@@ -102,6 +102,8 @@ transport = MCP::Server::Transports::StdioTransport.new(server)
 transport.open
 ```
 
+`StdioTransport.new` accepts an optional `max_line_bytes:` keyword that caps the byte length of a single newline-delimited request frame. A frame that reaches this limit without a newline is rejected and the connection is closed, preventing unbounded memory growth from a peer that never emits a newline. It defaults to `4 * 1024 * 1024` (4 MiB).
+
 You can run this script and then type in requests to the server at the command line.
 
 ```console
@@ -128,6 +130,27 @@ 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:
@@ -1967,6 +1990,7 @@ Use the `MCP::Client::Stdio` transport to interact with MCP servers running as s
 | `args:` | No | An array of arguments passed to the command. Defaults to `[]`. |
 | `env:` | No | A hash of environment variables to set for the server process. Defaults to `nil`. |
 | `read_timeout:` | No | Timeout in seconds for waiting for a server response. Defaults to `nil` (no timeout). |
+| `max_line_bytes:` | No | Maximum byte length of a single newline-delimited response frame. A frame that reaches this limit without a newline is rejected as a transport error, preventing unbounded memory growth from a server that never emits a newline. Defaults to `4 * 1024 * 1024` (4 MiB). |
 
 Example usage:
 
 
@@ -65,6 +65,7 @@ stdio_transport.close
 | `args:` | No | An array of arguments passed to the command. Defaults to `[]`. |
 | `env:` | No | A hash of environment variables for the server process. Defaults to `nil`. |
 | `read_timeout:` | No | Timeout in seconds for waiting for a server response. Defaults to `nil`. |
+| `max_line_bytes:` | No | Maximum byte length of a single newline-delimited response frame. A frame that reaches this limit without a newline is rejected as a transport error, preventing unbounded memory growth from a server that never emits a newline. Defaults to `4 * 1024 * 1024` (4 MiB). |
 
