ADR-0014: Security Headers and Environment-Aware CORS Middleware¶

• Status: Accepted
• Date: 2025-08-17
• Deciders: Core Engineering Team
• Issues: #344, #533
• Related: Addresses all 9 security headers identified by nodejsscan

Context¶

The MCP Gateway needed comprehensive security headers and proper CORS configuration to prevent common web attacks including XSS, clickjacking, MIME sniffing, and cross-origin attacks. Additionally, the nodejsscan static analysis tool identified 9 missing security headers specifically for the Admin UI and static assets.

The previous implementation had: - Basic CORS middleware with wildcard origins in some configurations - Limited security headers only in the DocsAuthMiddleware - No comprehensive security header implementation - Manual CORS origin configuration without environment awareness - Admin UI cookie settings without proper security attributes - No static analysis tool compatibility

Security requirements included: - Essential security headers for all responses (issue #344) - Configurable security headers for Admin UI and static assets (issue #533) - Environment-aware CORS configuration for development vs production - Secure cookie handling for authentication - Admin UI compatibility with Content Security Policy - Static analysis compatibility for nodejsscan and similar tools - Backward compatibility with existing configurations

Decision¶

We implemented a comprehensive security middleware solution with the following components:

1. SecurityHeadersMiddleware¶

Created mcpgateway/middleware/security_headers.py that automatically adds essential security headers to all responses:

# Essential security headers
response.headers["X-Content-Type-Options"] = "nosniff"
response.headers["X-Frame-Options"] = "DENY"
response.headers["X-XSS-Protection"] = "0"  # Modern browsers use CSP
response.headers["X-Download-Options"] = "noopen"  # Prevent IE downloads
response.headers["Referrer-Policy"] = "strict-origin-when-cross-origin"

# Content Security Policy (Admin UI compatible)
csp_directives = [
    "default-src 'self'",
    "script-src 'self' 'unsafe-inline' 'unsafe-eval' https://cdnjs.cloudflare.com https://cdn.tailwindcss.com https://cdn.jsdelivr.net",
    "style-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com",
    "img-src 'self' data: https:",
    "font-src 'self' data:",
    "connect-src 'self' ws: wss: https:",
    "frame-ancestors 'none'"
]

# HSTS for HTTPS connections
if request.url.scheme == "https" or request.headers.get("X-Forwarded-Proto") == "https":
    response.headers["Strict-Transport-Security"] = "max-age=31536000; includeSubDomains"

# Remove sensitive headers
del response.headers["X-Powered-By"]  # if present
del response.headers["Server"]        # if present

2. Environment-Aware CORS Configuration¶

Enhanced CORS setup in mcpgateway/main.py with automatic origin configuration:

Development Environment: - Automatically configures origins for common development ports: localhost:3000, localhost:8080, gateway port - Includes both localhost and 127.0.0.1 variants - Allows HTTP origins for development convenience

Production Environment: - Constructs HTTPS origins from APP_DOMAIN setting - Creates origins: https://{domain}, https://app.{domain}, https://admin.{domain} - Enforces HTTPS-only origins - Never uses wildcard origins

3. Secure Cookie Utilities¶

Added mcpgateway/utils/security_cookies.py with functions for secure authentication:

def set_auth_cookie(response: Response, token: str, remember_me: bool = False):
    use_secure = (settings.environment == "production") or settings.secure_cookies
    response.set_cookie(
        key="jwt_token",
        value=token,
        max_age=30 * 24 * 3600 if remember_me else 3600,
        httponly=True,      # Prevents JavaScript access
        secure=use_secure,  # HTTPS only in production
        samesite=settings.cookie_samesite,  # CSRF protection
        path="/"
    )

4. Configurable Security Headers¶

Added comprehensive configuration options to mcpgateway/config.py for all security headers:

# Environment awareness
environment: str = Field(default="development", env="ENVIRONMENT")
app_domain: str = Field(default="localhost", env="APP_DOMAIN")

# Cookie Security
secure_cookies: bool = Field(default=True, env="SECURE_COOKIES")
cookie_samesite: str = Field(default="lax", env="COOKIE_SAMESITE")

# CORS Configuration
cors_allow_credentials: bool = Field(default=True, env="CORS_ALLOW_CREDENTIALS")

# Security Headers Configuration (issue #533)
security_headers_enabled: bool = Field(default=True, env="SECURITY_HEADERS_ENABLED")
x_frame_options: str = Field(default="DENY", env="X_FRAME_OPTIONS")
x_content_type_options_enabled: bool = Field(default=True, env="X_CONTENT_TYPE_OPTIONS_ENABLED")
x_xss_protection_enabled: bool = Field(default=True, env="X_XSS_PROTECTION_ENABLED")
x_download_options_enabled: bool = Field(default=True, env="X_DOWNLOAD_OPTIONS_ENABLED")
hsts_enabled: bool = Field(default=True, env="HSTS_ENABLED")
hsts_max_age: int = Field(default=31536000, env="HSTS_MAX_AGE")
hsts_include_subdomains: bool = Field(default=True, env="HSTS_INCLUDE_SUBDOMAINS")
remove_server_headers: bool = Field(default=True, env="REMOVE_SERVER_HEADERS")

