267b8fa628
## Motivation and Context CRuby's `IO#gets` without a limit reads until the next newline with no upper bound. A peer that never emits a newline can therefore grow a single String until the process is killed by the OS OOM killer. This is a denial-of-service exposure (GHSA-7683-3w9x-ch42), identified during a cross-SDK audit of the same unbounded-buffer pattern across the MCP SDKs. It is most relevant when a host application spawns a sandboxed or untrusted MCP server and pipes its stdout into an unconstrained host process, where a memory-capped peer can exhaust the host. Each frame is now capped at a configurable `max_line_bytes` (default 4 MiB, large enough for any realistic frame including base64-embedded images, and the value recommended in the advisory). A frame that reaches the limit without a newline is treated as a transport error: the client raises, and the server reports the error and closes the connection. A short final frame that ends at EOF without a trailing newline is still accepted, so legitimate traffic is unaffected. Both constructors validate `max_line_bytes:` as a positive Integer, since `nil` or a non-positive value makes `IO#gets` read without an upper bound and would silently disable the protection. ## How Has This Been Tested? Server tests cover the `open` read loop stopping and reporting on an over-limit frame, the `send_request` read loop raising on an over-limit response, and a valid final frame without a trailing newline still being processed. Client tests cover `read_response` raising on an over-limit response frame and a normal response still round-tripping within a custom `max_line_bytes`. Both transports also have tests rejecting invalid `max_line_bytes:` values (`nil`, zero, negative, non-Integer) with an `ArgumentError`. The full test suite and RuboCop pass. ## Breaking Changes None. The default 4 MiB limit is far larger than any realistic JSON-RPC frame, so existing well-behaved traffic is unaffected. The new `max_line_bytes:` keyword on both transport constructors is purely additive and optional.
532 lines
17 KiB
Ruby
532 lines
17 KiB
Ruby
# frozen_string_literal: true
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require "test_helper"
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module MCP
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class Server
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module Transports
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class StdioTransportTest < ActiveSupport::TestCase
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include InstrumentationTestHelper
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setup do
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configuration = MCP::Configuration.new
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configuration.instrumentation_callback = instrumentation_helper.callback
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@server = Server.new(name: "test_server", configuration: configuration)
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@transport = StdioTransport.new(@server)
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end
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test "initializes with server and closed state" do
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server = @transport.instance_variable_get(:@server)
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assert_equal @server.object_id, server.object_id
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refute @transport.instance_variable_get(:@open)
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end
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test "processes JSON-RPC requests from stdin and sends responses to stdout" do
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request = {
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jsonrpc: "2.0",
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method: "ping",
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id: "123",
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}
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input = StringIO.new(JSON.generate(request) + "\n")
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = input
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$stdout = output
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thread = Thread.new { @transport.open }
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sleep(0.1)
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@transport.close
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thread.join
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response = JSON.parse(output.string, symbolize_names: true)
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assert_equal("2.0", response[:jsonrpc])
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assert_equal("123", response[:id])
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assert_equal({}, response[:result])
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refute(@transport.instance_variable_get(:@open))
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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end
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end
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test "sends string responses to stdout" do
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output = StringIO.new
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original_stdout = $stdout
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begin
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$stdout = output
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@transport.send_response("test response")
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assert_equal("test response\n", output.string)
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ensure
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$stdout = original_stdout
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end
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end
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test "sends JSON responses to stdout" do
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output = StringIO.new
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original_stdout = $stdout
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begin
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$stdout = output
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response = { key: "value" }
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@transport.send_response(response)
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assert_equal(JSON.generate(response) + "\n", output.string)
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ensure
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$stdout = original_stdout
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end
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end
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test "handles valid JSON-RPC requests" do
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request = {
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jsonrpc: "2.0",
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method: "ping",
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id: "123",
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}
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output = StringIO.new
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original_stdout = $stdout
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begin
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$stdout = output
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@transport.send(:handle_request, JSON.generate(request))
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response = JSON.parse(output.string, symbolize_names: true)
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assert_equal("2.0", response[:jsonrpc])
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assert_nil(response[:id])
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assert_nil(response[:result])
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ensure
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$stdout = original_stdout
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end
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end
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test "open creates a ServerSession and processes requests through it" do
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request = {
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jsonrpc: "2.0",
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method: "initialize",
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id: "1",
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params: {
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protocolVersion: "2025-11-25",
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clientInfo: { name: "stdio-client", version: "1.0" },
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},
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}
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input = StringIO.new(JSON.generate(request) + "\n")
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = input
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$stdout = output
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@transport.open
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# Verify a session was created.
