fix: validate JWT algorithms against verification keys

Author: atesgoral

docs/installation.md

@@ -45,6 +45,8 @@ See [Authorization](/server/authorization/).
 gem "jwt"
 ```
 
+jwt 3.3.0 or later is recommended for its upstream key-type fix. Older versions remain supported.
+
 The HTTP client transport requires the `faraday` gem:
 
 ```ruby

lib/mcp/server/oauth/jwt_verifier.rb

@@ -39,6 +39,15 @@ class JWKSFetchError < StandardError; end
 
         DEFAULT_ALGORITHMS = ["RS256", "RS384", "RS512", "PS256", "PS384", "PS512", "ES256", "ES384", "ES512", "EdDSA"].freeze
 
+        EC_CURVE_ALGORITHMS = {
+          "prime256v1" => "ES256",
+          "secp256r1" => "ES256",
+          "secp384r1" => "ES384",
+          "secp521r1" => "ES512",
+          "secp256k1" => "ES256K",
+        }.freeze
+        private_constant :EC_CURVE_ALGORITHMS
+
         HMAC_ALGORITHM_PATTERN = /\AHS\d+\z/i.freeze
         private_constant :HMAC_ALGORITHM_PATTERN
 
@@ -126,12 +135,32 @@ def decode(token)
             leeway: @leeway,
           }
 
-          if @key
-            JWT.decode(token, @key, true, options)
-          elsif @static_jwks
-            JWT.decode(token, nil, true, options.merge(jwks: @static_jwks))
+          JWT.decode(token, nil, true, options) do |header|
+            raise InvalidTokenError, "Invalid token" unless @key || header["kid"]
+
+            key = @key || JWT::JWK::KeyFinder.new(jwks: @static_jwks || jwk_loader).key_for(header["kid"])
+            keys = Array(key).select { |candidate| algorithm_matches_key?(header["alg"], candidate) }
+            raise InvalidTokenError, "Invalid token" if keys.empty?
+
+            keys
+          end
+        end
+
+        def algorithm_matches_key?(algorithm, key)
+          case key
+          when OpenSSL::PKey::RSA
+            algorithm.match?(/\A(?:RS|PS)(?:256|384|512)\z/i)
+          when OpenSSL::PKey::EC, OpenSSL::PKey::EC::Point
+            curve_algorithm = EC_CURVE_ALGORITHMS[key.group.curve_name]
+            curve_algorithm && curve_algorithm.casecmp?(algorithm)
+          when String
+            algorithm.match?(/\AHS(?:256|384|512)\z/i)
+          when JWT::JWK::KeyBase
+            algorithm_matches_key?(algorithm, key.verify_key)
           else
-            JWT.decode(token, nil, true, options.merge(jwks: jwk_loader))
+            ed25519_key = (defined?(Ed25519::VerifyKey) && key.is_a?(Ed25519::VerifyKey)) ||
+              (defined?(RbNaCl::Signatures::Ed25519::VerifyKey) && key.is_a?(RbNaCl::Signatures::Ed25519::VerifyKey))
+            ed25519_key && ["EdDSA", "Ed25519"].any? { |name| name.casecmp?(algorithm) }
           end
         end
 

