MCP Gem Streamable HTTP Transport Vulnerable to DNS-Rebinding Attacks
The mcp gem version 0.18.0 is confirmed to have a vulnerability in its Streamable HTTP transport that lacks DNS-rebinding protection, enabling cross-origin attacks to enumerate and invoke tools and exfiltrate data.
What happened
The vulnerability, tracked as GHSA-rjr6-rcgv-9m7m, allows attackers to bypass DNS-rebinding protections in the Streamable HTTP transport of the mcp gem. This can enable cross-origin attacks that enumerate and invoke tools, as well as exfiltrate sensitive data from affected systems.
The vulnerability affects version 0.18.0 of the mcp gem. Users of this version should assess their exposure and upgrade to a patched version once available. Local MCP server configurations should also be reviewed for potential exposure.
For more details, consult the primary sources: [GHSA-rjr6-rcgv-9m7m](https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-rcgv-9m7m) and [CVE-2026-12940](https://nvd.nist.gov/vuln/detail/CVE-2026-12940).
How 0Day mitigates this
mcp is anywhere in your dependency tree, the engineers who own the affected repositories get a push alert the moment it is flagged — no manual audit to remember to run.Read how this differs from waiting on a scanner to catch a known advisory, or see the exact, read-only access 0Day needs to do this for an organization.