CVE-2026-67430: MCP Ruby SDK DoS Vulnerability

CVE-2026-67430 Overview

CVE-2026-67430 affects the MCP Ruby SDK, the official Ruby SDK for Model Context Protocol servers and clients. The MCP::Server::Transports::StreamableHTTPTransport class in the mcp gem does not expire sessions by default. Repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. The flaw is classified as a missing release of memory after effective lifetime [CWE-401]. Version 0.23.0 resolves the issue by introducing session expiration.

Critical Impact
Unauthenticated remote attackers can exhaust the memory of MCP servers using StreamableHTTPTransport, resulting in denial of service against Ruby-based Model Context Protocol deployments.

Affected Products

MCP Ruby SDK (mcp gem) versions prior to 0.23.0
Applications using MCP::Server::Transports::StreamableHTTPTransport
Model Context Protocol server deployments written in Ruby

Discovery Timeline

2026-07-29 - CVE-2026-67430 published to NVD
2026-07-29 - Last updated in NVD database
v0.23.0 - Fix released in the GitHub Release v0.23.0

Technical Details for CVE-2026-67430

Vulnerability Analysis

The MCP Ruby SDK provides transports that expose Model Context Protocol servers over HTTP. The StreamableHTTPTransport class maintains per-client state in ServerSession objects. Each initialize request from a client allocates a new session object stored in server memory.

Before version 0.23.0, the transport does not enforce a session expiration policy. Sessions accumulate in memory for the lifetime of the process. An attacker who repeatedly sends initialize requests forces the server to allocate additional ServerSession instances without releasing older ones.

The behavior maps to [CWE-401]: missing release of memory after effective lifetime. Sustained request volume drives memory consumption until the Ruby process is terminated by the operating system or becomes unresponsive.

Root Cause

The root cause is the absence of a default session lifetime in MCP::Server::Transports::StreamableHTTPTransport. Session objects have no timeout, no idle expiry, and no upper bound on the number of concurrently retained instances. The transport treats every initialize call as legitimate long-lived state.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker sends repeated HTTP initialize requests to the MCP endpoint. Each request produces a new ServerSession retained in memory. Over time or at high request rates, the process exhausts available memory. See the GitHub Security Advisory GHSA-52jp-gj8w-j6xh for advisory details.

No public proof-of-concept exploit is listed for CVE-2026-67430. The vulnerability description in prose is sufficient for reproduction: repeated MCP initialize calls against a vulnerable StreamableHTTPTransport endpoint drive unbounded session allocation.

Detection Methods for CVE-2026-67430

Indicators of Compromise

Steady growth in resident memory of Ruby processes hosting MCP servers with no correlated increase in legitimate client activity.
High volume of HTTP requests to MCP endpoints containing the initialize method, particularly from a small set of source addresses.
Ruby processes terminated by the OOM killer or restarted repeatedly by a supervisor such as systemd or foreman.

Detection Strategies

Instrument the MCP server to log the count of active ServerSession objects and alert when the count exceeds a defined threshold.
Correlate application logs for initialize request rates against unique client identifiers to identify anomalous session churn.
Use runtime memory profiling (for example rbtrace or derailed_benchmarks) to attribute heap growth to ServerSession retention.

Monitoring Recommendations

Track process-level resident set size (RSS) for all Ruby processes running the mcp gem and alert on sustained upward trends.
Monitor HTTP request rates and payload types at the reverse proxy or ingress in front of MCP endpoints.
Forward MCP server logs and host metrics to a centralized analytics platform to enable long-horizon trend analysis and anomaly identification.

How to Mitigate CVE-2026-67430

Immediate Actions Required

Upgrade the mcp gem to version 0.23.0 or later on every host that runs a Ruby MCP server.
Inventory Ruby services that instantiate MCP::Server::Transports::StreamableHTTPTransport and prioritize internet-exposed instances.
Restart affected Ruby processes after upgrading to release accumulated ServerSession objects.

Patch Information

The fix is available in version 0.23.0 of the mcp gem. Details of the code change are in the GitHub Commit Update. Release notes are documented in the GitHub Release v0.23.0. Update the gem constraint in Gemfile to gem 'mcp', '>= 0.23.0' and run bundle update mcp.

Workarounds

Place a reverse proxy in front of the MCP endpoint and enforce per-source rate limits on initialize requests.
Configure a process supervisor to cap memory usage and recycle Ruby workers that exceed a defined RSS threshold.
Restrict network access to MCP endpoints so that only trusted clients can reach the StreamableHTTPTransport listener.

bash

# Configuration example
bundle update mcp --conservative
bundle list | grep mcp
# Verify version is >= 0.23.0, then restart the MCP server process
systemctl restart mcp-server