MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)

Exploitation

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

CVSS

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

EPSS

• CVE-2026-63119: 0.19% estimated exploitation activity in the next 30 days; percentile 8.48 (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 The stdio transports in MCP::Server::Transports::StdioTransport and MCP::Client::Stdio read newline-delimited JSON-RPC frames using IO#gets with no limit argument. CRuby's IO#gets with no limit reads from the current position until the next separator (\n) with no upper bound on the returned string length. A peer that streams bytes without ever emitting a newline causes gets to accumulate the entire stream in a single Ruby String until the process is killed by the operating-system OOM killer. This is the same vulnerability class tracked in sibling MCP SDKs as GHSA-74gp-qhv5-v493 (Kotlin), GHSA-wqgc-pwpr-pq7r (TypeScript), GHSA-655q-2283-6jgj (Python), and others. It was identified during a cross-SDK audit; ruby-sdk had no prior report. Affected code Verified at main @ cf44475c. Server transport — lib/mcp/server/transports/stdiotransport.rb $stdin is the raw process global (only setencoding is applied at line 16); there is no wrapper imposing a length limit. Client transport —…

Published by the source

30 Jul 2026, 14:41 UTC

Last modified by the source

30 Jul 2026, 14:45 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-7683-3w9x-ch42

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-7683-3w9x-ch42

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# Patch Next advisory brief: CVE-2026-63119

Advisory: MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)
IDs: CVE-2026-63119, GHSA-7683-3w9x-ch42
Record: https://patchnext.com/advisories/cve-2026-63119 (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 6.2)
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-7683-3w9x-ch42 (OSV record GHSA-7683-3w9x-ch42 lists affected ranges: RubyGems mcp >=0 <0.23.0 (fixed))
- Verified: https://github.com/advisories/GHSA-7683-3w9x-ch42 (GitHub-reviewed advisory GHSA-7683-3w9x-ch42 lists affected ranges: RubyGems mcp >=0 <=0.22.0)

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