CVE-2026-63118: MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection

Severity: medium (CVSS 6.9 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-29 · updated 2026-10-24
Weakness: CWE-346

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.

Affected products

modelcontextprotocol ruby-sdk: before 0.23.0

References

https://github.com/modelcontextprotocol/ruby-sdk/commit/ba543083a7594e7892b29464b89091816446ff7a
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0
https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-rcgv-9m7m
https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-rcgv-9m7m

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