Cookie Security Design

Cookie Security Design

Overview

This document explains the design decisions behind the session cookie security implementation in the MCP Gateway Registry, particularly regarding the use of domain cookies for cross-subdomain authentication.

Background

The MCP Gateway Registry supports authentication through both traditional username/password and OAuth2 providers. In deployments where the auth server and registry are on different subdomains (e.g., auth.example.com and registry.example.com), session cookies must be shared across these subdomains for seamless authentication.

Design Decision: Single-Tenant Architecture

This implementation is designed for single-tenant deployments where:

• All subdomains are owned and controlled by a single organization
• Cross-subdomain cookie sharing is a desired feature, not a security risk
• Users authenticate once and access multiple services on different subdomains

Cookie Security Configuration

Environment Variables

Two key environment variables control cookie security behavior:

1. SESSION_COOKIE_SECURE (default: false)

   • Set to true in production deployments with HTTPS
   • When true, cookies are only transmitted over HTTPS connections
   • Prevents man-in-the-middle (MITM) attacks and session hijacking
   • Production Requirement: MUST be set to true when deployed with HTTPS

2. SESSION_COOKIE_DOMAIN (default: None or empty string)

   • MUST be explicitly configured - no automatic domain inference
   • When set (e.g., .example.com), enables cross-subdomain cookie sharing
   • Must start with a dot (.) to match all subdomains
   • When None or empty, cookies are scoped to the exact host that sets them
   • Format: .example.com (note the leading dot)
   • Important: Set to empty string "" for single-domain deployments
   • Examples:
     • Single domain (mcpgateway.ddns.net): Leave unset or set to ""
     • Cross-subdomain (auth.example.com, registry.example.com): Set to .example.com
     • Multi-level domains (registry.region-1.corp.company.internal): Set to .corp.company.internal if cross-subdomain sharing needed

HTTPS Termination Detection

Critical Implementation Detail: The auth server intelligently handles HTTPS termination at load balancers (ALB, nginx, etc.):

• Backend sees HTTP but load balancer terminates HTTPS → Common in AWS ALB, nginx reverse proxy
• Solution: Auth server checks X-Forwarded-Proto header to detect original protocol
• Behavior: Cookie secure flag is set based on original request protocol, not backend protocol

Code Logic (auth_server/server.py:1797-1803):

x_forwarded_proto = request.headers.get("x-forwarded-proto", "")
is_https = x_forwarded_proto == "https" or request.url.scheme == "https"

# Only set secure=True if the original request was HTTPS
cookie_secure_config = OAUTH2_CONFIG.get("session", {}).get("secure", False)
cookie_secure = cookie_secure_config and is_https

Important:

• If SESSION_COOKIE_SECURE=true but is_https=False, the secure flag will NOT be set
• This prevents login failures when HTTPS termination is misconfigured
• Check server logs for is_https=True in production to verify HTTPS detection is working

Cookie Security Flags

The implementation sets the following security flags on all session cookies:

Security Considerations

✅ Safe Deployment Scenarios

This design is SAFE for:

1. Single-Tenant Production Deployments

   • Example: auth.company.com and registry.company.com
   • All subdomains owned by the same organization
   • Configuration:

     SESSION_COOKIE_SECURE=true
     SESSION_COOKIE_DOMAIN=.company.com
     

2. Local Development (localhost)

   • Local development on localhost via HTTP
   • Configuration:

     SESSION_COOKIE_SECURE=false  # MUST be false for HTTP
     SESSION_COOKIE_DOMAIN=       # Leave unset/empty
     

   • Important: Setting SESSION_COOKIE_SECURE=true on localhost will cause login to fail because cookies with secure=true are only sent over HTTPS, and localhost typically runs over HTTP.

⚠️ Unsafe Deployment Scenarios

This design is NOT SAFE for:

1. Multi-Tenant SaaS Deployments

   • Example: customer1.saas-platform.com and customer2.saas-platform.com
   • Risk: Setting SESSION_COOKIE_DOMAIN=.saas-platform.com would allow:
     • Customer A to access Customer B's sessions
     • Cross-tenant authentication bypass
     • Serious data breach potential

2. Shared Hosting Environments

   • Multiple organizations sharing the same root domain
   • Risk: Similar to multi-tenant scenario

Alternative Solutions for Multi-Tenant

If you need multi-tenant deployment, consider these alternatives:

1. Token-Based Authentication

   • Use JWT tokens passed via headers instead of cookies
   • Tokens explicitly scoped to each tenant
   • No domain-sharing concerns

2. Separate Auth Domains per Tenant

   • customer1-auth.platform.com and customer1-app.platform.com
   • Different root domains prevent cookie sharing between tenants

3. Reverse Proxy with Path-Based Routing

   • Single domain with path-based service routing
   • Example: platform.com/auth and platform.com/registry
   • No cross-subdomain cookie requirements

4. Centralized OAuth Flow

   • OAuth server on separate domain
   • Token exchange instead of session cookies
   • Better tenant isolation

