Fall Back to Legacy 2025-03-26 OAuth Discovery for Servers Without PRM

README.md

@@ -1986,6 +1986,9 @@ pass an `MCP::Client::OAuth::Provider` to the transport instead of a static `Aut
 - Send `Authorization: Bearer <access_token>` on every request when a token is available.
 - On a `401 Unauthorized`, parse the `WWW-Authenticate` header, discover the authorization server (Protected Resource Metadata + RFC 8414 Authorization Server Metadata),
   perform Dynamic Client Registration if needed, run the OAuth 2.1 Authorization Code flow with PKCE (S256), and retry the failed request with the acquired token.
+- Fall back to the legacy 2025-03-26 discovery when the server publishes no Protected Resource Metadata, matching the TypeScript and Python SDKs: the MCP server's origin acts
+  as the authorization base URL, its metadata is fetched from `<origin>/.well-known/oauth-authorization-server` without the RFC 8414 issuer byte-match (which the legacy spec predates),
+  and when even that is absent the spec's default endpoints `/authorize`, `/token`, and `/register` at the origin are used with PKCE S256 assumed.
 - On subsequent 401s with a saved `refresh_token`, exchange it at the token endpoint before falling back to the full interactive flow (RFC 6749 Section 6).
 - On a `403 Forbidden` whose `WWW-Authenticate` header carries `error="insufficient_scope"` (OAuth 2.0 step-up, RFC 6750 Section 3.1 and the MCP scope-selection-strategy),
   run a fresh authorization request for the union of the currently granted scope and the scope named in the challenge, then retry the failed request once.

conformance/expected_failures.yml

@@ -5,7 +5,5 @@ client:
   # TODO: Elicitation not implemented in Ruby client.
   - elicitation-sep1034-client-defaults
   # TODO: Remaining OAuth/auth scenarios not yet implemented in Ruby client.
-  - auth/2025-03-26-oauth-metadata-backcompat
-  - auth/2025-03-26-oauth-endpoint-fallback
   - auth/client-credentials-jwt
   - auth/cross-app-access-complete-flow

lib/mcp/client/oauth/flow.rb

@@ -38,22 +38,18 @@ def run!(server_url:, resource_metadata_url: nil, scope: nil)
             ensure_secure_url!(resource_metadata_url, label: "WWW-Authenticate resource_metadata URL")
           end
 
-          prm = fetch_protected_resource_metadata(
+          prm, authorization_server = locate_authorization_server(
             server_url: server_url,
             resource_metadata_url: resource_metadata_url,
           )
-          authorization_server = first_authorization_server(prm)
-          ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
 
           # Per RFC 8707 + MCP authorization, the canonical MCP server URI is sent on
           # both the authorization and token requests. When PRM advertises a `resource`,
           # it MUST identify the same MCP server we are talking to; otherwise we are
           # being redirected to credentials minted for a different audience.
-          resource = canonical_resource(server_url: server_url, prm_resource: prm["resource"])
+          resource = canonical_resource(server_url: server_url, prm_resource: prm&.dig("resource"))
 
-          as_metadata = fetch_authorization_server_metadata(issuer_url: authorization_server)
-          ensure_issuer_matches!(expected: authorization_server, returned: as_metadata["issuer"])
-          ensure_secure_endpoints!(as_metadata)
+          as_metadata = authorization_server_metadata(authorization_server: authorization_server, legacy: prm.nil?)
 
           if provider_authorization_flow == :client_credentials
             return run_client_credentials!(as_metadata: as_metadata, prm: prm, resource: resource, scope: scope)
@@ -63,7 +59,7 @@ def run!(server_url:, resource_metadata_url: nil, scope: nil)
 
           client_info = ensure_client_registered(as_metadata: as_metadata)
 
-          effective_scope = resolve_scope(scope: scope, prm: prm)
+          effective_scope = resolve_scope(scope: scope, prm: prm || {})
           effective_scope = normalize_offline_access_scope(effective_scope, as_metadata: as_metadata)
           pkce = PKCE.generate
           state = SecureRandom.urlsafe_base64(32)
@@ -158,18 +154,14 @@ def refresh!(server_url:, resource_metadata_url: nil)
             ensure_secure_url!(resource_metadata_url, label: "WWW-Authenticate resource_metadata URL")
           end
 
-          prm = fetch_protected_resource_metadata(
+          prm, authorization_server = locate_authorization_server(
             server_url: server_url,
             resource_metadata_url: resource_metadata_url,
           )
-          authorization_server = first_authorization_server(prm)
-          ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
 
