CVE-2026-67432

## 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`) ```ruby 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(