mastodon/config/initializers/content_security_policy.rb

# Define an application-wide content security policy
# For further information see the following documentation
# https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy

assets_host = Rails.configuration.action_controller.asset_host || "https://#{ENV['WEB_DOMAIN'] || ENV['LOCAL_DOMAIN']}"

Rails.application.config.content_security_policy do |p|
  p.base_uri        :none
  p.default_src     :none
  p.frame_ancestors :none
  p.script_src      :self, assets_host
  p.font_src        :self, assets_host
  p.img_src         :self, :https, :data, :blob
  p.style_src       :self, :unsafe_inline, assets_host
  p.media_src       :self, :data, assets_host
  p.frame_src       :self, :https
  p.connect_src     :self, assets_host, Rails.configuration.x.streaming_api_base_url
end

# Report CSP violations to a specified URI
# For further information see the following documentation:
# https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only
# Rails.application.config.content_security_policy_report_only = true