-          resource = canonical_resource(server_url: server_url, prm_resource: prm["resource"])
+          resource = canonical_resource(server_url: server_url, prm_resource: prm&.dig("resource"))
 
-          as_metadata = fetch_authorization_server_metadata(issuer_url: authorization_server)
-          ensure_issuer_matches!(expected: authorization_server, returned: as_metadata["issuer"])
-          ensure_secure_endpoints!(as_metadata)
+          as_metadata = authorization_server_metadata(authorization_server: authorization_server, legacy: prm.nil?)
 
           client_info = if have_stored_client_info
             # Pre-registered / DCR-issued `client_information` always wins: if the user picked an explicit identity,
@@ -221,6 +213,87 @@ def fetch_protected_resource_metadata(server_url:, resource_metadata_url:)
           fetch_metadata_json(urls, label: "protected resource metadata")
         end
 
+        # Locates the authorization server for `server_url` and returns `[prm, authorization_server]`.
+        #
+        # Modern path (2025-06-18+): Protected Resource Metadata names the authorization server in
+        # `authorization_servers`.
+        #
+        # Legacy path (2025-03-26 backwards compatibility): when the server publishes no PRM, `prm` is nil
+        # and the MCP server's own origin acts as the authorization base URL, matching the TypeScript and Python SDKs.
+        # Any PRM discovery failure (404s, network errors, malformed documents) selects the legacy path, mirroring both SDKs' behavior.
+        # https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization#fallbacks-for-servers-without-metadata-discovery
+        def locate_authorization_server(server_url:, resource_metadata_url:)
+          prm = begin
+            fetch_protected_resource_metadata(
+              server_url: server_url,
+              resource_metadata_url: resource_metadata_url,
+            )
+          rescue AuthorizationError
+            nil
+          end
+
+          if prm
+            authorization_server = first_authorization_server(prm)
+            ensure_secure_url!(authorization_server, label: "PRM `authorization_servers` entry")
+            [prm, authorization_server]
+          else
+            authorization_base = server_origin!(server_url)
+            ensure_secure_url!(authorization_base, label: "MCP server origin (legacy authorization base URL)")
+            [nil, authorization_base]
+          end
+        end
+
+        # Fetches and validates the authorization server's RFC 8414 metadata.
+        #
+        # On the modern path the metadata `issuer` must be byte-identical to the discovery URL (RFC 8414 Section 3.3).
+        # On the legacy 2025-03-26 path that validation is skipped: the legacy spec predates the requirement,
+        # and a pre-PRM server may host its OAuth endpoints under a path prefix whose `issuer` legitimately differs from
+        # the origin the metadata was discovered at (neither the TypeScript nor the Python SDK validates the issuer on this path).
+        # When even the metadata document is absent, the legacy spec's default endpoints are used.
+        def authorization_server_metadata(authorization_server:, legacy:)
+          metadata = if legacy
+            begin
+              fetch_authorization_server_metadata(issuer_url: authorization_server)
+            rescue AuthorizationError
+              default_legacy_metadata(authorization_server)
+            end
+          else
+            fetch_authorization_server_metadata(issuer_url: authorization_server).tap do |fetched|
+              ensure_issuer_matches!(expected: authorization_server, returned: fetched["issuer"])
+            end
+          end
+
+          ensure_secure_endpoints!(metadata)
+          metadata
+        end
+
+        # The 2025-03-26 spec's "Fallbacks for Servers without Metadata Discovery": clients MUST use these default endpoint paths
+        # relative to the authorization base URL. PKCE S256 is assumed because the legacy spec mandates PKCE and there is no metadata
+        # to advertise it (the TypeScript and Python SDKs hardcode S256 on this path too).
+        def default_legacy_metadata(authorization_base)
+          {
+            "issuer" => authorization_base,
+            "authorization_endpoint" => "#{authorization_base}/authorize",
+            "token_endpoint" => "#{authorization_base}/token",
+            "registration_endpoint" => "#{authorization_base}/register",
+            "code_challenge_methods_supported" => ["S256"],
+          }
+        end
+
+        # Returns `scheme://host[:port]` of `server_url`, the legacy 2025-03-26 authorization base URL for servers without PRM.
+        def server_origin!(server_url)
+          uri = URI.parse(server_url.to_s)
+          unless uri.is_a?(URI::HTTP) && uri.host
+            raise AuthorizationError,
+              "Cannot derive a legacy authorization base URL from MCP server URL #{server_url.inspect}."
+          end
+
+          port_part = uri.port == uri.default_port ? "" : ":#{uri.port}"
+          "#{uri.scheme}://#{uri.host}#{port_part}"
+        rescue URI::InvalidURIError => e
+          raise AuthorizationError, "MCP server URL #{server_url.inspect} is not a valid URI: #{e.message}."
+        end
+
         def fetch_authorization_server_metadata(issuer_url:)
           urls = Discovery.authorization_server_metadata_urls(issuer_url)
           fetch_metadata_json(urls, label: "authorization server metadata")

