Validate Host and Origin Headers to Prevent DNS Rebinding per MCP 2025-11-25

## Motivation and Context

MCP 2025-11-25 (modelcontextprotocol/modelcontextprotocol#1439) requires servers to validate
the `Origin` header on incoming Streamable HTTP connections and recommends binding only to localhost,
to prevent DNS rebinding attacks against locally bound MCP servers.
`StreamableHTTPTransport` previously validated neither `Host` nor `Origin`,
so a malicious web page could rebind its own hostname to 127.0.0.1 and reach a local server cross-origin,
invoking tools and exfiltrating their output. This is disclosed as GHSA-rjr6-rcgv-9m7m.

The TypeScript SDK (`enableDnsRebindingProtection` defaults to `false`, and is deprecated in favor of
external middleware) and the Python SDK (`transport_security.py` disables protection by default)
both ship this off for backwards compatibility, a constraint they cannot escape after 1.0.
Because this gem is still pre-1.0, it adopts the secure default now so protection is baked into the stable baseline:

- `Host` is validated against the loopback defaults (`127.0.0.1`, `::1`, `localhost`) plus any `allowed_hosts:`,
  compared case-insensitively; each entry matches the bare host name (any port) or a full `host:port`.
  This check is the primary anti-rebinding control, because a rebound request carries a foreign `Host`.
  A request without a `Host` header (e.g. HTTP/1.0) is allowed because the rebinding vector always sends one.
- `Origin`, when present, must be same-origin or in `allowed_origins:`. Same-origin compares authority (host:port)
  case-insensitively and uses the `Origin`'s own scheme to drop a redundant default port, so `http://example.com` matches
  `Host: example.com:80`. Non-browser MCP clients that send no `Origin` header are unaffected.
- `dns_rebinding_protection: false` disables the check when an upstream proxy or middleware already validates these headers.

A rejected request returns HTTP 403 with a JSON-RPC error response carrying no `id`, as the spec permits.

## How Has This Been Tested?

New tests in `test/mcp/server/transports/streamable_http_transport_test.rb`:

- a cross-origin `Origin` is rejected by default, and a same-origin browser request is allowed
- a rebound foreign `Host` is rejected with the `Invalid Host header` body, while loopback values
  (`127.0.0.1`, `localhost`, `[::1]`) are allowed
- an upper-cased loopback `Host` is accepted (case-insensitive matching)
- a non-loopback `Host` listed in `allowed_hosts:` is allowed
- a request with no `Host` header, and one with no `Origin` header, are allowed
- a default-port `Origin` (`http://localhost`) matches `Host: localhost:80`
- opaque and malformed `Origin`s (`null`, `file://`, a path suffix) are rejected
- a `host:port` `allowed_hosts:` entry matches that port only
- `Host` validation also applies to GET requests
- `dns_rebinding_protection: false` lets a forged `Host`/`Origin` through

## Breaking Changes

`StreamableHTTPTransport` now rejects non-loopback `Host` and cross-origin `Origin` requests by default.
A deployment behind a reverse proxy or bound to a non-loopback interface must set `allowed_hosts:` / `allowed_origins:`,
or pass `dns_rebinding_protection: false` when validation is handled upstream. Loopback servers and non-browser clients
are unaffected.

Co-authored-by: tonghuaroot <tonghuaroot@gmail.com>

README.md

@@ -128,6 +128,27 @@ The following examples show two common integration styles in Rails.
 >
 > Stateless mode (`stateless: true`) does not use sessions and works with any server configuration.
 
+> [!IMPORTANT]
+> Per MCP 2025-11-25, `StreamableHTTPTransport` validates the `Host` and `Origin` headers by default to
+> prevent DNS rebinding attacks against locally bound servers, rejecting unauthorized values with HTTP 403.
+> `Host` is allowed for the loopback defaults (`127.0.0.1`, `::1`, `localhost`), and an `Origin` header,
+> when present, must be same-origin or explicitly allow-listed. Non-browser clients that send no `Origin`
+> header are unaffected.
+>
+> Deployments behind a reverse proxy or bound to a non-loopback interface must widen the allow lists:
+>
+> ```ruby
+> transport = MCP::Server::Transports::StreamableHTTPTransport.new(
+>   server,
+>   allowed_hosts: ["mcp.example.com"],
+>   allowed_origins: ["https://app.example.com"],
+> )
+> ```
+>
+> An `allowed_hosts:` entry matches either the bare host name (any port) or the full `host:port` value,
+> so both `"mcp.example.com"` and `"mcp.example.com:8443"` work. Pass `dns_rebinding_protection: false`
+> to disable the check entirely (e.g., when an upstream proxy or middleware already validates `Host`/`Origin`).
+
 ##### Rails (mount)
 
