CVE-2026-63119

Deferred Deferred - Pending Action

Memory Exhaustion in MCP Ruby SDK via Unbounded IO Reads

Publication date: 2026-07-29

Last updated on: 2026-07-29

Assigner: GitHub, Inc.

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-07-29

Last Modified

2026-07-29

Generated

2026-08-19

AI Q&A

2026-07-30

EPSS Evaluated

2026-08-18

NVD

CVE-2026-63119

Affected Vendors & Products

Vendor	Product	Version / Range
mcp	ruby_sdk	0.23.0

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-400	The product does not properly control the allocation and maintenance of a limited resource.
CWE-770	The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

Attack-Flow Graph

Executive Summary

The vulnerability in the MCP Ruby SDK before version 0.23.0 involves the use of IO#gets without a byte limit in MCP::Server::Transports::StdioTransport and MCP::Client::Stdio. This allows a malicious peer to send data without a newline, causing the process to consume excessive memory and potentially exhaust system resources.

Detection Guidance

The vulnerability can be detected by checking the version of the mcp gem installed on your system. Run 'gem list mcp' to see the installed version. If it is below 0.23.0, the system is vulnerable.

Impact Analysis

This vulnerability can lead to denial-of-service conditions by crashing applications or systems due to memory exhaustion. It may disrupt services relying on the MCP Ruby SDK, cause application instability, or require manual intervention to restart affected processes.

Compliance Impact

This vulnerability could potentially impact compliance with GDPR and HIPAA by enabling denial-of-service attacks that exhaust system memory, leading to service disruptions. Unauthorized memory exhaustion may violate availability requirements in these regulations.

Mitigation Strategies

Upgrade the mcp gem to version 0.23.0 or later using the command 'gem install mcp --version 0.23.0'. After upgrading, verify the installation with 'gem list mcp' to ensure the correct version is running.

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