MCP Authentication: OAuth Security Guide

MCP authentication governs how clients access MCP servers, using OAuth 2.1-based authorization for HTTP transports and narrowly scoped environment credentials for local stdio servers. Under the July 2026 specification, protected servers publish RFC 9728 metadata, clients discover and validate authorization-server metadata, use PKCE and RFC 8707 resource indicators, validate authorization-response issuers, and present bearer tokens whose audience is checked by the receiving server. The guidance prohibits token passthrough to upstream services because it can enable confused-deputy attacks, recommends minimal scopes and step-up authorization for additional permissions, and distinguishes 401 responses for missing or invalid tokens from 403 responses for insufficient scopes. It also highlights a September 2026 OAuth client vulnerability in affected MCP Python SDK versions that could expose client secrets, authorization codes, and PKCE verifiers through malicious discovery metadata; fixes were released in versions 1.30.0 and 2.2.0, alongside recommendations to bind registrations to issuers, clear old registrations, and rotate potentially exposed credentials. Beyond OAuth configuration, the material notes that token theft, excessive permissions, prompt injection, and unapproved MCP servers remain risks because authentication establishes identity and scope but cannot determine whether an agent’s requested action is appropriate.

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