CVE-2026-63118 (High) detected in mcp-0.9.0.gem

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
75/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
ruby
Domain
security

Research direction

Start with Gemfile.lock and the vendored mcp-0.9.0.gem at vendor/cache/mcp-0.9.0.gem, then inspect how the dependency is resolved through the listed RuboCop packages. Upgrade mcp to 0.23.0 and refresh the dependency files so the vulnerable version is no longer present; verify the resulting bundle and dependency scan.

Written by the indexing model from the issue text.

Description

Mend: dependency security vulnerability

CVE-2026-63118 - 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 validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.

Publish Date: 2026-07-29

URL: CVE-2026-63118

CVSS 3 Score Details (7.4)

Base Score Metrics:

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

For more information on CVSS3 Scores, click here.

Suggested Fix

Type: Upgrade version

Origin: https://github.com/advisories/GHSA-rjr6-rcgv-9m7m

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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