 ## HTTP Transport
 
 
@@ -59,6 +59,8 @@ transport = MCP::Server::Transports::StdioTransport.new(server)
 transport.open
 ```
 
+`StdioTransport.new` accepts an optional `max_line_bytes:` keyword that caps the byte length of a single newline-delimited request frame. A frame that reaches this limit without a newline is rejected and the connection is closed, preventing unbounded memory growth from a peer that never emits a newline. It defaults to `4 * 1024 * 1024` (4 MiB).
+
 ## Streamable HTTP Transport
 
 `MCP::Server::Transports::StreamableHTTPTransport` is a standard Rack app, so it can be mounted in any Rack-compatible framework.
 
@@ -19,13 +19,28 @@ class Stdio
       CLOSE_TIMEOUT = 2
       STDERR_READ_SIZE = 4096
 
+      # Default upper bound on a single newline-delimited frame read from the
+      # server's stdout. CRuby's `IO#gets` without a limit accumulates bytes until a
+      # newline arrives, so a spawned server that never emits one can grow a single
+      # String until the host process is OOM-killed. 4 MiB is large enough for any
+      # realistic JSON-RPC frame, including base64-embedded images.
+      MAX_LINE_BYTES = 4 * 1024 * 1024
+
       attr_reader :command, :args, :env, :server_info
 
-      def initialize(command:, args: [], env: nil, read_timeout: nil)
+      def initialize(command:, args: [], env: nil, read_timeout: nil, max_line_bytes: MAX_LINE_BYTES)
+        # Reject `nil` or non-positive values: `IO#gets("\n", nil)` and a negative
+        # limit read without an upper bound, which would silently disable the
+        # protection this option exists to provide.
+        unless max_line_bytes.is_a?(Integer) && max_line_bytes > 0
+          raise ArgumentError, "max_line_bytes must be a positive Integer"
+        end
+
         @command = command
         @args = args
         @env = env
         @read_timeout = read_timeout
+        @max_line_bytes = max_line_bytes
         @stdin = nil
         @stdout = nil
         @stderr = nil
@@ -239,7 +254,7 @@ def read_response(request)
         loop do
           ensure_running!
           wait_for_readable!(method, params) if @read_timeout
-          line = @stdout.gets
+          line = read_line(method, params)
           raise_connection_error!(method, params) if line.nil?
 
           parsed = JSON.parse(line.strip)
@@ -278,6 +293,22 @@ def wait_for_readable!(method, params)
         )
       end
 
+      # Reads one newline-delimited frame from the server's stdout, bounded by `@max_line_bytes`.
+      # Returns the line (including its trailing newline) or `nil` at EOF. Raises when the limit
+      # is reached before a newline arrives, which signals a server streaming an unbounded frame.
+      # A short final frame without a trailing newline (EOF) is still returned, since its length
+      # stays under the limit.
+      def read_line(method, params)
+        line = @stdout.gets("\n", @max_line_bytes)
+        return line unless line && !line.end_with?("\n") && line.bytesize >= @max_line_bytes
+
+        raise RequestHandlerError.new(
+          "Server response frame exceeds #{@max_line_bytes} bytes without a newline",
+          { method: method, params: params },
+          error_type: :internal_error,
+        )
+      end
+
       def raise_connection_error!(method, params)
         raise RequestHandlerError.new(
           "Server process closed stdout unexpectedly",
 
@@ -9,18 +9,46 @@ module Transports
       class StdioTransport < Transport
         STATUS_INTERRUPTED = Signal.list["INT"] + 128
 
-        def initialize(server)
+        # Default upper bound on a single newline-delimited frame. CRuby's `IO#gets`
+        # without a limit accumulates bytes until a newline arrives, so a peer that
+        # never emits one can grow a single String until the process is OOM-killed.
+        # 4 MiB is large enough for any realistic JSON-RPC frame, including
+        # base64-embedded images.
+        MAX_LINE_BYTES = 4 * 1024 * 1024
+
+        def initialize(server, max_line_bytes: MAX_LINE_BYTES)
           super(server)
           @open = false
           @session = nil
+          # Reject `nil` or non-positive values: `IO#gets("\n", nil)` and a negative
+          # limit read without an upper bound, which would silently disable the
+          # protection this option exists to provide.
+          unless max_line_bytes.is_a?(Integer) && max_line_bytes > 0
+            raise ArgumentError, "max_line_bytes must be a positive Integer"
+          end
+
+          @max_line_bytes = max_line_bytes
           $stdin.set_encoding("UTF-8")
           $stdout.set_encoding("UTF-8")
         end
 
         def open
           @open = true
           @session = ServerSession.new(server: @server, transport: self)
-          while @open && (line = $stdin.gets)
+          while @open
+            begin
+              line = read_line($stdin)
+            rescue RequestHandlerError => e
+              # Stop accumulating and end the connection gracefully rather than
+              # letting an unbounded read exhaust memory or escape as an uncaught
+              # backtrace. Scoped to the read so genuine request errors raised while
+              # handling a frame are not swallowed here.
+              @open = false
+              MCP.configuration.exception_reporter.call(e, { error: "stdio frame exceeds limit" })
+              break
+            end
+            break if line.nil?
+
             response = @session.handle_json(line.strip)
             send_response(response) if response
           end
@@ -73,7 +101,7 @@ def send_request(method, params = nil)
             raise
           end
 
-          while @open && (line = $stdin.gets)
+          while @open && (line = read_line($stdin))
             begin
               parsed = JSON.parse(line.strip, symbolize_names: true)
             rescue JSON::ParserError => e
@@ -95,6 +123,26 @@ def send_request(method, params = nil)
 
           raise "Transport closed while waiting for response to #{method} request."
         end
+
+        private
+
+        # Reads one newline-delimited frame, bounded by `@max_line_bytes`. Returns
+        # the line (including its trailing newline) or `nil` at EOF. Raises when the
+        # limit is reached before a newline arrives, which signals a peer streaming
+        # an unbounded frame. A short final frame without a trailing newline (EOF) is
+        # still returned, since its length stays under the limit.
+        def read_line(io)
+          line = io.gets("\n", @max_line_bytes)
+          if line && !line.end_with?("\n") && line.bytesize >= @max_line_bytes
+            raise RequestHandlerError.new(
+              "stdio frame exceeds #{@max_line_bytes} bytes without a newline",
+              nil,
+              error_type: :internal_error,
+            )
+          end
+
+          line
+        end
       end
     end
   end
 
@@ -39,15 +39,47 @@ class InvalidJsonError < StandardError; end
         UNSET_IDLE_TIMEOUT = Object.new.freeze
         private_constant :UNSET_IDLE_TIMEOUT
 
-        def initialize(server, stateless: false, enable_json_response: false,
-          session_idle_timeout: UNSET_IDLE_TIMEOUT, max_sessions: DEFAULT_MAX_SESSIONS)
+        # 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; defaults
+        #   to `DEFAULT_SESSION_IDLE_TIMEOUT` (1800) in stateful mode, and an explicit `nil` disables
+        #   expiry. Not supported in stateless mode.
+        # @param max_sessions [Integer, nil] cap on the concurrent session count in stateful mode; a new
+        #   `initialize` past the cap is rejected with HTTP 503, and `nil` disables the cap.
+        # @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: UNSET_IDLE_TIMEOUT,
+          max_sessions: DEFAULT_MAX_SESSIONS,
+          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 = {}
           @mutex = Mutex.new
 
           @stateless = stateless
           @enable_json_response = enable_json_response
+          @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 = {}
 
           # Resolve the idle timeout: an explicit value (including `nil` to opt out) wins; otherwise apply the secure default,
@@ -82,12 +114,19 @@ def initialize(server, stateless: false, enable_json_response: false,
         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)
@@ -863,6 +902,74 @@ def session_active?(session_id)
           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,