diff --git a/config/initializers/content_security_policy.rb b/config/initializers/content_security_policy.rb
index b3076b38..d85e080b 100644
--- a/config/initializers/content_security_policy.rb
+++ b/config/initializers/content_security_policy.rb
@@ -1,25 +1,114 @@
 # 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
+
+  # Per-install CSP extras.
+  #
+  # Writebook is a ONCE product: each customer self-hosts it on their own domain
+  # and an admin may embed or connect to external hosts — video/embed providers,
+  # image CDNs, analytics, form or webhook endpoints — that vary per install and
+  # are unknown at build time. `:self` already tracks this install's own origin;
+  # these ENV knobs let an admin allow additional hosts without editing this file
+  # (and without which enforcement would break their legitimate integrations).
+  #
+  # Each is a comma-, semicolon-, or whitespace-separated list of CSP source
+  # expressions, e.g.
+  #
+  #   CSP_EXTRA_FRAME_SRC="https://www.youtube.com https://player.vimeo.com"
+  #
+  # Leave them unset (the default) to keep each directive at :self only.
+  EXTRA_ENV = {
+    script_src:  "CSP_EXTRA_SCRIPT_SRC",
+    style_src:   "CSP_EXTRA_STYLE_SRC",
+    img_src:     "CSP_EXTRA_IMG_SRC",
+    connect_src: "CSP_EXTRA_CONNECT_SRC",
+    frame_src:   "CSP_EXTRA_FRAME_SRC",
+    form_action: "CSP_EXTRA_FORM_ACTION"
+  }.freeze
+
+  # Parse one ENV knob into a list of extra host sources.
+  #
+  # Semicolons are tokenized like commas/whitespace: because the nonce forces a
+  # per-request policy build, a stray `;` in a value (a plausible operator paste,
+  # e.g. "https://youtube.com; https://vimeo.com") would otherwise land inside a
+  # single source token and make Rails raise InvalidDirectiveError on every
+  # request — a site-wide 500 even in report-only mode. Splitting on `;` yields
+  # valid tokens instead; Rails still validates each one, so no injection is
+  # introduced (a token with an embedded space still fails validation).
+  def self.extra(directive)
+    ENV[EXTRA_ENV.fetch(directive)].to_s.split(/[,;\s]+/).reject(&:blank?)
+  end
+
+  # Build the baseline policy. Kept as a reusable method so it can be exercised
+  # in isolation by tests as well as at boot.
+  def self.apply(policy)
+    policy.default_src     :self
+    policy.script_src      :self, *extra(:script_src)  # 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, *extra(:style_src)
+    # frame_src / img_src / connect_src start at :self plus any per-install extras
+    # and get tuned against violation reports during the report-only window.
+    policy.img_src         :self, :data, :blob, *extra(:img_src)
+    policy.connect_src     :self, *extra(:connect_src)
+    policy.frame_src       :self, *extra(:frame_src)
+    policy.frame_ancestors :self
+    policy.base_uri        :self
+    policy.form_action     :self, *extra(:form_action)
+    policy.object_src      :none
+    # Specify URI for violation reports once a report sink is available
+    # policy.report_uri "/csp-violation-report-endpoint"
+  end
+end
+
+Rails.application.configure do
+  config.content_security_policy { |policy| CSP.apply(policy) }
+
+  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
diff --git a/test/integration/csp_nonce_test.rb b/test/integration/csp_nonce_test.rb
new file mode 100644
index 00000000..2edb3626
--- /dev/null
+++ b/test/integration/csp_nonce_test.rb
@@ -0,0 +1,103 @@
+require "test_helper"
+
+class CspNonceTest < ActionDispatch::IntegrationTest
+  test "policy is delivered Report-Only, not enforced" do
+    sign_in :david
+    get root_path
+
+    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_path
+    nonce1 = report_only_nonce
+
+    get root_path
+    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_path
+    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_path
+
+    assert_response :success
+    nonce = report_only_nonce
+    assert_select "script[type='importmap'][nonce=?]", nonce
+  end
+
+  test "a per-install ENV extra host is appended to its directive" do
+    with_env "CSP_EXTRA_FRAME_SRC" => "https://player.vimeo.com https://www.youtube.com",
+             "CSP_EXTRA_IMG_SRC"   => "https://cdn.example.test" do
+      header = ActionDispatch::ContentSecurityPolicy.new { |p| CSP.apply(p) }.build
+
+      assert_match %r{frame-src[^;]*\bhttps://player\.vimeo\.com\b}, header
+      assert_match %r{frame-src[^;]*\bhttps://www\.youtube\.com\b}, header
+      assert_match %r{img-src[^;]*\bhttps://cdn\.example\.test\b}, header
+      # :self is preserved alongside the extras.
+      assert_match %r{frame-src 'self'}, header
+    end
+  end
+
+  test "a semicolon-separated ENV extra tokenizes into valid sources without raising" do
+    # A plausible operator paste separates hosts with "; ". Because the nonce
+    # forces a per-request policy build, a semicolon left inside a single source
+    # token would make Rails raise InvalidDirectiveError on every request — a
+    # site-wide 500 even in report-only mode. Splitting on ';' must yield both
+    # hosts as valid tokens and never raise.
+    with_env "CSP_EXTRA_FRAME_SRC" => "https://a.example; https://b.example" do
+      header = nil
+      assert_nothing_raised do
+        header = ActionDispatch::ContentSecurityPolicy.new { |p| CSP.apply(p) }.build
+      end
+
+      assert_match %r{frame-src[^;]*\bhttps://a\.example\b}, header
+      assert_match %r{frame-src[^;]*\bhttps://b\.example\b}, header
+      # Both hosts share the one frame-src directive; the semicolon did not leak
+      # a second directive into the policy.
+      assert_equal 1, header.scan(/(?:^|;\s*)frame-src\b/).size,
+        "Expected exactly one frame-src directive"
+    end
+  end
+
+  test "directives default to :self only when no ENV extras are set" do
+    with_env "CSP_EXTRA_FRAME_SRC" => nil, "CSP_EXTRA_IMG_SRC" => nil do
+      header = ActionDispatch::ContentSecurityPolicy.new { |p| CSP.apply(p) }.build
+
+      assert_match %r{frame-src 'self'(;|\z)}, header
+    end
+  end
+
+  private
+    def with_env(vars)
+      original = {}
+      vars.each_key { |k| original[k] = ENV.key?(k) ? ENV[k] : :__unset__ }
+      vars.each { |k, v| v.nil? ? ENV.delete(k) : ENV[k] = v }
+      yield
+    ensure
+      original.each { |k, v| v == :__unset__ ? ENV.delete(k) : ENV[k] = v }
+    end
+
+    def report_only_nonce
+      response.headers["Content-Security-Policy-Report-Only"].to_s[/'nonce-([^']+)'/, 1]
+    end
+end