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Fencer Vulns
high 7.5

CVE-2026-63128 json

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

published 2026-09-16 53 d ago modified 2026-09-30 39 d ago also GHSA-9pj6-vhgr-3mwh , EUVD-2026-80823

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 45%
public exploits
0
none known

Summary

RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's stateful Streamable HTTP server in crates/rmcp/src/transport/streamable_http_server/tower.rs allows an unauthenticated client to send a well-formed JSON-RPC POST that is not an initialization request, or an initialization request with a mismatched protocol header, causing StreamableHttpService::handle_post to call LocalSessionManager.create_session before validating the message. An early validation failure returns without removing the inserted LocalSessionHandle from LocalSessionManager.sessions, permanently retaining session and channel state for the server process lifetime. Repeated requests can grow the shared session table without bound, degrade legitimate-client latency through lock contention, exhaust memory, and terminate the server. This issue is fixed in version 2.0.0.

Patch / fix

  • crates.io rmcp 2.0.0
  • goose RHEL 10.2 1.45.0-1.el10_2
  • goose RHEL 9 1.45.0-1.el9_8

red hat advisories RHSA-2026:80765RHSA-2026:80764

  • openai-codex openSUSE Tumbleweed 0.155.1-1.1

opensuse advisories openSUSE-SU-2026:11849-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-63128 updated cvss 7.5

rmcp: RMCP: Denial of Service via unauthenticated session table leak

  • Red Hat Enterprise Linux 9 goose, goose 1.45.0-1.el10_2 Red Hat Enterprise Linux 10 goose 1.45.0-1.el10_2
    affected: workaround, no fix available

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
53 d ago

Exploit prediction

0.53% chance of exploitation in the next 30 days, higher than 45% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-9pj6-vhgr-3mwh referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
crates.io rmcp
<2.0.0
2.0.0

Affected products

  • modelcontextprotocol rust-sdk

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption
  • CWE-401 Missing Release of Memory after Effective Lifetime
  • CWE-772 Missing Release of Resource after Effective Lifetime

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.