RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-service

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

• CVE-2026-63128: 0.53% estimated exploitation activity in the next 30 days; percentile 45.21 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary An unauthenticated remote attacker can leak one entry per HTTP request out of the in-memory session table of LocalSessionManager by sending a well-formed JSON-RPC POST that is not an InitializeRequest. The Streamable HTTP server's handlepost allocates the session before it validates the body, then early-returns on the validation failure without calling closesession. The LocalSessionHandle (and the tokio mpsc channel internals it holds) is never released for the remainder of the process's lifetime — turning a ~250-byte request into a permanent ~400–550-byte server-side allocation that scales linearly with request volume and eventually exhausts memory. In the verified reproduction below, a single Python client sustains over 2 000 leak requests per second; that translates to roughly 170 million leaked entries per day, equivalent to ≈75 GB of resident memory just from the session table. Details The bug lives in crates/rmcp/src/transport/streamablehttpserver/tower.rs inside…

Published by the source

16 Sep 2026, 22:13 UTC

Last modified by the source

16 Sep 2026, 22:30 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-9pj6-vhgr-3mwh

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-9pj6-vhgr-3mwh

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# Patch Next advisory brief: CVE-2026-63128

Advisory: RMCP: Unauthenticated permanent session-table leak in rmcp Streamable HTTP server transport leads to remote denial-of-service
IDs: CVE-2026-63128, GHSA-9pj6-vhgr-3mwh
Record: https://patchnext.com/advisories/cve-2026-63128 (revision 2, source checked 26 Oct 2026, 15:43 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 2.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rmcp: affected < 2.0.0; fixed in 2.0.0

## 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/GHSA-9pj6-vhgr-3mwh (OSV record GHSA-9pj6-vhgr-3mwh lists affected ranges: crates.io rmcp >=0 <2.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-9pj6-vhgr-3mwh (GitHub-reviewed advisory GHSA-9pj6-vhgr-3mwh lists affected ranges: crates.io rmcp >=0 <2.0.0)

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