Shield
Authentication protocol for use in your routing and model context
What is Shield?
Shield is a Model Context Protocol (MCP) server that allows AI assistants like Claude, Cursor, and VS Code to authentication protocol for use in your routing and model context
Authentication protocol for use in your routing and model context
This server falls under the Security category on MCPgee, the world's largest MCP server directory with 33,000+ servers.
Features
- Authentication protocol for use in your routing and model co
Use Cases
Maintainer
Works with
Installation
Manual Installation
npx shieldConfiguration
Configuration Details
claude_desktop_config.json
Performance
Response Metrics
Resource Usage
How to Set Up and Use Shield
Shield is a lightweight Ruby authentication library (~110 lines of code) that provides a clean protocol for adding authentication to Rack-based web applications such as Sinatra, Cuba, or Rails. It supplies three composable modules — Shield::Model for password hashing and user lookup, Shield::Helpers for login/logout session management, and Shield::Middleware for automatic HTTP 401 redirect handling — all without imposing a specific ORM. Developers use it to secure MCP routing layers and model context communication endpoints where a minimal, auditable auth layer is preferred over a full-featured framework.
Prerequisites
- Ruby 2.0+ runtime
- A Rack-compatible web framework (Sinatra, Cuba, Rails, or similar)
- An ORM or data layer with a User model (Sequel, Ohm, ActiveRecord, etc.)
- RubyGems package manager
- Session middleware configured (e.g., Rack::Session::Cookie)
Install the gem
Add Shield to your Gemfile or install it directly via RubyGems.
gem install shield
# Or add to Gemfile:
# gem 'shield'Implement Shield::Model on your User model
Include Shield::Model in your User class and implement the fetch class method that looks up a user by login string. Shield then provides authenticate and password= automatically.
class User
include Shield::Model
# Return a user by email or username
def self.fetch(login)
first(email: login.to_s.downcase)
end
endInclude Shield::Helpers in your application
Mix Shield::Helpers into your Rack app or controller so you have access to login, logout, authenticated, and remember helpers.
class App < Sinatra::Base
include Shield::Helpers
# Rack session is required
use Rack::Session::Cookie, secret: ENV['SESSION_SECRET']
endAdd login and logout routes
Use the provided helpers inside your route handlers to authenticate users and manage sessions.
post '/login' do
if login(User, params[:login], params[:password])
redirect(params[:return] || '/')
else
redirect '/login'
end
end
get '/logout' do
logout(User)
redirect '/login'
endOptionally add Shield::Middleware for automatic 401 handling
Mount the middleware to intercept any HTTP 401 response from your application and redirect the user to the login page, preserving the original URL as a return parameter.
use Shield::Middleware, '/login'Shield Examples
Minimal Sinatra app with Shield
A complete minimal example showing Shield integrated into a Sinatra application with session management.
{
"note": "Shield is a Ruby gem, not an MCP server with a JSON config. Integrate via Gemfile.",
"gemfile": "gem 'shield'",
"usage": "include Shield::Model in User, include Shield::Helpers in App"
}Prompts to try
Shield is a Ruby library, not an interactive MCP tool. Typical developer queries when working with it:
- "How do I implement Shield::Model for a Sequel-backed User class?"
- "Show me how to add remember-me functionality with Shield::Helpers"
- "How do I protect all routes behind Shield::Middleware?"
- "Explain how Shield hashes passwords using Armor/PBKDF2"Troubleshooting Shield
login() always returns false even with correct credentials
Ensure User.fetch returns an object that responds to crypted_password and that the password was originally set via the password= setter (which hashes it with Armor). Direct assignment of plain-text passwords will not work.
Shield::Middleware redirects loop indefinitely
Make sure the login path itself does not require authentication. Shield::Middleware intercepts 401 responses; if your login route also returns 401, you will get a redirect loop. Whitelist the login path in your access controls.
session not persisting between requests
Verify that Rack::Session::Cookie (or your chosen session middleware) is mounted before Shield::Helpers methods are called, and that the SESSION_SECRET environment variable is set to a stable value — rotating it invalidates all existing sessions.
Frequently Asked Questions about Shield
What is Shield?
Shield is a Model Context Protocol (MCP) server that authentication protocol for use in your routing and model context It connects AI assistants to external tools and data sources through a standardized interface.
How do I install Shield?
Follow the installation instructions on the Shield GitHub repository. Clone the repo, install dependencies, and add the server config to your AI client.
Which AI clients work with Shield?
Shield works with all major MCP-compatible AI clients including Claude Desktop, Claude Code, Cursor, VS Code (GitHub Copilot), Windsurf, and Cline.
Is Shield free to use?
Yes, Shield is open source and available under the MIT license. You can use it freely in both personal and commercial projects.
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