Fall Back to Legacy 2025-03-26 OAuth Discovery for Servers Without PRM
## Motivation and Context The MCP 2025-03-26 authorization spec predates Protected Resource Metadata: the MCP server itself acted as the authorization base URL, its RFC 8414 metadata was fetched from the server origin, and the spec's "Fallbacks for Servers without Metadata Discovery" section required clients to use the default endpoints `/authorize`, `/token`, and `/register` relative to the authorization base URL when no metadata was published. Both the TypeScript SDK (`discoverOAuthServerInfo` falls back to the server base URL on PRM failure; `startAuthorization` / `executeTokenRequest` / `registerClient` default the endpoint paths) and the Python SDK (`build_oauth_authorization_server_metadata_discovery_urls(None, ...)` plus the same endpoint defaults) keep this as client-side backwards compatibility, and the `auth/2025-03-26-oauth-metadata-backcompat` and `auth/2025-03-26-oauth-endpoint-fallback` conformance scenarios exercise it. The Ruby flow previously raised as soon as PRM discovery failed, so both scenarios were expected conformance failures. This change adds the same fallback: - `Flow#locate_authorization_server` first attempts PRM discovery; on any discovery failure (404s, network errors, malformed documents, matching the TypeScript and Python SDKs' broad fallback) the MCP server's origin becomes the legacy authorization base URL. The Communication Security check (HTTPS or loopback) still applies to that origin, and PRM documents that parse correctly keep their strict shape validation. - `Flow#authorization_server_metadata` fetches RFC 8414 metadata from the base URL. On the legacy path the `issuer` byte-match is skipped: the 2025-03-26 spec predates that requirement, and a pre-PRM server may host its OAuth endpoints under a path prefix whose `issuer` legitimately differs from the discovery origin (neither reference SDK validates the issuer on this path). On the modern path the byte-match is unchanged. - When even the metadata document is absent, synthetic metadata carrying the legacy spec's default endpoints is used, with PKCE S256 assumed (the legacy spec mandates PKCE; the TypeScript and Python SDKs hardcode S256 on this path too). Endpoint HTTPS checks still apply. - `run!` and `refresh!` share the new discovery helpers, and both 2025-03-26 scenarios are removed from `conformance/expected_failures.yml`. ## How Has This Been Tested? New tests in `test/mcp/client/oauth/flow_test.rb`: - the metadata-backcompat shape: no PRM, AS metadata at the server origin with `/oauth`-prefixed endpoints and a mismatched `issuer` completes the flow against those endpoints (no issuer error) - the endpoint-fallback shape: no metadata at all registers, authorizes, and exchanges the code at `/register`, `/authorize`, and `/token` on the origin, still sending `code_challenge` (S256) and `code_verifier` - a remote plain-http origin is rejected as the legacy authorization base - strict-mode regression: with PRM present, a mismatched `issuer` still aborts the flow - a malformed PRM (top-level JSON array) now selects the legacy path instead of leaking a `TypeError`, and surfaces a domain error when that path also dead-ends Two existing tests that asserted PRM failure was terminal were updated to dead-end the legacy path explicitly, preserving their original intent (single flow attempt, domain error type). ## Breaking Changes None for spec-compliant 2025-06-18+ servers, whose PRM-based discovery and issuer validation are unchanged. Servers that previously failed hard during PRM discovery now get one legacy discovery attempt before the flow errors, which can only turn previously failing flows into working ones.
This commit is contained in:
Koichi ITO
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1 changed files with 246 additions and 21 deletions
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@ -38,22 +38,18 @@ module MCP
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ensure_secure_url!(resource_metadata_url, label: "WWW-Authenticate resource_metadata URL")
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end
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prm = fetch_protected_resource_metadata(
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prm, authorization_server = locate_authorization_server(
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server_url: server_url,
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resource_metadata_url: resource_metadata_url,
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)
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authorization_server = first_authorization_server(prm)
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ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
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# Per RFC 8707 + MCP authorization, the canonical MCP server URI is sent on
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# both the authorization and token requests. When PRM advertises a `resource`,
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# it MUST identify the same MCP server we are talking to; otherwise we are
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# being redirected to credentials minted for a different audience.
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resource = canonical_resource(server_url: server_url, prm_resource: prm["resource"])
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resource = canonical_resource(server_url: server_url, prm_resource: prm&.dig("resource"))
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as_metadata = fetch_authorization_server_metadata(issuer_url: authorization_server)
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ensure_issuer_matches!(expected: authorization_server, returned: as_metadata["issuer"])
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ensure_secure_endpoints!(as_metadata)
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as_metadata = authorization_server_metadata(authorization_server: authorization_server, legacy: prm.nil?)
