Add a Report-Only Content-Security-Policy baseline

config/initializers/content_security_policy.rb

@@ -1,25 +1,25 @@
 # Be sure to restart your server when you modify this file.
 
-# Define an application-wide content security policy.
+# Baseline application-wide Content-Security-Policy, deployed in Report-Only
+# mode: browsers evaluate the policy and report violations (once a report
+# endpoint is wired up) but enforce nothing, so rendering cannot break.
+# Tune the policy against observed violations, then flip
+# `content_security_policy_report_only` off to enforce it.
+#
 # 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)
-#
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+Rails.application.configure do
+  config.content_security_policy do |policy|
+    policy.default_src     :self
+    policy.object_src      :none
+    policy.base_uri        :self
+    policy.frame_ancestors :self
+    policy.form_action     :self
+    # Specify URI for violation reports once a report sink is available
+    # policy.report_uri "/csp-violation-report-endpoint"
+  end
+
+  # Report violations without enforcing the policy.
+  config.content_security_policy_report_only = true
+end