### Summary In affected versions, servers put no limit on how many array elements and object keys they validated in one request. One HTTP request within the default 4 MiB size limit could exhaust the server's memory, and the process aborted with `JavaScript heap out of memory`. No credentials are needed, unless you authenticate requests before they reach the SDK. ### Am I affected? Yes, if clients you do not fully trust can reach your server over HTTP. The risk is highest for `McpServer` tools whose input contains an array or a record. stdio-only servers are not affected. Affected versions: - `@modelcontextprotocol/sdk` 1.3.0 through 1.31.0 - `@modelcontextprotocol/server` 2.0.0 through 2.2.0 ### Fix Upgrade to: - 1.x: `@modelcontextprotocol/sdk` 1.32.0 or later - 2.x: `@modelcontextprotocol/server` 2.3.0 or later **Upgrading alone changes nothing.** The patched versions add an `McpServer` option, `maxToolInputElements`, that is off by default. Make both of these changes: 1. Set `maxToolInputElements` just above the largest input your tools need, for example `new McpServer(info, { maxToolInputElements: 10_000 })`. It counts array elements and object keys in a call's arguments. A call over the limit gets an `isError: true` result. 2. Lower the request size limit to what your tools need. The option covers tool arguments only, and the size limit covers everything else. Set `maxRequestBodySize` on the transport or `createMcpHandler()`. If you parse the body yourself, for example with `express.json()` or Fastify, set that parser's limit. If you cannot upgrade yet, make the second change. `maxRequestBodySize` needs `@modelcontextprotocol/sdk` 1.30.1 or `@modelcontextprotocol/server` 2.1.0. Below that, use a reverse proxy.