GHSA-h669-8m4g-r2hc

Summary

An unauthenticated remote attacker can force any MCP Ruby SDK server using MCP::Server::Transports::StreamableHTTPTransport to allocate gigabytes of memory by sending a single oversized JSON-RPC POST. The transport reads the entire HTTP body into a Ruby String and parses it with JSON.parse(body, symbolize_names: true) with no size limit, no Content-Length pre-check, and no streaming parser, allowing trivial denial of service against the worker process.

Affected component

lib/mcp/server/transports/streamable_http_transport.rb, method handle_post:

• Line 341: body_string = request.body.read — reads the full HTTP body into memory with no upper bound.
• Lines 531–535: JSON.parse(body_string, symbolize_names: true) — fully materialises the parsed object graph; with symbolize_names: true every JSON key also allocates a Ruby symbol.

The vulnerable path runs before session validation, so it is reachable in both the default stateful mode and in stateless: true mode, without an Mcp-Session-Id header and without any prior authentication.

A second instance of the same root cause exists in lib/mcp/server/transports/stdio_transport.rb:23 ($stdin.gets with no limit: argument). The practical impact there is limited because the stdio peer is normally a trusted parent process, but the fix should cover both transports.

Proof of concept

Both files below are self-contained. Save them anywhere on disk, run the server in one terminal and the client in another. The only dependencies are the SDK's existing Gemfile entries (rack ~> 3.2, rackup >= 2.1.0, webrick ~> 1.9) and Python's standard library.

Server (oom_poc_server.rb)

require "bundler/setup"
require "mcp"
require "mcp/server/transports/streamable_http_transport"
require "rackup"
require "webrick"
require "rackup/handler/webrick"

server = MCP::Server.new(name: "oom-poc-target", tools: [])
transport = MCP::Server::Transports::StreamableHTTPTransport.new(...