Vulnerability RUSTSEC-2026-0461

From 3.0.0 until 3.6.0, StreamableHttpService cached the result of ServerHandler::get_tool in a process-wide map keyed by the tool name of incoming tools/call requests. The map had no size limit and no eviction, and it also cached unknown tool names.

An entry was added while the request headers were validated, before the request could be rejected, so requests answered with HTTP 400 still grew the map. A client that can reach the endpoint can send tools/call requests with distinct tool names, each up to the default 4 MiB body limit, until the server runs out of memory. rmcp does not authenticate requests itself, so servers without an authentication layer in front of the endpoint are exposed to unauthenticated clients.

Only servers that use the Streamable HTTP server transport and receive requests at protocol version 2026-07-28 or later are affected.

The flaw was corrected in 3.6.0 by removing the cache. The transport now calls ServerHandler::get_tool on each request.

If you cannot upgrade, put authentication in front of the endpoint, or reject tools/call requests for unknown tool names before they reach StreamableHttpService.