DEV Community

CVE Reports
CVE Reports

Posted on • Originally published at cvereports.com

CVE-2026-67430: CVE-2026-67430: Denial of Service via Unbounded Session Retention in Model Context Protocol Ruby SDK

#security #cve #cybersecurity

CVE-2026-67430: Denial of Service via Unbounded Session Retention in Model Context Protocol Ruby SDK

Vulnerability ID: CVE-2026-67430
CVSS Score: 5.3
Published: 2026-07-30

CVE-2026-67430 is a medium-severity Denial of Service (DoS) vulnerability in the Model Context Protocol (MCP) Ruby SDK (packaged as the mcp gem) versions prior to 0.23.0. In stateful deployments using the StreamableHTTPTransport class, client session states are retained in an in-memory hash map. Because the transport implements a nil idle timeout by default, the background scavenger process is suppressed. Remote, unauthenticated attackers can flood the endpoint with initialize requests, rapidly consuming system memory and triggering an Out-of-Memory (OOM) crash.

TL;DR

Unauthenticated remote attackers can crash stateful MCP Ruby SDK servers by repeatedly triggering new initialize requests. Because the SDK lacks default session limits and idle timeouts, every connection allocates a persistent context in RAM, leading to memory exhaustion and service crashes.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-770
  • Attack Vector: Network (AV:N)
  • CVSS v3.1 Score: 5.3
  • EPSS Score: 0.00291
  • EPSS Percentile: 21.41%
  • Impact: Denial of Service (DoS) via memory exhaustion (OOM)
  • Exploit Status: PoC Available
  • KEV Status: Not Listed

Affected Systems

  • Model Context Protocol Ruby SDK (mcp gem) running stateful transports
  • mcp (Ruby SDK): >= 0.6.0, < 0.23.0 (Fixed in: 0.23.0)

Code Analysis

Commit: afb968c

Enforce secure session idle timeouts and concurrent session limits inside StreamableHTTPTransport

Exploit Details

Mitigation Strategies

  • Upgrade the mcp gem to version 0.23.0 or later.
  • Configure tight session_idle_timeout and max_sessions values on StreamableHTTPTransport initialization.
  • Utilize stateless: true if stateful session management is not required.
  • Deploy a reverse proxy (e.g., Nginx, HAProxy) in front of the application to enforce request and connection rate-limiting.

Remediation Steps:

  1. Update your Gemfile to reference mcp version 0.23.0 or later.
  2. Run bundle update mcp to retrieve and apply the security patch.
  3. Optionally, explicitly set session_idle_timeout and max_sessions parameters in your StreamableHTTPTransport.new initializer block.
  4. Restart the Ruby application server to apply the code changes.

References

Read the full report for CVE-2026-67430 on our website for more details including interactive diagrams and full exploit analysis.

Top comments (0)