MCP SDKs: resource exhaustion

Summary

In affected versions, MCP servers reachable over HTTP put no limit on the size of a POST body or on the number of messages in a JSON-RPC batch. Large requests could exhaust the server's memory, and one long batch could block it from serving anyone else. No credentials were needed, unless your application authenticates requests before they reach the SDK.

Am I affected?

Yes, if clients you do not fully trust can reach your server over Streamable HTTP. A body parser such as express.json() does not protect you, because it does not limit batch length. Servers that only use stdio are not affected.

Affected versions:

• @modelcontextprotocol/sdk 1.10.0 through 1.30.0
• @modelcontextprotocol/server 2.0.0
• @modelcontextprotocol/node 2.0.0
• @modelcontextprotocol/hono 2.0.0

Fix

Upgrade to:

• 1.x: @modelcontextprotocol/sdk 1.30.1 or later
• 2.x: @modelcontextprotocol/server 2.1.0, @modelcontextprotocol/node 2.1.0 and @modelcontextprotocol/hono 2.0.1 or later. Upgrade every one you have installed.

Bodies over 4 MiB now get 413, and batches over 100 messages get 400. If your clients send larger bodies, raise the new maxRequestBodySize option.

If you cannot upgrade yet, cap request body size in a reverse proxy.