RUSTSEC-2026-0462: Unbounded response body buffering in Streamable HTTP clients

Before 3.9.0, the built-in Streamable HTTP clients in rmcp read non-streaming HTTP response bodies into memory with no size limit. Both the reqwest client (transport-streamable-http-client-reqwest) and the Unix socket client (transport-streamable-http-client-unix-socket) were affected:

- application/json responses to POST requests were read in full before parsing. - Non-2xx responses were read in full for error reporting.

A malicious or compromised MCP server can answer with a body that never ends, for example a chunked application/json response that streams whitespace. The client buffers it until the process runs out of memory. This works during initialize, before any authentication. On desktop or agent hosts, this terminates the whole host application.

Server-sent event streams are not affected. Each event is already capped by maxsseeventsize, and events are not accumulated.

The flaw was corrected in 3.9.0. Successful JSON bodies are limited to 16 MiB and non-success bodies to 64 KiB. Bytes are counted while streaming, after decompression. An oversized Content-Length is rejected before the body is read, and an oversized body is rejected with StreamableHttpError::ResponseBodyTooLarge instead of being truncated.

If you cannot upgrade, use a custom StreamableHttpClient that enforces a response body limit, or connect only to trusted MCP servers.