Add a Report-Only Content-Security-Policy baseline

config/initializers/content_security_policy.rb

@@ -1,25 +1,73 @@
 # 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
+# Session-stable nonce for permitted inline scripts (the importmap JSON + shim,
+# auto-nonced by importmap-rails).
 #
-#   # 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)
+# The nonce is stable across a session's requests so Turbo snapshot restores
+# don't replay a stale nonce and trip CSP. It's the HMAC of a stable cookie
+# value keyed by the server secret: the cookie is client-settable, but the
+# client can't predict the resulting nonce without knowing secret_key_base.
 #
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+# Writebook sets no per-session verification cookie, so the lightweight
+# nonce_id cookie — set on first visit, present for every session including
+# unauthenticated ones — is the sole identifier.
+module CSP
+  module Nonce
+    COOKIE = "writebook_csp_nonce_id"
+
+    def self.generate(request)
+      hmac(nonce_id(request))
+    end
+
+    def self.hmac(identifier)
+      OpenSSL::HMAC.hexdigest("SHA256", Rails.application.secret_key_base, identifier)
+    end
+
+    # Read or initialize a stable nonce identifier cookie.
+    def self.nonce_id(request)
+      request.cookies[COOKIE] || set_nonce_id(request)
+    end
+
+    def self.set_nonce_id(request)
+      value = SecureRandom.base64(16)
+      request.cookie_jar[COOKIE] = { value: value, httponly: true, same_site: :lax }
+      value
+    end
+  end
+end
+
+Rails.application.configure do
+  config.content_security_policy do |policy|
+    policy.default_src     :self
+    policy.script_src      :self  # nonce auto-appended via nonce_directives below
+    # unsafe_inline retained: many style="…" attributes and the per-user
+    # hide_from_user_style_tag can't be nonced yet.
+    policy.style_src       :self, :unsafe_inline
+    # frame_src / img_src / connect_src start at :self and get tuned against
+    # violation reports during the report-only window.
+    policy.img_src         :self, :data, :blob
+    policy.connect_src     :self
+    policy.frame_src       :self
+    policy.frame_ancestors :self
+    policy.base_uri        :self
+    policy.form_action     :self
+    policy.object_src      :none
+    # Specify URI for violation reports once a report sink is available
+    # policy.report_uri "/csp-violation-report-endpoint"
+  end
+
+  config.content_security_policy_nonce_generator = ->(request) { CSP::Nonce.generate(request) }
+  config.content_security_policy_nonce_directives = %w[ script-src ]
+
+  # Report violations without enforcing the policy.
+  config.content_security_policy_report_only = true
+end

test/integration/csp_nonce_test.rb

@@ -0,0 +1,52 @@
+require "test_helper"
+
+class CspNonceTest < ActionDispatch::IntegrationTest
+  test "policy is delivered Report-Only, not enforced" do
+    sign_in :david
+    get root_url
+
+    assert_response :success
+    assert response.headers["Content-Security-Policy-Report-Only"].present?,
+      "Expected a Report-Only CSP header"
+    assert_nil response.headers["Content-Security-Policy"],
+      "Policy must not be enforced yet"
+  end
+
+  test "nonce is stable across requests so Turbo restores don't trip CSP" do
+    sign_in :david
+
+    get root_url
+    nonce1 = report_only_nonce
+
+    get root_url
+    nonce2 = report_only_nonce
+
+    assert nonce1.present?, "Expected a nonce in the Report-Only CSP header"
+    assert_equal nonce1, nonce2, "Nonce must be stable across requests"
+  end
+
+  test "client-set identifier still yields an unpredictable HMAC nonce" do
+    fake_id = "attacker-controlled-value"
+    cookies[CSP::Nonce::COOKIE] = fake_id
+
+    get root_url
+    nonce = report_only_nonce
+
+    assert_equal CSP::Nonce.hmac(fake_id), nonce,
+      "Nonce must be HMAC-SHA256 of the identifier keyed by secret_key_base"
+  end
+
+  test "importmap script tag carries the nonce" do
+    sign_in :david
+    get root_url
+
+    assert_response :success
+    nonce = report_only_nonce
+    assert_select "script[type='importmap'][nonce=?]", nonce
+  end
+
+  private
+    def report_only_nonce
+      response.headers["Content-Security-Policy-Report-Only"].to_s[/'nonce-([^']+)'/, 1]
+    end
+end