 `StreamableHTTPTransport` is a Rack app that can be mounted directly in Rails routes:

lib/mcp/server/transports/streamable_http_transport.rb

@@ -24,7 +24,31 @@ module MCP
           "Connection" => "keep-alive",
         }.freeze
 
-        def initialize(server, stateless: false, enable_json_response: false, session_idle_timeout: nil)
+        # Creates a Streamable HTTP transport that can be mounted as a Rack app.
+        #
+        # @param server [MCP::Server] the server whose requests this transport dispatches.
+        # @param stateless [Boolean] when `true`, no session is issued and each POST is self-contained.
+        # @param enable_json_response [Boolean] when `true`, a request is answered with a single JSON
+        #   object instead of an SSE stream.
+        # @param session_idle_timeout [Numeric, nil] seconds before an idle session is reaped; `nil`
+        #   disables expiry. Not supported in stateless mode.
+        # @param allowed_origins [Array<String>, nil] extra `Origin` values accepted in addition to
+        #   same-origin requests, for DNS rebinding protection.
+        # @param allowed_hosts [Array<String>, nil] extra `Host` values accepted beyond the loopback
+        #   defaults (`127.0.0.1`, `::1`, `localhost`); each entry matches a bare host name (any port)
+        #   or a full `host:port`.
+        # @param dns_rebinding_protection [Boolean] when `true` (default), validates the `Host` and
+        #   `Origin` headers to prevent DNS rebinding; pass `false` when an upstream proxy already
+        #   validates them.
+        def initialize(
+          server,
+          stateless: false,
+          enable_json_response: false,
+          session_idle_timeout: nil,
+          allowed_origins: nil,
+          allowed_hosts: nil,
+          dns_rebinding_protection: true
+        )
           super(server)
           # Maps `session_id` to `{ get_sse_stream: stream_object, server_session: ServerSession, last_active_at: float_from_monotonic_clock }`.
           @sessions = {}
@@ -33,6 +57,11 @@ module MCP
           @stateless = stateless
           @enable_json_response = enable_json_response
           @session_idle_timeout = session_idle_timeout
+          @dns_rebinding_protection = dns_rebinding_protection
+
+          # Host names are case-insensitive, so the allow lists are compared down-cased.
+          @allowed_hosts = (DEFAULT_LOOPBACK_HOSTS + Array(allowed_hosts)).map(&:downcase).freeze
+          @allowed_origins = Array(allowed_origins).map(&:downcase).freeze
           @pending_responses = {}
 
           if @session_idle_timeout
@@ -52,12 +81,19 @@ module MCP
         STREAM_WRITE_ERRORS = [IOError, Errno::EPIPE, Errno::ECONNRESET].freeze
         SESSION_REAP_INTERVAL = 60
 
+        # Loopback hosts always accepted by DNS rebinding protection. A locally bound MCP server (the canonical pattern) is
+        # protected out of the box; non-loopback deployments widen the list via `allowed_hosts:`.
+        DEFAULT_LOOPBACK_HOSTS = ["127.0.0.1", "::1", "localhost"].freeze
+
         # Rack app interface. This transport can be mounted as a Rack app.
         def call(env)
           handle_request(Rack::Request.new(env))
         end
 
         def handle_request(request)
+          rebinding_error = validate_dns_rebinding(request)
+          return rebinding_error if rebinding_error
+
           case request.env["REQUEST_METHOD"]
           when "POST"
             handle_post(request)
@@ -808,6 +844,74 @@ module MCP
           active
         end
 
