MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport

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-2026-67432: 0.78% estimated exploitation activity in the next 30 days; percentile 56.10 (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 force any MCP Ruby SDK server using MCP::Server::Transports::StreamableHTTPTransport to allocate gigabytes of memory by sending a single oversized JSON-RPC POST. The transport reads the entire HTTP body into a Ruby String and parses it with JSON.parse(body, symbolizenames: true) with no size limit, no Content-Length pre-check, and no streaming parser, allowing trivial denial of service against the worker process. Affected component lib/mcp/server/transports/streamablehttptransport.rb, method handlepost: - Line 341: bodystring = request.body.read — reads the full HTTP body into memory with no upper bound. - Lines 531–535: JSON.parse(bodystring, symbolizenames: true) — fully materialises the parsed object graph; with symbolizenames: true every JSON key also allocates a Ruby symbol. The vulnerable path runs before session validation, so it is reachable in both the default stateful mode and in stateless: true mode, without an Mcp-Session-Id…

Published by the source

30 Jul 2026, 14:44 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-h669-8m4g-r2hc

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-h669-8m4g-r2hc

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# Patch Next advisory brief: CVE-2026-67432

Advisory: MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport
IDs: CVE-2026-67432, GHSA-h669-8m4g-r2hc
Record: https://patchnext.com/advisories/cve-2026-67432 (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: 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: 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-h669-8m4g-r2hc (OSV record GHSA-h669-8m4g-r2hc lists affected ranges: RubyGems mcp >=0 <0.23.0 (fixed))
- Verified: https://github.com/advisories/GHSA-h669-8m4g-r2hc (GitHub-reviewed advisory GHSA-h669-8m4g-r2hc lists affected ranges: RubyGems mcp >=0 <=0.22.0)

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