CVE-2026-63119 MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)
A malicious peer can exhaust process memory in the MCP Ruby SDK by sending data without newlines, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63119?
CVE-2026-63119 is a medium-severity denial of service vulnerability in modelcontextprotocol ruby-sdk, published on 29 Jul 2026. It can only be exploited from the same machine or local network, 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::StdioTransport and MCP::Client::Stdio in the mcp gem use IO#gets without a byte limit, allowing a peer that sends data without a newline to exhaust process memory. This issue is fixed in version 0.23.0.
Which versions are affected by CVE-2026-63119?
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-2026-63119?
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-2026-63119 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63119 exists.
Sources
Also known as GHSA-7683-3w9x-ch42.