+        # Per MCP 2025-11-25, servers MUST validate the `Origin` header and SHOULD bind only to localhost
+        # to prevent DNS rebinding attacks against locally bound MCP servers. Protection is on by default;
+        # pass `dns_rebinding_protection: false` to disable it (e.g. when an upstream proxy or middleware already
+        # performs the check). The `Host` header is validated against the loopback defaults plus `allowed_hosts:`,
+        # and the `Origin` header, when present, must be same-origin or in `allowed_origins:`.
+        def validate_dns_rebinding(request)
+          return unless @dns_rebinding_protection
+
+          validate_host(request) || validate_origin(request)
+        end
+
+        # Rejects a rebound `Host` (e.g. `evil.example.com` re-pointed at 127.0.0.1).
+        # A request without a `Host` header (e.g. HTTP/1.0) is allowed; the rebinding vector this guards against always carries one.
+        def validate_host(request)
+          host = request.env["HTTP_HOST"]
+          return if host.nil?
+
+          # An `allowed_hosts:` entry matches either the bare host name (any port)
+          # or the full `host:port` value, so both `"app.example.com"` and
+          # `"app.example.com:8443"` can be configured.
+          normalized = host.downcase
+          return if @allowed_hosts.include?(request_hostname(normalized)) || @allowed_hosts.include?(normalized)
+
+          forbidden_response("Forbidden: Invalid Host header")
+        end
+
+        # A request without an `Origin` header (typical for non-browser MCP clients) is allowed. A browser cross-origin request is
+        # rejected unless the origin is same-origin or explicitly allow-listed via `allowed_origins:`.
+        def validate_origin(request)
+          origin = request.env["HTTP_ORIGIN"]
+          return if origin.nil?
+          return if same_origin?(origin, request)
+          return if @allowed_origins.include?(origin.downcase)
+
+          forbidden_response("Forbidden: Invalid Origin header")
+        end
+
+        # Extracts the host name from a `Host` header value, stripping any port and IPv6 brackets
+        # (`[::1]:8080` becomes `::1`, `127.0.0.1:8080` becomes `127.0.0.1`).
+        def request_hostname(host)
+          return host[/\A\[([^\]]+)\]/, 1] if host.start_with?("[")
+
+          host.split(":").first
+        end
+
+        # Compares the `Origin` authority (host:port) against the request's own `Host`.
+        # Scheme is not compared (the `Host` header carries none, and `request.scheme` is unreliable behind proxies),
+        # but the `Origin`'s scheme is used to drop a redundant default port (`:80` for http, `:443` for https) from
+        # both sides so `http://example.com` matches `Host: example.com:80`. Comparison is case-insensitive.
+        def same_origin?(origin, request)
+          host = request.env["HTTP_HOST"]
+          return false if host.nil?
+
+          normalized = origin.downcase
+          default_port = normalized.start_with?("https://") ? ":443" : ":80"
+          authority = normalized.sub(%r{\Ahttps?://}, "")
+
+          authority.delete_suffix(default_port) == host.downcase.delete_suffix(default_port)
+        end
+
+        def forbidden_response(message)
+          json_rpc_error_response(
+            status: 403,
+            code: JsonRpcHandler::ErrorCode::INVALID_REQUEST,
+            message: message,
+          )
+        end
+
         def method_not_allowed_response
           json_rpc_error_response(
             status: 405,

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -4328,6 +4328,306 @@ module MCP
           refute headers.frozen?, "SSE response headers should not be frozen"
         end
 
