Content Security Policy (#1964)

Default to a very narrow policy since there are no CDNs or third-party
resources to contend with.

Configurable via:
- config.x.content_security_policy.* for fizzy-saas gem overrides
- DISABLE_CSP to skip entirely
- CSP_REPORT_ONLY to enable report-only mode
- CSP_REPORT_URI for violation reporting

Author: jeremy

Gemfile.saas.lock

@@ -21,7 +21,7 @@ GIT
 
 GIT
   remote: https://github.com/basecamp/fizzy-saas
-  revision: 97d126e0a6084905aceaadf4facaa2a48b44e124
+  revision: 3b30f3d56d8318bb9256469cb91c06a5a5c760a6
   specs:
     fizzy-saas (0.1.0)
       audits1984

config/initializers/content_security_policy.rb

@@ -1,25 +1,34 @@
 # Be sure to restart your server when you modify this file.
 
-# Define an application-wide content security policy.
-# See the Securing Rails Applications Guide for more information:
-# https://guides.rubyonrails.org/security.html#content-security-policy-header
+# Define an application-wide Content Security Policy.
+# https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy
 
-# 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)
-#
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+Rails.application.configure do
+  # Configure from environment variables (fizzy-saas can override by setting config.x values first)
+  config.x.content_security_policy.report_uri ||= ENV["CSP_REPORT_URI"]
+  config.x.content_security_policy.report_only ||= ENV["CSP_REPORT_ONLY"] == "true"
+
+  # Generate nonces for importmap and inline scripts
+  config.content_security_policy_nonce_generator = ->(request) { SecureRandom.base64(16) }
+  config.content_security_policy_nonce_directives = %w[ script-src ]
+
+  config.content_security_policy do |policy|
+    policy.script_src :self
+    policy.style_src :self, :unsafe_inline
+    policy.img_src :self, "blob:", "data:", "https:"
+    policy.font_src :self
+    policy.object_src :none
+
+    policy.base_uri :none
+    policy.form_action :self
+    policy.frame_ancestors :self
+
+    # Specify URI for violation reports (e.g., Sentry CSP endpoint)
+    if report_uri = config.x.content_security_policy.report_uri
+      policy.report_uri report_uri
+    end
+  end
+
+  # Report violations without enforcing the policy.
+  config.content_security_policy_report_only = config.x.content_security_policy.report_only
+end unless ENV["DISABLE_CSP"]