CVE-2026-63118

Received Received - Intake

MCP Ruby SDK Host Header Validation Bypass via DNS Rebinding

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::StreamableHTTPTransport in the mcp gem does not validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.

CVSS Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-07-29

Last Modified

2026-07-29

Generated

2026-07-30

AI Q&A

2026-07-30

EPSS Evaluated

N/A

NVD

CVE-2026-63118

Affected Vendors & Products

Currently, no data is known.

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-346	The product does not properly verify that the source of data or communication is valid.
CWE-350	The product performs reverse DNS resolution on an IP address to obtain the hostname and make a security decision, but it does not properly ensure that the IP address is truly associated with the hostname.

Attack-Flow Graph

Executive Summary

The MCP Ruby SDK before version 0.23.0 has a vulnerability in its StreamableHTTPTransport component. It fails to validate the HTTP Host or Origin headers, allowing a malicious browser page to use DNS rebinding to connect to a locally running MCP server and execute exposed tools.

Detection Guidance

Check if the MCP Ruby SDK version is below 0.23.0 by running gem list mcp. If the version is older, the system is vulnerable. Inspect HTTP headers for Host or Origin values that do not match expected domains, which may indicate DNS rebinding attempts.

Impact Analysis

An attacker could exploit this to access tools on your local MCP server from a remote browser page. This could lead to unauthorized actions, data exposure, or further network compromise depending on the exposed tools.

Compliance Impact

This vulnerability could lead to unauthorized access and data breaches, violating GDPR's data protection requirements and HIPAA's safeguards for protected health information. Compliance may be compromised if sensitive data is exposed.

Mitigation Strategies

Upgrade the MCP Ruby SDK to version 0.23.0 or later using gem update mcp. Ensure your MCP server is not exposed to untrusted networks. Monitor network traffic for unexpected connections to localhost.

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