diff --git a/app/controllers/api/forward_auth_controller.rb b/app/controllers/api/forward_auth_controller.rb
index 2b722df..5a06ea2 100644
--- a/app/controllers/api/forward_auth_controller.rb
+++ b/app/controllers/api/forward_auth_controller.rb
@@ -135,9 +135,6 @@ module Api
     def render_unauthorized(reason = nil)
       Rails.logger.info "ForwardAuth: Unauthorized - #{reason}"
 
-      # Set header to help with debugging
-      response.headers["X-Auth-Reason"] = reason if reason
-
       # Get the redirect URL from query params or construct default
       redirect_url = validate_redirect_url(params[:rd])
       base_url = redirect_url || "https://clinch.aapamilne.com"
@@ -179,9 +176,6 @@ module Api
     def render_forbidden(reason = nil)
       Rails.logger.info "ForwardAuth: Forbidden - #{reason}"
 
-      # Set header to help with debugging
-      response.headers["X-Auth-Reason"] = reason if reason
-
       # Return 403 Forbidden
       head :forbidden
     end
diff --git a/config/environments/production.rb b/config/environments/production.rb
index 6211472..af3d06b 100644
--- a/config/environments/production.rb
+++ b/config/environments/production.rb
@@ -81,11 +81,37 @@ Rails.application.configure do
   config.active_record.attributes_for_inspect = [ :id ]
 
   # Enable DNS rebinding protection and other `Host` header attacks.
-  # config.hosts = [
-  #   "example.com",     # Allow requests from example.com
-  #   /.*\.example\.com/ # Allow requests from subdomains like `www.example.com`
-  # ]
-  #
+  # Configure allowed hosts based on deployment scenario
+  allowed_hosts = [
+    ENV.fetch('CLINCH_HOST', 'auth.aapamilne.com'),  # External domain
+    /.*#{ENV.fetch('CLINCH_HOST', 'aapamilne\.com').gsub('.', '\.')}/  # Subdomains
+  ]
+
+  # Allow Docker service names if running in same compose
+  if ENV['CLINCH_DOCKER_SERVICE_NAME']
+    allowed_hosts << ENV['CLINCH_DOCKER_SERVICE_NAME']
+  end
+
+  # Allow internal IP access for cross-compose or host networking
+  if ENV['CLINCH_ALLOW_INTERNAL_IPS'] == 'true'
+    # Specific host IP
+    allowed_hosts << '192.168.2.246'
+
+    # Private IP ranges for internal network access
+    allowed_hosts += [
+      /192\.168\.\d+\.\d+/,    # 192.168.0.0/16 private network
+      /10\.\d+\.\d+\.\d+/,     # 10.0.0.0/8 private network
+      /172\.(1[6-9]|2[0-9]|3[0-1])\.\d+\.\d+/  # 172.16.0.0/12 private network
+    ]
+  end
+
+  # Local development fallbacks
+  if ENV['CLINCH_ALLOW_LOCALHOST'] == 'true'
+    allowed_hosts += ['localhost', '127.0.0.1', '0.0.0.0']
+  end
+
+  config.hosts = allowed_hosts
+
   # Skip DNS rebinding protection for the default health check endpoint.
-  # config.host_authorization = { exclude: ->(request) { request.path == "/up" } }
+  config.host_authorization = { exclude: ->(request) { request.path == "/up" } }
 end
diff --git a/config/initializers/content_security_policy.rb b/config/initializers/content_security_policy.rb
index d51d713..10b1ccc 100644
--- a/config/initializers/content_security_policy.rb
+++ b/config/initializers/content_security_policy.rb
@@ -4,26 +4,74 @@
 # See the Securing Rails Applications Guide for more information:
 # https://guides.rubyonrails.org/security.html#content-security-policy-header
 
