## Motivation and Context
SEP-2468 (modelcontextprotocol/modelcontextprotocol#2468, merged for the 2026-07-28 spec release) recommends
that authorization servers include the RFC 9207 `iss` parameter in authorization responses and requires MCP clients
to validate a present `iss` against the expected issuer before redeeming the authorization code.
This mitigates mix-up attacks in multi-IdP setups, where an attacker-controlled authorization server could otherwise capture
a code minted by an honest server.
This mirrors the TypeScript SDK's approach (typescript-sdk#2272, with typescript-sdk#1957 as the reference implementation),
adapted to the Ruby provider contract:
- `Provider`'s `callback_handler` may now return `[code, state, iss]` in addition to the existing `[code, state]`.
The 3-element form opts into validation; returning `iss = nil` asserts the caller inspected the authorization response
and found no `iss`.
- `Flow#run!` validates the issuer after the `state` check and before `exchange_authorization_code`,
implementing the RFC 9207 Section 2.4 rules: a present `iss` must equal the AS metadata `issuer` exactly
(the value `ensure_issuer_matches!` already pinned to the discovery URL); a nil `iss` from a 3-element callback is rejected
when the AS metadata advertises `authorization_response_iss_parameter_supported: true`; a legacy 2-element callback skips the check,
so every existing integration keeps working. On any violation the flow aborts with `AuthorizationError` before
the code reaches a token endpoint.
- The conformance client now returns the 3-element form, plumbing `iss` through from the redirect URL.
Resolves#386.
## How Has This Been Tested?
New tests in `test/mcp/client/oauth/flow_test.rb`:
- a matching `iss` completes the flow and persists tokens
- a mismatched `iss` raises `AuthorizationError` and never contacts the token endpoint (`assert_not_requested` on the token POST)
- a nil `iss` with `authorization_response_iss_parameter_supported: true` raises without contacting the token endpoint (fail closed)
- a legacy 2-element callback completes even when the AS advertises `iss` support (backward compatibility)
- a nil `iss` without the advertisement completes
- a mismatched `iss` is rejected even without the advertisement (validate-when-present is unconditional)
`bundle exec rake` (tests, RuboCop, and conformance baseline with the 3-element conformance callback) passes.
## Breaking Changes
None. The 2-element `callback_handler` contract continues to work unchanged; issuer validation activates only
when a callback opts in by returning the 3-element form.
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