Unbounded tool schema cache in Streamable HTTP server transport

Exploitation

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

CVSS

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

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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…

Published by the source

20 Oct 2026, 12:00 UTC

Last modified by the source

9 Nov 2026, 10:15 UTC

Source last checked

10 Nov 2026, 06:45 UTC

1. Revision 1 · created · 10 Nov 2026

   Published from osv record RUSTSEC-2026-0461

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# Patch Next advisory brief: GHSA-4432-vg92-fqh6

Advisory: Unbounded tool schema cache in Streamable HTTP server transport
IDs: GHSA-4432-vg92-fqh6
Record: https://patchnext.com/advisories/rustsec-2026-0461 (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: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rmcp 3.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected >= 3.0.0, < 3.6.0; fixed in 3.6.0

## 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-2026-0461 (OSV record RUSTSEC-2026-0461 lists affected ranges: crates.io rmcp >=3.0.0 <3.6.0 (fixed))

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