CSP: full config with env vars per source (#2069)

* Configure all sources with CSP_* vars. Space separated source list.
* Fall back to `config.x.content_security_policy.*`
* Move our sources to fizzy-saas

References https://github.com/basecamp/fizzy-saas/pull/24

Gemfile.saas.lock

@@ -21,7 +21,7 @@ GIT
 
 GIT
   remote: https://github.com/basecamp/fizzy-saas
-  revision: 43dbc896ce7b6a08194a92ddd1695d3f1ebf554b
+  revision: 509c9ddecde0cbcc7752dd023a9c73cc50accdc5
   specs:
     fizzy-saas (0.1.0)
       audits1984

config/initializers/content_security_policy.rb

@@ -2,9 +2,40 @@
 
 # Define an application-wide Content Security Policy.
 # https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy
+#
+# Directives are configurable via environment variables with fallback to config.x
+# settings. This allows fizzy-saas (or other deployments) to extend the base policy
+# without duplicating it.
+#
+# ENV vars (space-separated sources):
+#   CSP_DEFAULT_SRC, CSP_SCRIPT_SRC, CSP_STYLE_SRC, CSP_CONNECT_SRC, CSP_FRAME_SRC,
+#   CSP_IMG_SRC, CSP_FONT_SRC, CSP_MEDIA_SRC, CSP_WORKER_SRC, CSP_FRAME_ANCESTORS,
+#   CSP_FORM_ACTION, CSP_REPORT_URI, CSP_REPORT_ONLY, DISABLE_CSP
+#
+# config.x.content_security_policy.* (string, space-separated string, or array):
+#   script_src, style_src, connect_src, frame_src, img_src, font_src, media_src,
+#   worker_src, frame_ancestors, form_action, report_uri, report_only
 
 Rails.application.configure do
-  # Configure with environment variables with fallback to config.x values (via fizzy-sass)
+  # Helper to get additional CSP sources from ENV or config.x.
+  # Supports: nil, string, space-separated string, or array.
+  sources = ->(directive) do
+    env_key = "CSP_#{directive.to_s.upcase}"
+    value = if ENV.key?(env_key)
+      ENV[env_key]
+    else
+      config.x.content_security_policy.send(directive)
+    end
+
+    case value
+    when nil then []
+    when Array then value
+    when String then value.split
+    else []
+    end
+  end
+
+  # Report URI and report-only mode
   report_uri = ENV.fetch("CSP_REPORT_URI") { config.x.content_security_policy.report_uri }
   report_only =
     if ENV.key?("CSP_REPORT_ONLY")
@@ -18,23 +49,25 @@ Rails.application.configure do
   config.content_security_policy_nonce_directives = %w[ script-src ]
 
   config.content_security_policy do |policy|
-    policy.default_src :self
-    policy.script_src :self, "https://challenges.cloudflare.com"
-    policy.connect_src :self, "https://storage.basecamp.com"
-    policy.frame_src :self, "https://challenges.cloudflare.com"
+    policy.default_src :self, *sources.(:default_src)
+    policy.script_src :self, *sources.(:script_src)
+    policy.connect_src :self, *sources.(:connect_src)
+    policy.frame_src :self, *sources.(:frame_src)
 
     # Don't fight user tools: permit inline styles, data:/https: sources, and
     # blob: workers for accessibility extensions, privacy tools, and custom fonts.
-    policy.style_src :self, :unsafe_inline
-    policy.img_src :self, "blob:", "data:", "https:"
-    policy.font_src :self, "data:", "https:"
-    policy.media_src :self, "blob:", "data:", "https:"
-    policy.worker_src :self, "blob:"
+    policy.style_src :self, :unsafe_inline, *sources.(:style_src)
+    policy.img_src :self, "blob:", "data:", "https:", *sources.(:img_src)
+    policy.font_src :self, "data:", "https:", *sources.(:font_src)
+    policy.media_src :self, "blob:", "data:", "https:", *sources.(:media_src)
+    policy.worker_src :self, "blob:", *sources.(:worker_src)
 
+    # Security-critical defaults (not configurable)
     policy.object_src :none
     policy.base_uri :none
-    policy.form_action :self
-    policy.frame_ancestors :self
+
+    policy.form_action :self, *sources.(:form_action)
+    policy.frame_ancestors :self, *sources.(:frame_ancestors)
 
     # Specify URI for violation reports (e.g., Sentry CSP endpoint)
     policy.report_uri report_uri if report_uri