope; client re-prompts user consent and retries.

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The text and code of this lesson are based on AI Engineering from Scratch by Rohit Ghumare, MIT licensed.