5. Static Analysis Tool Compatibility¶

Added security meta tags to mcpgateway/templates/admin.html for static analysis tool compatibility:

<!-- Security meta tags for static analysis tools (complement HTTP headers) -->
<meta http-equiv="Content-Security-Policy" content="..." />
<meta http-equiv="X-Frame-Options" content="DENY" />
<meta http-equiv="X-Content-Type-Options" content="nosniff" />
<meta http-equiv="X-XSS-Protection" content="1; mode=block" />
<meta http-equiv="X-Download-Options" content="noopen" />

6. Enhanced Static Analysis¶

Updated Makefile to scan both static files and templates:

nodejsscan:
    @$(VENV_DIR)/bin/nodejsscan --directory ./mcpgateway/static --directory ./mcpgateway/templates || true

Consequences¶

✅ Benefits¶

• Comprehensive Protection: All responses include essential security headers
• Automatic Configuration: CORS origins are automatically configured based on environment
• Admin UI Compatible: CSP allows required CDN resources while maintaining security
• Production Ready: Secure defaults for production deployments
• Development Friendly: Permissive localhost origins for development
• Backward Compatible: Existing configurations continue to work
• Cookie Security: Authentication cookies automatically configured with security flags
• HTTPS Detection: HSTS header added automatically when HTTPS is detected

❌ Trade-offs¶

• CSP Flexibility: Using 'unsafe-inline' and 'unsafe-eval' for Admin UI compatibility
• CDN Dependencies: CSP allows specific external CDN domains
• Configuration Complexity: More environment variables to configure
• Development Overhead: Additional middleware processing on every request

🔄 Maintenance¶

• CSP Updates: May need updates if Admin UI adds new external dependencies
• CDN Changes: CSP must be updated if CDN URLs change
• Security Reviews: Periodic review of CSP directives for security improvements
• Browser Updates: Monitor browser CSP implementation changes

Alternatives Considered¶

Implementation Details¶

Middleware Order¶

# Order matters - security headers should be added after CORS
app.add_middleware(CORSMiddleware, ...)      # 1. CORS first
app.add_middleware(SecurityHeadersMiddleware) # 2. Security headers
app.add_middleware(DocsAuthMiddleware)       # 3. Auth protection

Environment Detection¶

• Uses ENVIRONMENT setting to determine development vs production mode
• Falls back to safe defaults if environment not specified
• Only applies automatic origins when using default configuration

CSP Design Decisions¶

• 'unsafe-inline': Required for Tailwind CSS inline styles and Alpine.js
• 'unsafe-eval': Required for some JavaScript frameworks used in Admin UI
• Specific CDN domains: Whitelisted known-good CDN sources instead of wildcard
• 'frame-ancestors none': Prevents all framing to prevent clickjacking

iframe Embedding Configuration¶

By default, iframe embedding is disabled for security via X-Frame-Options: DENY and frame-ancestors 'none'. To enable iframe embedding:

1. Same-domain embedding: Set X_FRAME_OPTIONS=SAMEORIGIN
2. Specific domain embedding: Set X_FRAME_OPTIONS=ALLOW-FROM https://trusted-domain.com
3. Disable frame protection: Set X_FRAME_OPTIONS="" (not recommended)

Note: When changing X-Frame-Options, also consider updating the CSP frame-ancestors directive for comprehensive browser support.

Testing Strategy¶

Implemented comprehensive test coverage (42 new tests): - Security headers validation across all endpoints - CORS behavior testing for allowed and blocked origins - Environment-aware configuration testing - Cookie security attributes validation - Production security posture verification - CSP directive structure validation - HSTS behavior testing

Future Enhancements¶

Potential improvements for future iterations: - CSP Nonces: Replace 'unsafe-inline' with nonces for dynamic content - Subresource Integrity: Add SRI for external CDN resources - CSP Violation Reporting: Implement CSP violation reporting endpoint - Per-Route CSP: Different CSP policies for different endpoints - Security Header Compliance: Monitoring dashboard for header compliance

Status¶

This security headers and CORS middleware implementation is accepted and implemented as of version 0.5.0, providing comprehensive security coverage while maintaining compatibility with existing functionality.