Attack Scenarios Mitigated

1. Session Hijacking (MITM)

• Threat: Attacker intercepts session cookies over unencrypted HTTP
• Mitigation: secure=True flag in production
• Status: ✅ Mitigated when SESSION_COOKIE_SECURE=true

2. Cross-Site Scripting (XSS)

• Threat: Malicious JavaScript reads session cookies
• Mitigation: httponly=True flag
• Status: ✅ Always mitigated

3. Cross-Site Request Forgery (CSRF)

• Threat: Malicious site triggers authenticated requests
• Mitigation: samesite="lax" flag
• Status: ✅ Always mitigated

4. Subdomain Cookie Theft (Single-Tenant)

• Threat: Attacker controls a subdomain and steals cookies
• Mitigation: Only valid in trusted single-tenant environments
• Status: ⚠️ Acceptable risk for single-tenant deployments

Production Deployment Checklist

Before deploying to production:

• Set SESSION_COOKIE_SECURE=true in environment (REQUIRED for HTTPS)
• Verify HTTPS is properly configured and enforced
• IMPORTANT: If using load balancer with HTTPS termination, ensure X-Forwarded-Proto header is set
• Set SESSION_COOKIE_DOMAIN appropriately:
  • Empty string or unset for single-domain deployments (RECOMMENDED - safest)
  • .example.com only if you need cross-subdomain authentication
• Confirm you are deploying in a single-tenant architecture (NOT multi-tenant SaaS)
• Test cross-subdomain authentication between auth and registry services (if using domain cookies)
• Verify cookies are NOT transmitted over HTTP in production
• Review server logs for cookie configuration at startup:
  • Check for Auth server setting session cookie: secure=True
  • Verify domain setting matches your configuration
  • Confirm is_https=True in production

Example Production Configurations

Single-Domain Deployment (RECOMMENDED - Most Secure):

# .env for production - single domain (e.g., mcpgateway.example.com)
SESSION_COOKIE_SECURE=true  # REQUIRED for HTTPS
SESSION_COOKIE_DOMAIN=      # Empty = exact host only (safest)
SESSION_COOKIE_NAME=mcp_gateway_session
SESSION_MAX_AGE_SECONDS=28800  # 8 hours
AUTH_SERVER_URL=http://auth-server:8888  # Internal URL
AUTH_SERVER_EXTERNAL_URL=https://mcpgateway.example.com  # External URL

Cross-Subdomain Deployment:

# .env for production - cross-subdomain (e.g., auth.example.com + registry.example.com)
SESSION_COOKIE_SECURE=true  # REQUIRED for HTTPS
SESSION_COOKIE_DOMAIN=.example.com  # Note the leading dot
SESSION_COOKIE_NAME=mcp_gateway_session
SESSION_MAX_AGE_SECONDS=28800  # 8 hours
AUTH_SERVER_URL=http://auth-server:8888  # Internal URL
AUTH_SERVER_EXTERNAL_URL=https://auth.example.com  # External URL

Code Implementation

The cookie security implementation is found in:

• Configuration: registry/core/config.py

  • session_cookie_secure: Controls HTTPS-only flag
  • session_cookie_domain: Controls cross-subdomain sharing

• Auth Server Cookie Setting: auth_server/server.py (lines 1800-1831)

  • X-Forwarded-Proto detection for HTTPS termination at load balancer
  • Explicit configuration only - no automatic domain inference
  • Conditional secure flag based on both config AND actual protocol
  • All security flags properly set

• Registry Cookie Setting: registry/auth/routes.py (lines 139-158)

  • Comprehensive security comments explaining single-tenant model
  • Conditional domain attribute application
  • All security flags properly set

Monitoring and Validation

Runtime Validation

The auth server logs detailed cookie configuration for debugging:

logger.info(f"Auth server setting session cookie: secure={cookie_secure} (config={cookie_secure_config}, is_https={is_https}), samesite={cookie_samesite}, domain={cookie_domain or 'not set'}, x-forwarded-proto={x_forwarded_proto}, request_scheme={request.url.scheme}")

Key logging details:

• secure: Final secure flag value (after protocol detection)
• config: Configured SESSION_COOKIE_SECURE value
• is_https: Whether the original request was HTTPS (based on X-Forwarded-Proto or request scheme)
• domain: Configured domain or "not set"
• x-forwarded-proto: Load balancer protocol header
• request_scheme: Direct request protocol

The registry logs successful login events:

logger.info(f"User '{username}' logged in successfully.")

Security Auditing

Periodically review:

1. Cookie flags are properly set in browser developer tools
2. Cookies are NOT transmitted over HTTP in production
3. secure flag is enabled in production environments
4. Domain scope matches your deployment architecture

Browser Developer Tools Verification

In your browser's developer tools (Application/Storage → Cookies), verify:

References

• OWASP Session Management Cheat Sheet
• MDN: Set-Cookie HTTP Header
• RFC 6265: HTTP State Management Mechanism

Contact

For questions or security concerns regarding this implementation, please:

• Open an issue in the GitHub repository
• Tag the issue with security label
• Provide details about your deployment scenario