CVE-2026-63118

MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection

State: Published
Severity: 6.9 Medium
Published: Jul 29, 2026
Updated: Jul 29, 2026
Reserved: Jul 15, 2026

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.

Scores

CVSS	Score	Vector	Source
4.0	6.9 Medium	CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N	GitHub_M

Weaknesses

CWE-346 Origin Validation Error
CWE-350 Reliance on Reverse DNS Resolution for a Security-Critical Action

Affected Products

Product	Versions
ruby-sdkmodelcontextprotocol	< 0.23.0Affected

References

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