CVE-2026-67430 MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood
Attackers can exhaust server memory by flooding initialize requests due to unbounded session retention in the MCP Ruby 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-2026-67430?
CVE-2026-67430 is a medium-severity denial of service 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 expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.
Which versions are affected by CVE-2026-67430?
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-2026-67430?
Upgrade mcp to 0.23.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-67430 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-67430 exists.
Sources
Also known as GHSA-52jp-gj8w-j6xh.