CVE-2026-67431 (High) detected in mcp-0.9.0.gem

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
68/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
ruby
Domain
security

Research direction

Start with /Gemfile.lock and the vendored /vendor/cache/mcp-0.9.0.gem, then inspect the dependency chain through rubocop-performance and rubocop. Upgrade mcp to 0.23.0 and confirm the lockfile and vendored contents no longer contain 0.9.0; run the repository’s existing checks to verify compatibility.

Written by the indexing model from the issue text.

Description

Mend: dependency security vulnerability

CVE-2026-67431 - High Severity Vulnerability

Vulnerable Library - mcp-0.9.0.gem

The official Ruby SDK for Model Context Protocol servers and clients

Library home page: https://rubygems.org/gems/mcp-0.9.0.gem

Path to dependency file: /Gemfile.lock

Path to vulnerable library: /vendor/cache/mcp-0.9.0.gem

Dependency Hierarchy:

  • manageiq-style-1.6.0.gem (Root Library)
    • rubocop-performance-1.26.1.gem
      • rubocop-1.85.1.gem
        • ❌ mcp-0.9.0.gem (Vulnerable Library)

Found in base branch: master

Vulnerability Details

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.

Publish Date: 2026-07-29

URL: CVE-2026-67431

CVSS 3 Score Details (8.2)

Base Score Metrics:

  • Exploitability Metrics:
    • Attack Vector: Network
    • Attack Complexity: Low
    • Privileges Required: None
    • User Interaction: None
    • Scope: Unchanged
  • Impact Metrics:
    • Confidentiality Impact: None
    • Integrity Impact: High
    • Availability Impact: Low

For more information on CVSS3 Scores, click here.

Suggested Fix

Type: Upgrade version

Origin: https://github.com/advisories/GHSA-5p9g-j988-pcwv

Release Date: 2026-07-29

Fix Resolution: mcp - 0.23.0

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Dominant language
Ruby
Stars
259
Forks
35
No merged PRs in 30d

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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