-# Rails.application.configure do
-#   config.content_security_policy do |policy|
-#     policy.default_src :self, :https
-#     policy.font_src    :self, :https, :data
-#     policy.img_src     :self, :https, :data
-#     policy.object_src  :none
-#     policy.script_src  :self, :https
-#     policy.style_src   :self, :https
-#     # Specify URI for violation reports
-#     # policy.report_uri "/csp-violation-report-endpoint"
-#   end
-#
-#   # Generate session nonces for permitted importmap, inline scripts, and inline styles.
-#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
-#   config.content_security_policy_nonce_directives = %w(script-src style-src)
-#
-#   # Automatically add `nonce` to `javascript_tag`, `javascript_include_tag`, and `stylesheet_link_tag`
-#   # if the corresponding directives are specified in `content_security_policy_nonce_directives`.
-#   # config.content_security_policy_nonce_auto = true
-#
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+Rails.application.configure do
+  config.content_security_policy do |policy|
+    # Default policy: only allow resources from same origin and HTTPS
+    policy.default_src :self, :https
+
+    # Scripts: strict security with nonce support for dynamic content
+    policy.script_src :self, :https, :strict_dynamic
+
+    # Styles: allow inline styles for CSS frameworks, but require HTTPS
+    policy.style_src :self, :https, :unsafe_inline
+
+    # Images: allow data URIs for inline images and HTTPS sources
+    policy.img_src :self, :https, :data
+
+    # Fonts: allow self-hosted and HTTPS fonts, plus data URIs
+    policy.font_src :self, :https, :data
+
+    # Media: allow self and HTTPS media sources
+    policy.media_src :self, :https
+
+    # Objects: block potentially dangerous plugins
+    policy.object_src :none
+
+    # Base URI: restrict base tag to same origin
+    policy.base_uri :self
+
+    # Form actions: only allow forms to submit to same origin
+    policy.form_action :self
+
+    # Frame ancestors: prevent clickjacking by disallowing framing
+    policy.frame_ancestors :none
+
+    # Frame sources: block iframes unless explicitly needed
+    policy.frame_src :none
+
+    # Connect sources: control where XHR/Fetch can connect
+    policy.connect_src :self, :https
+
+    # Manifest: only allow same-origin manifest files
+    policy.manifest_src :self
+
+    # Worker sources: control web worker origins
+    policy.worker_src :self, :https
+
+    # Report URI: send violation reports to our monitoring endpoint
+    if Rails.env.production?
+      policy.report_uri "/api/csp-violation-report"
+    end
+  end
+
+  # Generate session nonces for permitted inline scripts and styles
+  config.content_security_policy_nonce_generator = ->(request) {
+    # Use a secure random nonce instead of session ID for better security
+    SecureRandom.base64(16)
+  }
+
+  # Apply nonces to script and style directives
+  config.content_security_policy_nonce_directives = %w(script-src style-src)
+
+  # Automatically add `nonce` attributes to script/style tags
+  config.content_security_policy_nonce_auto = true
+
+  # Enforce CSP in production, but use report-only in development for debugging
+  if Rails.env.production?
+    # Enforce the policy in production
+    config.content_security_policy_report_only = false
+  else
+    # Report violations only in development (helps with debugging)
+    config.content_security_policy_report_only = true
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index 8decd97..71d05d1 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -31,6 +31,7 @@ Rails.application.routes.draw do
   # ForwardAuth / Trusted Header SSO
   namespace :api do
     get "/verify", to: "forward_auth#verify"
+    post "/csp-violation-report", to: "csp#violation_report"
   end
 