+        test "handle_request returns 403 for POST when Origin is not in allowed_origins" do
+          transport = StreamableHTTPTransport.new(@server, allowed_origins: ["https://app.example.com"])
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_ORIGIN" => "https://evil.example.com",
+            },
+            { jsonrpc: "2.0", method: "ping", id: "1" }.to_json,
+          )
+
+          status, headers, body = transport.handle_request(request)
+          assert_equal(403, status)
+          assert_equal("application/json", headers["Content-Type"])
+          parsed = JSON.parse(body.first)
+          assert_equal(JsonRpcHandler::ErrorCode::INVALID_REQUEST, parsed["error"]["code"])
+          assert_equal("Forbidden: Invalid Origin header", parsed["error"]["message"])
+        ensure
+          transport&.close
+        end
+
+        test "handle_request returns 403 for GET when Origin is not in allowed_origins" do
+          transport = StreamableHTTPTransport.new(@server, allowed_origins: ["https://app.example.com"])
+
+          request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_ORIGIN" => "https://evil.example.com" },
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(403, status)
+        ensure
+          transport&.close
+        end
+
+        test "handle_request returns 403 for DELETE when Origin is not in allowed_origins" do
+          transport = StreamableHTTPTransport.new(@server, allowed_origins: ["https://app.example.com"])
+
+          request = create_rack_request(
+            "DELETE",
+            "/",
+            {
+              "HTTP_ORIGIN" => "https://evil.example.com",
+              "HTTP_MCP_SESSION_ID" => "any",
+            },
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(403, status)
+        ensure
+          transport&.close
+        end
+
+        test "handle_request allows POST when Origin is in allowed_origins" do
+          transport = StreamableHTTPTransport.new(@server, allowed_origins: ["https://app.example.com"])
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_ORIGIN" => "https://app.example.com",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(200, status)
+        ensure
+          transport&.close
+        end
+
+        test "handle_request allows POST when Origin header is absent (e.g. non-browser client)" do
+          transport = StreamableHTTPTransport.new(@server, allowed_origins: ["https://app.example.com"])
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(200, status)
+        ensure
+          transport&.close
+        end
+
+        test "handle_request rejects a cross-origin request by default (secure-by-default)" do
+          # No allowed_origins configured: a browser cross-origin Origin is still rejected.
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_ORIGIN" => "https://anywhere.example.com",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = @transport.handle_request(request)
+          assert_equal 403, status
+        end
+
+        test "handle_request allows a same-origin browser request" do
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_HOST" => "localhost:3000",
+              "HTTP_ORIGIN" => "http://localhost:3000",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = @transport.handle_request(request)
+          assert_equal 200, status
+        end
+
+        test "handle_request rejects a rebound Host by default" do
+          # DNS rebinding: a foreign Host re-pointed at the loopback server is rejected.
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_HOST" => "evil.example.com",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, headers, body = @transport.handle_request(request)
+          assert_equal(403, status)
+          assert_equal("application/json", headers["Content-Type"])
+          parsed = JSON.parse(body.first)
+          assert_equal(JsonRpcHandler::ErrorCode::INVALID_REQUEST, parsed["error"]["code"])
+          assert_equal("Forbidden: Invalid Host header", parsed["error"]["message"])
+        end
+
+        test "handle_request allows loopback Host values by default" do
+          ["127.0.0.1:8080", "localhost:3000", "[::1]:8080"].each do |host|
+            transport = StreamableHTTPTransport.new(@server)
+            request = create_rack_request(
+              "POST",
+              "/",
+              {
+                "CONTENT_TYPE" => "application/json",
+                "HTTP_HOST" => host,
+              },
+              { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+            )
+
+            status, = transport.handle_request(request)
+            assert_equal(200, status, "expected #{host} to be allowed")
+          ensure
+            transport.close
+          end
+        end
+
+        test "handle_request matches Host case-insensitively" do
+          # Host names are case-insensitive, so an upper-cased loopback Host is allowed.
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_HOST" => "LOCALHOST:3000",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = @transport.handle_request(request)
+          assert_equal(200, status)
+        end
+
+        test "handle_request allows a non-loopback Host listed in allowed_hosts" do
+          transport = StreamableHTTPTransport.new(@server, allowed_hosts: ["app.example.com"])
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_HOST" => "app.example.com",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(200, status)
+        ensure
+          transport.close
+        end
+
+        test "handle_request matches a host:port entry in allowed_hosts exactly" do
+          transport = StreamableHTTPTransport.new(@server, allowed_hosts: ["app.example.com:8443"])
+
+          allowed = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_HOST" => "app.example.com:8443" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          rejected = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_HOST" => "app.example.com:9000" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          assert_equal(200, transport.handle_request(allowed).first)
+          assert_equal(403, transport.handle_request(rejected).first)
+        ensure
+          transport.close
+        end
+
+        test "handle_request treats a default port as same-origin" do
+          # `Origin: http://localhost` (no port) is same-origin with `Host: localhost:80`.
+          [
+            ["http://localhost", "localhost:80"],
+            ["https://localhost", "localhost:443"],
+          ].each do |origin, host|
+            transport = StreamableHTTPTransport.new(@server)
+            request = create_rack_request(
+              "POST",
+              "/",
+              { "CONTENT_TYPE" => "application/json", "HTTP_HOST" => host, "HTTP_ORIGIN" => origin },
+              { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+            )
+
+            assert_equal(200, transport.handle_request(request).first, "expected #{origin} / #{host} to match")
+          ensure
+            transport.close
+          end
+        end
+
+        test "handle_request rejects opaque and malformed Origins" do
+          ["null", "file:///etc/passwd", "http://localhost:3000/path"].each do |origin|
+            transport = StreamableHTTPTransport.new(@server)
+            request = create_rack_request(
+              "POST",
+              "/",
+              { "CONTENT_TYPE" => "application/json", "HTTP_HOST" => "localhost:3000", "HTTP_ORIGIN" => origin },
+              { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+            )
+
+            assert_equal(403, transport.handle_request(request).first, "expected #{origin} to be rejected")
+          ensure
+            transport.close
+          end
+        end
+
+        test "handle_request enforces Host validation on GET requests" do
+          request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_HOST" => "evil.example.com" },
+          )
+
+          status, = @transport.handle_request(request)
+          assert_equal(403, status)
+        end
+
+        test "handle_request allows a request with no Host header" do
+          # Non-browser clients may omit Host; the rebinding vector always carries one.
+          request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = @transport.handle_request(request)
+          assert_equal(200, status)
+        end
+
+        test "handle_request skips DNS rebinding checks when protection is disabled" do
+          transport = StreamableHTTPTransport.new(@server, dns_rebinding_protection: false)
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_HOST" => "evil.example.com",
+              "HTTP_ORIGIN" => "https://evil.example.com",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+
+          status, = transport.handle_request(request)
+          assert_equal(200, status)
+        ensure
+          transport.close
+        end
+
         private
 
         def create_rack_request(method, path, headers, body = nil)