test/mcp/server/oauth/jwt_verifier_key_compatibility_test.rb

@@ -0,0 +1,208 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "jwt"
+require "openssl"
+require "base64"
+require "open3"
+require "webmock/minitest"
+
+module MCP
+  class Server
+    module OAuth
+      class JWTVerifierKeyCompatibilityTest < Minitest::Test
+        ISSUER = "https://as.example.com"
+        AUDIENCE = "https://mcp.example.com/mcp"
+        JWKS_URI = "https://as.example.com/.well-known/jwks.json"
+        METADATA = ProtectedResourceMetadata.new(resource: AUDIENCE, authorization_servers: [ISSUER])
+
+        def setup
+          @claims = { "iss" => ISSUER, "aud" => AUDIENCE, "exp" => Time.now.to_i + 300 }
+        end
+
+        def test_valid_rsa_algorithms_are_accepted
+          key = OpenSSL::PKey::RSA.new(2048)
+          ["RS256", "RS384", "RS512", "PS256", "PS384", "PS512"].each do |algorithm|
+            assert_valid_pair(algorithm, key)
+          end
+        end
+
+        def test_valid_ec_algorithms_and_curves_are_accepted
+          { "ES256" => "prime256v1", "ES384" => "secp384r1", "ES512" => "secp521r1" }.each do |algorithm, curve|
+            assert_valid_pair(algorithm, OpenSSL::PKey::EC.generate(curve))
+          end
+        end
+
+        def test_ec_curve_algorithm_matches_and_mismatches
+          { "prime256v1" => "ES256", "secp256r1" => "ES256", "secp384r1" => "ES384", "secp521r1" => "ES512" }.each do |curve, algorithm|
+            assert_curve_algorithm(curve, algorithm)
+          end
+        end
+
+        def test_valid_es256k_pair_is_accepted
+          skip "secp256k1 is not available" unless OpenSSL::PKey::EC.builtin_curves.any? { |name, _comment| name == "secp256k1" }
+
+          assert_curve_algorithm("secp256k1", "ES256K")
+          assert_valid_pair("ES256K", OpenSSL::PKey::EC.generate("secp256k1"))
+        end
+
+        def test_valid_hmac_algorithms_are_accepted
+          ["HS256", "HS384", "HS512"].each do |algorithm|
+            assert_valid_pair(algorithm, "shared-key" * 8)
+          end
+        end
+
+        def test_valid_eddsa_key_is_accepted
+          require_eddsa
+          assert_valid_pair("EdDSA", Ed25519::SigningKey.generate)
+        end
+
+        def test_builtin_eddsa_key_is_accepted_without_the_plugin
+          script = <<~'RUBY'
+            gem "jwt", "= #{ARGV.fetch(0)}"
+            require "jwt"
+            exit 77 unless defined?(RbNaCl::Signatures::Ed25519::SigningKey)
+
+            require "mcp"
+            key = RbNaCl::Signatures::Ed25519::SigningKey.generate
+            claims = { iss: "https://as.example.com", aud: "https://mcp.example.com/mcp", exp: Time.now.to_i + 300 }
+            token = JWT.encode(claims, key, "EdDSA", { kid: "built-in" })
+            jwk = JWT::JWK.new(key.verify_key, { kid: "built-in" }).export
+            metadata = MCP::Server::OAuth::ProtectedResourceMetadata.new(
+              resource: "https://mcp.example.com/mcp",
+              authorization_servers: ["https://as.example.com"],
+            )
+            [{ key: key.verify_key }, { jwks: { keys: [jwk] } }].each do |source|
+              verifier = MCP::Server::OAuth::JWTVerifier.new(resource_metadata: metadata, **source)
+              raise "Valid built-in EdDSA token was not returned" unless verifier.verify(token).token == token
+            end
+            puts "accepted"
+          RUBY
+          env = { "RUBYOPT" => nil, "BUNDLER_SETUP" => nil, "GEM_PATH" => Gem.path.join(File::PATH_SEPARATOR) }
+          lib = File.expand_path("../../../../lib", __dir__)
+          version = Gem.loaded_specs.fetch("jwt").version.to_s
+          stdout, stderr, status = Open3.capture3(env, RbConfig.ruby, "-I", lib, "-e", script, version)
+          skip "Built-in RbNaCl EdDSA is not available" if status.exitstatus == 77
+
+          assert_predicate(status, :success?, stderr)
+          assert_equal("accepted\n", stdout)
+        end
+
+        def test_valid_rsa_key_in_an_array_is_accepted
+          first = OpenSSL::PKey::RSA.new(2048)
+          second = OpenSSL::PKey::RSA.new(2048)
+          token = JWT.encode(@claims, second, "RS256")
+          verifier = JWTVerifier.new(resource_metadata: METADATA, key: [first.public_key, second.public_key])
+
+          assert_equal(token, verifier.verify(token).token)
+        end
+
+        def test_es256_with_an_rsa_key_returns_401
+          assert_algorithm_mismatch("ES256", OpenSSL::PKey::RSA.new(2048))
+        end
+
+        def test_rs256_with_an_ec_key_returns_401