   # Authenticated routes
diff --git a/docs/forward-auth.md b/docs/forward-auth.md
index 5939ad8..907cfe9 100644
--- a/docs/forward-auth.md
+++ b/docs/forward-auth.md
@@ -193,17 +193,179 @@ curl -v http://localhost:9000/api/verify?rd=https://clinch.example.com
 # Or 200 OK if you have a valid session cookie
 ```
 
+## Security Considerations
+
+### Content Security Policy (CSP)
+
+Clinch includes a comprehensive Content Security Policy to prevent Cross-Site Scripting (XSS) attacks by controlling which resources can be loaded by the browser.
+
+**What CSP Prevents:**
+- Malicious script injection attacks
+- Unauthorized resource loading
+- Clickjacking through iframe protection
+- Data exfiltration through unauthorized connections
+
+**CSP Features:**
+- **Strict script control**: Only allows scripts from same origin or HTTPS
+- **Nonce support**: Allows specific inline scripts with cryptographic nonces
+- **Frame protection**: Prevents clickjacking attacks
+- **Resource restrictions**: Controls images, fonts, styles, and media sources
+- **Violation reporting**: Monitors and logs attempted XSS attacks
+
+**Development vs Production:**
+- **Development**: Report-only mode for debugging CSP violations
+- **Production**: Full enforcement with violation logging
+
+### DNS Rebinding Protection
+
+Clinch includes built-in DNS rebinding protection for enhanced security in all deployment scenarios.
+
+**What is DNS Rebinding?**
+DNS rebinding attacks trick a victim's browser into accessing internal network resources by manipulating DNS responses, potentially allowing attackers to probe your authentication system.
+
+**Clinch's Protection Layers:**
+1. **Rails Host Validation**: Blocks unauthorized domains at the application level
+2. **Infrastructure Security**: Caddy/Reverse proxy provides additional protection
+3. **Environment-Specific Configuration**: Adapts to your deployment scenario
+
+### Deployment Scenarios
+
+#### Scenario 1: Same Docker Compose (Recommended)
+```yaml
+# docker-compose.yml
+services:
+  caddy:
+    # ... caddy configuration
+
+  clinch:
+    image: reg.tbdb.info/clinch:latest
+    environment:
+      - CLINCH_HOST=auth.aapamilne.com
+      - CLINCH_DOCKER_SERVICE_NAME=clinch  # Enable service name access
+      - CLINCH_ALLOW_INTERNAL_IPS=true     # Allow backup IP access
+      - CLINCH_ALLOW_LOCALHOST=false
+```
+
+**Caddy Configuration:**
+```caddyfile
+metube.aapamilne.com {
+    forward_auth clinch:3000 {  # Docker service name (preferred)
+        uri /api/verify
+        copy_headers Remote-User Remote-Email Remote-Groups Remote-Admin
+    }
+
+    handle {
+        reverse_proxy * {
+            to http://192.168.2.223:8081
+        }
+    }
+}
+```
+
+**Security Benefits:**
+- ✅ Docker network isolation prevents external access
+- ✅ Service names resolve to unpredictable internal IPs
+- ✅ Natural DNS rebinding protection
+- ✅ Application-level host validation as backup
+
+#### Scenario 2: Separate Docker Composes (Current Setup)
+```yaml
+# clinch-compose/.env
+CLINCH_HOST=auth.aapamilne.com
+CLINCH_ALLOW_INTERNAL_IPS=true
+CLINCH_ALLOW_LOCALHOST=false
+CLINCH_DOCKER_SERVICE_NAME=
+```
+
+**Caddy Configuration:**
+```caddyfile
+metube.aapamilne.com {
+    forward_auth 192.168.2.246:3000 {  # IP access across composes
+        uri /api/verify
+        copy_headers Remote-User Remote-Email Remote-Groups Remote-Admin
+    }
+}
+```
+
+**Security Benefits:**
+- ✅ Rails host validation blocks unauthorized domains
+- ✅ Only allows private IP ranges and your domain
+- ✅ Defense in depth (application + infrastructure security)
+
+#### Scenario 3: External Deployment
+```yaml
+# Production environment
+environment:
+  - CLINCH_HOST=auth.example.com
+  - CLINCH_ALLOW_INTERNAL_IPS=false  # Stricter for external
+  - CLINCH_ALLOW_LOCALHOST=false
+```
+
+**Caddy Configuration:**
+```caddyfile
+app.example.com {
+    forward_auth auth.example.com:3000 {  # External domain only
+        uri /api/verify
+        copy_headers Remote-User Remote-Email Remote-Groups Remote-Admin
+    }
+}
+```
+
+**Security Benefits:**
+- ✅ Only allows your external domain
+- ✅ Blocks internal IP access
+- ✅ Maximum security for public deployments
+
+### Host Validation Environment Variables
+
+| Variable | Default | Purpose | Recommended Setting |
+|----------|---------|---------|-------------------|
+| `CLINCH_HOST` | `auth.aapamilne.com` | Primary domain | Always set to your auth domain |
+| `CLINCH_DOCKER_SERVICE_NAME` | `nil` | Docker service name | Set to service name in same compose |
+| `CLINCH_ALLOW_INTERNAL_IPS` | `true` | Allow private IPs | `true` for internal, `false` for external |
+| `CLINCH_ALLOW_LOCALHOST` | `false` | Allow localhost access | `true` for development only |
+
+### Security Architecture
+
+Clinch provides **defense in depth** security with multiple protection layers:
+
+**Application-Level Security:**
+- Host validation prevents unauthorized domain access
+- Session-based authentication with secure cookies
+- Rate limiting on sensitive endpoints
+- Input validation and sanitization
+- Content Security Policy (CSP) prevents XSS attacks
+
+**Infrastructure Security:**
+- Docker network isolation
+- Reverse proxy access control
+- SSL/TLS encryption
+- Private network restrictions
+
+**Benefits of Multi-Layer Security:**
+- If infrastructure security fails, application security still protects
+- Flexible deployment options without compromising security
+- Environment-specific configuration for different threat models
+
 ## Troubleshooting
 
 ### Common Issues
 
 1. **Authentication Loop**: Check that cookies are set on the root domain
 2. **Session Not Shared**: Verify `extract_root_domain` is working correctly
-3. **Caddy Connection**: Ensure `clinch:9000` resolves from your Caddy container
+3. **Caddy Connection**: Ensure service name/IP resolves from your Caddy container
 4. **Race Condition After Authentication**:
    - **Problem**: Forward auth fails immediately after login due to cookie timing
    - **Solution**: One-time tokens automatically bridge this gap
    - **Debug**: Look for "ForwardAuth: Valid one-time token used" in logs
+5. **Host Validation Errors**:
+   - **Problem**: "Blocked host: [host]" errors in logs
+   - **Solution**: Check `CLINCH_HOST` and other environment variables
+   - **Debug**: Verify your Caddy configuration matches allowed hosts
+6. **DNS Rebinding Protection**:
+   - **Problem**: Legitimate requests blocked as "unauthorized host"
+   - **Solution**: Ensure your deployment scenario matches environment variables
+   - **Debug**: Check Rails logs for host validation messages
 
 ### Debug Logging