Vulnerability · Published Jul 29, 2026 · Assigned by GitHub_M

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

Medium 6.9

In plain English

A medium-severity security flaw 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 15% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)6.9Medium

Exploit odds (30 days)0.2%top 85.0% 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::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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
RubyGems	mcp	0.23.0	GHSA-rjr6-rcgv-9m7m

Affected products (as reported)

Vendor	Product	Versions
modelcontextprotocol	ruby-sdk	< 0.23.0

Weakness: CWE-346, CWE-350 · Vector: 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

References

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