+          assert_algorithm_mismatch("RS256", OpenSSL::PKey::EC.generate("prime256v1"))
+        end
+
+        def test_eddsa_with_an_rsa_key_returns_401
+          require_eddsa
+          assert_algorithm_mismatch("EdDSA", OpenSSL::PKey::RSA.new(2048))
+        end
+
+        def test_ec_curve_mismatches_return_401
+          { "ES256" => "secp384r1", "ES384" => "prime256v1", "ES512" => "prime256v1" }.each do |algorithm, curve|
+            assert_algorithm_mismatch(algorithm, OpenSSL::PKey::EC.generate(curve))
+          end
+        end
+
+        def test_hs256_with_an_rsa_key_returns_401
+          assert_algorithm_mismatch("HS256", OpenSSL::PKey::RSA.new(2048))
+        end
+
+        def test_rs256_with_an_oct_jwks_returns_401
+          assert_algorithm_mismatch("RS256", "shared-key" * 8, direct: false)
+        end
+
+        def test_es256_with_an_oct_jwks_returns_401
+          assert_algorithm_mismatch("ES256", "shared-key" * 8, direct: false)
+        end
+
+        def test_disallowed_algorithm_does_not_fetch_a_key
+          token = JWT.encode(@claims, OpenSSL::PKey::RSA.new(2048), "RS512", { kid: "compat-key" })
+          verifier = JWTVerifier.new(resource_metadata: METADATA, jwks_uri: JWKS_URI, algorithms: ["RS256"])
+
+          assert_raises(InvalidTokenError) { verifier.verify(token) }
+          assert_not_requested(:get, JWKS_URI)
+        end
+
+        private
+
+        def assert_curve_algorithm(curve, algorithm)
+          key = OpenSSL::PKey::EC.generate(curve == "secp256r1" ? "prime256v1" : curve)
+          if curve == "secp256r1"
+            group = key.group
+            group.stubs(:curve_name).returns(curve)
+            key.stubs(:group).returns(group)
+          end
+          verifier = JWTVerifier.new(resource_metadata: METADATA, key: key, algorithms: [algorithm])
+
+          assert(verifier.send(:algorithm_matches_key?, algorithm, key), curve)
+          (["ES256", "ES384", "ES512", "ES256K"] - [algorithm]).each do |other|
+            refute(verifier.send(:algorithm_matches_key?, other, key), "#{curve}: #{other}")
+          end
+        end
+
+        def assert_valid_pair(algorithm, signing_key)
+          jwk = JWT::JWK.new(signing_key, { kid: "compat-key" }).export(include_private: signing_key.is_a?(String))
+          key = JWT::JWK.new(jwk).verify_key
+          token = JWT.encode(@claims, signing_key, algorithm, { kid: "compat-key", typ: "at+jwt" })
+
+          key_sources(key, jwk).each do |source|
+            verifier = JWTVerifier.new(resource_metadata: METADATA, algorithms: [algorithm], **source)
+
+            assert_equal(token, verifier.verify(token).token, "#{algorithm}: #{source.keys}")
+          end
+        end
+
+        def assert_algorithm_mismatch(algorithm, key, direct: true)
+          jwk = JWT::JWK.new(key, { kid: "compat-key" }).export(include_private: key.is_a?(String))
+          header = Base64.urlsafe_encode64(JSON.generate(alg: algorithm, kid: "compat-key", typ: "at+jwt"), padding: false)
+          payload = Base64.urlsafe_encode64(JSON.generate(@claims), padding: false)
+          signature = Base64.urlsafe_encode64("\0" * 64, padding: false)
+          token = "#{header}.#{payload}.#{signature}"
+          sources = key_sources(key, jwk)
+          sources.shift unless direct
+
+          sources.each do |source|
+            verifier = JWTVerifier.new(resource_metadata: METADATA, algorithms: [algorithm], **source)
+            app = mock
+            app.expects(:call).never
+            middleware = Middleware.new(app, token_verifier: verifier, resource_metadata: METADATA)
+            reports = []
+            MCP.configuration.stubs(:exception_reporter).returns(->(error, _context) { reports << error })
+
+            status, headers, body = middleware.call({ "HTTP_AUTHORIZATION" => "Bearer #{token}" })
+
+            assert_equal(401, status, "#{algorithm}: #{source.keys}")
+            assert_includes(headers.fetch("www-authenticate"), 'error="invalid_token"')
+            assert_equal("invalid_token", JSON.parse(body.join).fetch("error"))
+            assert_empty(reports)
+          end
+        end
+
+        def key_sources(key, jwk)
+          jwks = { keys: [jwk] }
+          stub_request(:get, JWKS_URI).to_return(body: JSON.generate(jwks), headers: { "content-type" => "application/json" })
+          [{ key: key }, { jwks: jwks }, { jwks_uri: JWKS_URI }]
+        end
+
+        def require_eddsa
+          require "jwt/eddsa"
+        rescue LoadError
+          skip "jwt-eddsa is not available"
+        end
+      end
+    end
+  end
+end

