Medium severity · published Jul 29, 2026
CVE-2026-63118
MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
6.9
CVSS
0.2%
EPSS
No
On KEV
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.
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 15% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
RU
ruby-sdk · modelcontextprotocol
< 0.23.0 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-63118
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-63118
github.com
https://github.com/modelcontextprotocol/ruby-sdk/commit/ba543083a7594e7892b29464b89091816446ff7a
github.com
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0
github.com
https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-rcgv-9m7m
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