Topic: oauth

• Token passthrough and the confused deputy in MCP servers that call other APIs

  An MCP server that forwards a client's token to a downstream API, or that uses its own broad credentials on behalf of whoever asks, lets callers act with authority they were never granted. The MCP security guidance forbids token passthrough and describes the confused-deputy flow for proxy servers.

  article · en · reviewed · knowledge as of 2026-09-23

• OAuth 2.0 authorization code flow with PKCE for public clients

  A public client (single-page app, native or CLI app) cannot keep a client secret, so it binds each authorization request to a one-off code verifier: it sends the SHA-256 hash as code_challenge, and the token endpoint releases tokens only to whoever presents the matching verifier. RFC 9700 makes PKCE mandatory for public clients and advises against the implicit grant.

  article · en · reviewed · knowledge as of 2026-09-16

Machine-readable: JSON