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if provider_authorization_flow == :client_credentials
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return run_client_credentials!(as_metadata: as_metadata, prm: prm, resource: resource, scope: scope)
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@ -63,7 +59,7 @@ module MCP
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client_info = ensure_client_registered(as_metadata: as_metadata)
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effective_scope = resolve_scope(scope: scope, prm: prm)
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effective_scope = resolve_scope(scope: scope, prm: prm || {})
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effective_scope = normalize_offline_access_scope(effective_scope, as_metadata: as_metadata)
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pkce = PKCE.generate
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state = SecureRandom.urlsafe_base64(32)
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@ -158,18 +154,14 @@ module MCP
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ensure_secure_url!(resource_metadata_url, label: "WWW-Authenticate resource_metadata URL")
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end
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prm = fetch_protected_resource_metadata(
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prm, authorization_server = locate_authorization_server(
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server_url: server_url,
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resource_metadata_url: resource_metadata_url,
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)
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authorization_server = first_authorization_server(prm)
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ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
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resource = canonical_resource(server_url: server_url, prm_resource: prm["resource"])
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resource = canonical_resource(server_url: server_url, prm_resource: prm&.dig("resource"))
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as_metadata = fetch_authorization_server_metadata(issuer_url: authorization_server)
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ensure_issuer_matches!(expected: authorization_server, returned: as_metadata["issuer"])
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ensure_secure_endpoints!(as_metadata)
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as_metadata = authorization_server_metadata(authorization_server: authorization_server, legacy: prm.nil?)
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client_info = if have_stored_client_info
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# Pre-registered / DCR-issued `client_information` always wins: if the user picked an explicit identity,
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@ -221,6 +213,87 @@ module MCP
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fetch_metadata_json(urls, label: "protected resource metadata")
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end
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# Locates the authorization server for `server_url` and returns `[prm, authorization_server]`.
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#
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# Modern path (2025-06-18+): Protected Resource Metadata names the authorization server in
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# `authorization_servers`.
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#
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# Legacy path (2025-03-26 backwards compatibility): when the server publishes no PRM, `prm` is nil
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# and the MCP server's own origin acts as the authorization base URL, matching the TypeScript and Python SDKs.
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# Any PRM discovery failure (404s, network errors, malformed documents) selects the legacy path, mirroring both SDKs' behavior.
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# https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#fallbacks-for-servers-without-metadata-discovery
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def locate_authorization_server(server_url:, resource_metadata_url:)
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prm = begin
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fetch_protected_resource_metadata(
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server_url: server_url,
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resource_metadata_url: resource_metadata_url,
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)
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rescue AuthorizationError
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nil
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end
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if prm
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authorization_server = first_authorization_server(prm)
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ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
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[prm, authorization_server]
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else
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authorization_base = server_origin!(server_url)
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ensure_secure_url!(authorization_base, label: "MCP server origin (legacy authorization base URL)")
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[nil, authorization_base]
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end
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end
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# Fetches and validates the authorization server's RFC 8414 metadata.
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#
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# On the modern path the metadata `issuer` must be byte-identical to the discovery URL (RFC 8414 Section 3.3).
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# On the legacy 2025-03-26 path that validation is skipped: the legacy spec predates the requirement,
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# and a pre-PRM server may host its OAuth endpoints under a path prefix whose `issuer` legitimately differs from
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# the origin the metadata was discovered at (neither the TypeScript nor the Python SDK validates the issuer on this path).
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# When even the metadata document is absent, the legacy spec's default endpoints are used.
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def authorization_server_metadata(authorization_server:, legacy:)
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metadata = if legacy
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begin
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fetch_authorization_server_metadata(issuer_url: authorization_server)
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rescue AuthorizationError
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default_legacy_metadata(authorization_server)
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end
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else
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fetch_authorization_server_metadata(issuer_url: authorization_server).tap do |fetched|
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ensure_issuer_matches!(expected: authorization_server, returned: fetched["issuer"])
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end
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end
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ensure_secure_endpoints!(metadata)
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metadata
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end
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# The 2025-03-26 spec's "Fallbacks for Servers without Metadata Discovery": clients MUST use these default endpoint paths
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# relative to the authorization base URL. PKCE S256 is assumed because the legacy spec mandates PKCE and there is no metadata
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# to advertise it (the TypeScript and Python SDKs hardcode S256 on this path too).
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def default_legacy_metadata(authorization_base)
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{
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"issuer" => authorization_base,
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"authorization_endpoint" => "#{authorization_base}/authorize",
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"token_endpoint" => "#{authorization_base}/token",
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"registration_endpoint" => "#{authorization_base}/register",
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"code_challenge_methods_supported" => ["S256"],
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}
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end
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# Returns `scheme://host[:port]` of `server_url`, the legacy 2025-03-26 authorization base URL for servers without PRM.
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def server_origin!(server_url)
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uri = URI.parse(server_url.to_s)
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unless uri.is_a?(URI::HTTP) && uri.host
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raise AuthorizationError,
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"Cannot derive a legacy authorization base URL from MCP server URL #{server_url.inspect}."
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end
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port_part = uri.port == uri.default_port ? "" : ":#{uri.port}"
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"#{uri.scheme}://#{uri.host}#{port_part}"
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rescue URI::InvalidURIError => e
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raise AuthorizationError, "MCP server URL #{server_url.inspect} is not a valid URI: #{e.message}."
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end
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def fetch_authorization_server_metadata(issuer_url:)
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urls = Discovery.authorization_server_metadata_urls(issuer_url)
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fetch_metadata_json(urls, label: "authorization server metadata")
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