CVE-2026-67431 MCP Ruby SDK: Ruby SSE Session Poisoning
Attackers with stolen session IDs can execute tools in victim sessions due to missing session ownership binding in the Ruby MCP SDK.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-67431?
CVE-2026-67431 is a high-severity missing authorization vulnerability in modelcontextprotocol ruby-sdk, published on 29 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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 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.
Weakness: CWE-284
Which versions are affected by CVE-2026-67431?
2 open source packages ship an affected version, including mcp.
| Package | Affected | Fixed in |
|---|---|---|
mcp RubyGems | All versions | 0.23.0 |
mcp RubyGems | All versions | 0.23.0 |
How do I fix CVE-2026-67431?
Upgrade mcp to 0.23.0 or later.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-67431 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-67431 exists.
Sources
Also known as GHSA-5p9g-j988-pcwv.