test/mcp/client/oauth/flow_test.rb

@@ -544,14 +544,20 @@ def test_run_raises_when_prm_resource_is_malformed_uri
           assert_match(/PRM `resource`|not a valid URI/i, error.message)
         end
 
-        def test_run_raises_when_prm_is_not_a_json_object
-          # Valid JSON but the wrong shape: indexing into a top-level array
-          # would otherwise raise `TypeError` and leak out of the SDK.
+        def test_run_falls_back_to_legacy_discovery_when_prm_is_not_a_json_object
+          # Valid JSON but the wrong shape. Any PRM discovery failure selects the legacy 2025-03-26 path
+          # (matching the TypeScript and Python SDKs); here the legacy path also dead-ends, surfacing
+          # a domain error rather than a raw `TypeError` from indexing the array.
           stub_request(:get, @prm_url).to_return(
             status: 200,
             headers: { "Content-Type" => "application/json" },
             body: "[]",
           )
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-protected-resource/mcp").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-protected-resource").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-authorization-server").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/openid-configuration").to_return(status: 404)
+          stub_request(:post, "https://srv.example.com/register").to_return(status: 404)
 
           provider = Provider.new(
             client_metadata: {
@@ -569,7 +575,147 @@ def test_run_raises_when_prm_is_not_a_json_object
             Flow.new(provider: provider).run!(server_url: @server_url, resource_metadata_url: @prm_url)
           end
 