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session = @transport.instance_variable_get(:@session)
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assert_instance_of(ServerSession, session)
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# Verify client info was stored on the session, not on the server.
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assert_equal({ name: "stdio-client", version: "1.0" }, session.client)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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end
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end
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test "rejects duplicate initialize on the same stdio session with -32600" do
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first = {
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jsonrpc: "2.0",
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method: "initialize",
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id: "first",
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params: {
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protocolVersion: "2025-11-25",
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clientInfo: { name: "original", version: "1.0" },
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},
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}
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second = {
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jsonrpc: "2.0",
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method: "initialize",
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id: "second",
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params: {
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protocolVersion: "2024-11-05",
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clientInfo: { name: "intruder", version: "9.9" },
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},
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}
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input = StringIO.new("#{JSON.generate(first)}\n#{JSON.generate(second)}\n")
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = input
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$stdout = output
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@transport.open
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lines = output.string.lines
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assert_equal(2, lines.length)
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first_response = JSON.parse(lines[0], symbolize_names: true)
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second_response = JSON.parse(lines[1], symbolize_names: true)
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assert_equal("first", first_response[:id])
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refute_nil(first_response[:result])
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assert_equal("second", second_response[:id])
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assert_equal(-32600, second_response[:error][:code])
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assert_equal("Invalid Request", second_response[:error][:message])
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session = @transport.instance_variable_get(:@session)
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assert_equal({ name: "original", version: "1.0" }, session.client)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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end
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end
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test "handles invalid JSON requests" do
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invalid_json = "invalid json"
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output = StringIO.new
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original_stdout = $stdout
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begin
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$stdout = output
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@transport.send(:handle_request, invalid_json)
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response = JSON.parse(output.string, symbolize_names: true)
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assert_equal("2.0", response[:jsonrpc])
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assert_nil(response[:id])
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assert_equal(-32600, response[:error][:code])
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assert_equal("Invalid Request", response[:error][:message])
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assert_equal("Request must be an array or a hash", response[:error][:data])
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ensure
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$stdout = original_stdout
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end
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end
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test "send_request sends request to stdout and waits for response" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Send response from client in a thread.
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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response = {
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jsonrpc: "2.0",
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id: request[:id],
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result: { content: "test response" },
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}
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writer.puts(response.to_json)
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writer.flush
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end
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result = @transport.send_request("test/method", { param: "value" })
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assert_equal({ content: "test response" }, result)
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# Verify request was sent.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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assert_equal("2.0", request[:jsonrpc])
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assert_equal("test/method", request[:method])
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assert_equal({ param: "value" }, request[:params])
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assert(request[:id])
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request raises on error response from client" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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error_response = {
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jsonrpc: "2.0",
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id: request[:id],
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error: { code: -1, message: "User rejected sampling request" },
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}
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writer.puts(error_response.to_json)
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writer.flush
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end
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error = assert_raises(StandardError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_equal("Client returned an error for sampling/createMessage request (code: -1): User rejected sampling request", error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request does not double-report intentional raises via exception_reporter" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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reported_errors = []
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original_reporter = MCP.configuration.exception_reporter
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begin
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MCP.configuration.exception_reporter = ->(e, ctx) { reported_errors << [e, ctx] }
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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error_response = {
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jsonrpc: "2.0",
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id: request[:id],
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error: { code: -1, message: "rejected" },
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}
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writer.puts(error_response.to_json)
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writer.flush
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end
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assert_raises(StandardError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_empty(reported_errors)
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ensure
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MCP.configuration.exception_reporter = original_reporter
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request processes interleaved requests via session" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Initialize a session so @session is set.
