Unbounded response body buffering in Streamable HTTP clients

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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…

Published by the source

5 Nov 2026, 12:00 UTC

Last modified by the source

9 Nov 2026, 10:30 UTC

Source last checked

10 Nov 2026, 06:45 UTC

1. Revision 1 · created · 10 Nov 2026

   Published from osv record RUSTSEC-2026-0462

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# Patch Next advisory brief: GHSA-c9f9-cp8h-jcj5

Advisory: Unbounded response body buffering in Streamable HTTP clients
IDs: GHSA-c9f9-cp8h-jcj5
Record: https://patchnext.com/advisories/rustsec-2026-0462 (revision 1, source checked 10 Nov 2026, 06:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rmcp 3.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected >= 0.2.0, < 3.9.0; fixed in 3.9.0

## Suggested inspection (read-only)
- crates.io rmcp: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rmcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/RUSTSEC-2026-0462 (OSV record RUSTSEC-2026-0462 lists affected ranges: crates.io rmcp >=0.2.0 <3.9.0 (fixed))

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