-          assert_match(/not a JSON object/i, error.message)
+          assert_match(/Dynamic client registration failed/i, error.message)
+          assert_requested(:get, "https://srv.example.com/.well-known/oauth-authorization-server")
+        end
+
+        # Builds a provider for the legacy-discovery tests, capturing the authorization URL so tests can assert
+        # which endpoint was used.
+        def build_legacy_discovery_provider(holder)
+          Provider.new(
+            client_metadata: {
+              redirect_uris: ["http://localhost:0/callback"],
+              grant_types: ["authorization_code"],
+              response_types: ["code"],
+              token_endpoint_auth_method: "none",
+            },
+            redirect_uri: "http://localhost:0/callback",
+            redirect_handler: ->(url) {
+              holder[:authorization_url] = url
+              holder[:state] = URI.decode_www_form(url.query).to_h.fetch("state")
+            },
+            callback_handler: -> { ["test-auth-code", holder[:state]] },
+          )
+        end
+
+        def stub_prm_not_found
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-protected-resource/mcp").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-protected-resource").to_return(status: 404)
+        end
+
+        def test_run_falls_back_to_server_origin_metadata_without_prm
+          # Legacy 2025-03-26 shape: no PRM, AS metadata served from the MCP server origin,
+          # OAuth endpoints under a path prefix whose `issuer` differs from the discovery origin.
+          # The legacy path must not apply the RFC 8414 issuer byte-match (the legacy spec predates it).
+          stub_prm_not_found
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-authorization-server").to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(
+              issuer: "https://srv.example.com/oauth",
+              authorization_endpoint: "https://srv.example.com/oauth/authorize",
+              token_endpoint: "https://srv.example.com/oauth/token",
+              registration_endpoint: "https://srv.example.com/oauth/register",
+              response_types_supported: ["code"],
+              code_challenge_methods_supported: ["S256"],
+              token_endpoint_auth_methods_supported: ["none"],
+            ),
+          )
+          stub_request(:post, "https://srv.example.com/oauth/register").to_return(
+            status: 201,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(client_id: "legacy-client"),
+          )
+          stub_request(:post, "https://srv.example.com/oauth/token").to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(access_token: "legacy-token", token_type: "Bearer", expires_in: 3600),
+          )
+
+          holder = {}
+          provider = build_legacy_discovery_provider(holder)
+
+          result = Flow.new(provider: provider).run!(server_url: @server_url)
+
+          assert_equal(:authorized, result)
+          assert_equal("legacy-token", provider.access_token)
+          assert_equal("/oauth/authorize", holder[:authorization_url].path)
+          assert_requested(:post, "https://srv.example.com/oauth/register")
+          assert_requested(:post, "https://srv.example.com/oauth/token")
+        end
+
+        def test_run_falls_back_to_default_endpoints_without_any_metadata
+          # Legacy 2025-03-26 "Fallbacks for Servers without Metadata Discovery": with no PRM and no AS metadata,
+          # the client MUST use /authorize, /token, and /register at the authorization base URL, still sending PKCE S256.
+          stub_prm_not_found
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-authorization-server").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/openid-configuration").to_return(status: 404)
+          stub_request(:post, "https://srv.example.com/register").to_return(
+            status: 201,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(client_id: "legacy-client"),
+          )
+          stub_request(:post, "https://srv.example.com/token").to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(access_token: "legacy-token", token_type: "Bearer", expires_in: 3600),
+          )
+
+          holder = {}
+          provider = build_legacy_discovery_provider(holder)
+
+          result = Flow.new(provider: provider).run!(server_url: @server_url)
+
+          assert_equal(:authorized, result)
+          assert_equal("/authorize", holder[:authorization_url].path)
+          query = URI.decode_www_form(holder[:authorization_url].query).to_h
+          assert_equal("S256", query["code_challenge_method"])
+          refute_empty(query["code_challenge"].to_s)
+          assert_requested(:post, "https://srv.example.com/register")
+          assert_requested(:post, "https://srv.example.com/token") do |req|
+            URI.decode_www_form(req.body).to_h["code_verifier"].to_s != ""
+          end
+        end
+
+        def test_run_legacy_fallback_rejects_insecure_authorization_base
+          # The Communication Security requirement still applies on the legacy path: a remote plain-http origin must not
+          # become the authorization base URL.
+          stub_request(:get, "http://internal.example.com/.well-known/oauth-protected-resource/mcp").to_return(status: 404)
+          stub_request(:get, "http://internal.example.com/.well-known/oauth-protected-resource").to_return(status: 404)
+
+          holder = {}
+          provider = build_legacy_discovery_provider(holder)
+
+          error = assert_raises(Flow::AuthorizationError) do
+            Flow.new(provider: provider).run!(server_url: "http://internal.example.com/mcp")
+          end
+
+          assert_match(/legacy authorization base URL/, error.message)
+        end
+
+        def test_run_keeps_strict_issuer_validation_when_prm_is_present
+          # The legacy issuer-check relaxation must not leak into the modern path: with PRM present,
+          # a mismatched issuer still aborts.
+          stub_request(:get, @as_metadata_url).to_return(
+            status: 200,
+            headers: { "Content-Type" => "application/json" },
+            body: JSON.generate(
+              issuer: "https://evil.example.com",
+              authorization_endpoint: "#{@auth_base}/authorize",
+              token_endpoint: "#{@auth_base}/token",
+              registration_endpoint: "#{@auth_base}/register",
+              code_challenge_methods_supported: ["S256"],
+            ),
+          )
+
+          holder = {}
+          provider = build_legacy_discovery_provider(holder)
+
+          error = assert_raises(Flow::AuthorizationError) do
+            Flow.new(provider: provider).run!(server_url: @server_url, resource_metadata_url: @prm_url)
+          end
+
+          assert_match(/`issuer` does not match/, error.message)
         end
 
         def test_run_raises_when_prm_authorization_servers_is_not_an_array

test/mcp/client/oauth/http_oauth_test.rb

@@ -1304,6 +1304,11 @@ def test_send_request_does_not_loop_when_oauth_flow_fails
             body: "",
           )
 
+          # With no PRM, discovery falls back to the legacy 2025-03-26 path; dead-end that too so the flow fails exactly once.
+          stub_request(:get, "https://srv.example.com/.well-known/oauth-authorization-server").to_return(status: 404)
+          stub_request(:get, "https://srv.example.com/.well-known/openid-configuration").to_return(status: 404)
+          stub_request(:post, "https://srv.example.com/register").to_return(status: 404)
+
           provider = Provider.new(
             client_metadata: { redirect_uris: ["http://localhost:0/callback"] },
             redirect_uri: "http://localhost:0/callback",