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session = MCP::ServerSession.new(server: @server, transport: @transport)
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@transport.instance_variable_set(:@session, session)
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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request = JSON.parse(output.string.lines.first, symbolize_names: true)
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# Send an interleaved ping request before the response.
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ping = { jsonrpc: "2.0", method: "ping", id: "ping-1" }
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writer.puts(ping.to_json)
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writer.flush
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sleep(0.05) # Wait for the ping to be processed.
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# Then send the actual response.
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response = {
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jsonrpc: "2.0",
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id: request[:id],
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result: { content: "done" },
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}
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writer.puts(response.to_json)
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writer.flush
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end
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result = @transport.send_request("test/method", { param: "value" })
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assert_equal({ content: "done" }, result)
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# Verify the interleaved ping was handled (response sent to output).
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lines = output.string.lines
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ping_response = lines.find { |l| l.include?("ping-1") }
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assert(ping_response, "Interleaved ping request should have been handled")
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue
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nil
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end
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begin
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reader.close
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rescue
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nil
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end
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end
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end
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test "send_request raises when transport is closed while waiting" do
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reader, writer = IO.pipe
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = reader
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$stdout = output
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@transport.instance_variable_set(:@open, true)
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# Close transport while waiting for response.
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Thread.new do
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sleep(0.05) # Wait for request to be written to `StringIO`.
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@transport.instance_variable_set(:@open, false)
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writer.close
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end
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error = assert_raises(RuntimeError) do
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@transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_equal("Transport closed while waiting for response to sampling/createMessage request.", error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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begin
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writer.close
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rescue IOError
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nil
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end
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begin
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reader.close
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rescue IOError
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nil
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end
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end
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end
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test "raises ArgumentError when max_line_bytes is not a positive Integer" do
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[nil, 0, -1, 1.5, "1024"].each do |invalid|
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error = assert_raises(ArgumentError) do
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StdioTransport.new(@server, max_line_bytes: invalid)
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end
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assert_equal("max_line_bytes must be a positive Integer", error.message)
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end
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end
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test "open stops and reports when a stdin frame exceeds max_line_bytes without a newline" do
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transport = StdioTransport.new(@server, max_line_bytes: 64)
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input = StringIO.new("A" * 200) # No newline: an unbounded frame.
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output = StringIO.new
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reported = []
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original_stdin = $stdin
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original_stdout = $stdout
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original_reporter = MCP.configuration.exception_reporter
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begin
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$stdin = input
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$stdout = output
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MCP.configuration.exception_reporter = ->(e, ctx) { reported << [e, ctx] }
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transport.open
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refute(transport.instance_variable_get(:@open))
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assert_equal(1, reported.size)
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error, _ctx = reported.first
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assert_instance_of(RequestHandlerError, error)
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assert_match(/exceeds 64 bytes without a newline/, error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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MCP.configuration.exception_reporter = original_reporter
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end
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end
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test "open processes a valid final frame that ends at EOF without a trailing newline" do
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request = { jsonrpc: "2.0", method: "ping", id: "eof" }
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input = StringIO.new(JSON.generate(request)) # No trailing newline.
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = input
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$stdout = output
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@transport.open
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response = JSON.parse(output.string, symbolize_names: true)
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assert_equal("eof", response[:id])
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assert_equal({}, response[:result])
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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end
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end
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test "send_request raises when a response frame exceeds max_line_bytes without a newline" do
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transport = StdioTransport.new(@server, max_line_bytes: 64)
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input = StringIO.new("A" * 200) # No newline: an unbounded frame.
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output = StringIO.new
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original_stdin = $stdin
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original_stdout = $stdout
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begin
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$stdin = input
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$stdout = output
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transport.instance_variable_set(:@open, true)
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error = assert_raises(RequestHandlerError) do
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transport.send_request("sampling/createMessage", { messages: [] })
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end
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assert_match(/exceeds 64 bytes without a newline/, error.message)
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ensure
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$stdin = original_stdin
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$stdout = original_stdout
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end
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end
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end
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end
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end
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end
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