MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood

Exploitation

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

CVSS

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

EPSS

• CVE-2026-67430: 0.37% estimated exploitation activity in the next 30 days; percentile 30.59 (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 In its default configuration, MCP::Server::Transports::StreamableHTTPTransport never expires sessions. Every successful initialize request stores a new ServerSession and a session record under a fresh UUID, and the only path that removes them is an explicit client-issued HTTP DELETE. An unauthenticated attacker can repeatedly initialize new sessions and immediately disconnect, forcing the server to retain an unbounded number of ServerSession objects until memory is exhausted. Affected component lib/mcp/server/transports/streamablehttptransport.rb: - Line 27, constructor: def initialize(server, stateless: false, enablejsonresponse: false, sessionidletimeout: nil) — the default for sessionidletimeout is nil. - Line 46: startreaperthread if @sessionidletimeout — when the timeout is nil, the reaper that prunes idle sessions is never started. - Lines 604–643 (handleinitialization): every successful initialize inserts a new session record; the only removal sites are handledelete…

Published by the source

30 Jul 2026, 14:43 UTC

Last modified by the source

30 Jul 2026, 15:00 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-52jp-gj8w-j6xh

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-52jp-gj8w-j6xh

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# Patch Next advisory brief: CVE-2026-67430

Advisory: MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood
IDs: CVE-2026-67430, GHSA-52jp-gj8w-j6xh
Record: https://patchnext.com/advisories/cve-2026-67430 (revision 2, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 0.23.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems mcp: affected <= 0.22.0; fixed in 0.23.0

## Suggested inspection (read-only)
- RubyGems mcp: 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 mcp.
- 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-52jp-gj8w-j6xh (OSV record GHSA-52jp-gj8w-j6xh lists affected ranges: RubyGems mcp >=0 <0.23.0 (fixed))
- Verified: https://github.com/advisories/GHSA-52jp-gj8w-j6xh (GitHub-reviewed advisory GHSA-52jp-gj8w-j6xh lists affected ranges: RubyGems mcp >=0 <=0.22.0)

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