Vulnerability · Published Jul 29, 2026 · Assigned by GitHub_M

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

Medium 6.2

In plain English

A medium-severity uncontrolled resource consumption in ruby-sdk from modelcontextprotocol. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.2Medium

Exploit odds (30 days)0.2%top 91.5% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
RubyGems	mcp	0.23.0	GHSA-7683-3w9x-ch42

Affected products (as reported)

Vendor	Product	Versions
modelcontextprotocol	ruby-sdk	< 0.23.0

Weakness: CWE-400, CWE-770 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

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