test/mcp/server/oauth/jwt_verifier_middleware_test.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "jwt"
+require "openssl"
+require "base64"
+require "webmock/minitest"
+
+module MCP
+  class Server
+    module OAuth
+      class JWTVerifierMiddlewareTest < Minitest::Test
+        ISSUER = "https://as.example.com"
+        AUDIENCE = "https://mcp.example.com/mcp"
+        JWKS_URI = "https://as.example.com/.well-known/jwks.json"
+        METADATA = ProtectedResourceMetadata.new(resource: AUDIENCE, authorization_servers: [ISSUER])
+
+        def setup
+          @key = OpenSSL::PKey::EC.generate("prime256v1")
+          @jwks = { keys: [JWT::JWK.new(@key, { kid: "ec-key", use: "sig", alg: "ES256" }).export] }
+          @public_key = JWT::JWK.new(@jwks[:keys].first).verify_key
+          @claims = { "iss" => ISSUER, "aud" => AUDIENCE, "exp" => Time.now.to_i + 300 }
+        end
+
+        def test_ps256_token_with_an_ec_key_returns_401
+          assert_ps256_rejected(key: @public_key)
+        end
+
+        def test_ps256_token_with_a_static_ec_jwks_returns_401
+          assert_ps256_rejected(jwks: @jwks)
+        end
+
+        def test_ps256_token_with_a_fetched_ec_jwks_returns_401
+          stub_request(:get, JWKS_URI).to_return(body: JSON.generate(@jwks), headers: { "content-type" => "application/json" })
+
+          assert_ps256_rejected(jwks_uri: JWKS_URI)
+          assert_requested(:get, JWKS_URI, times: 1)
+        end
+
+        private
+
+        def assert_ps256_rejected(**key_source)
+          verifier = JWTVerifier.new(resource_metadata: METADATA, **key_source)
+          app = mock
+          app.expects(:call).once.returns([200, {}, ["authenticated"]])
+          middleware = Middleware.new(app, token_verifier: verifier, resource_metadata: METADATA)
+          reports = []
+          MCP.configuration.stubs(:exception_reporter).returns(->(error, _context) { reports << error })
+          valid_token = JWT.encode(@claims, @key, "ES256", { kid: "ec-key" })
+
+          status, = middleware.call({ "HTTP_AUTHORIZATION" => "Bearer #{valid_token}" })
+
+          assert_equal(200, status)
+
+          header = Base64.urlsafe_encode64(JSON.generate(alg: "PS256", kid: "ec-key"), padding: false)
+          payload = Base64.urlsafe_encode64(JSON.generate(@claims), padding: false)
+          token = "#{header}.#{payload}.AAAA"
+
+          status, headers, body = middleware.call({ "HTTP_AUTHORIZATION" => "Bearer #{token}" })
+
+          assert_equal(401, status)
+          assert_includes(headers.fetch("www-authenticate"), 'error="invalid_token"')
+          assert_equal("invalid_token", JSON.parse(body.join).fetch("error"))
+          assert_empty(reports)
+        end
+      end
+    end
+  end
+end