diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 9ad6c7c..b73e592 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -1,4 +1,4 @@
 class ApplicationController < ActionController::Base
-  include AllowBrowser, Authentication, Authorization, BlockBannedRequests, SetCurrentRequest, SetPlatform, TrackedRoomVisit, VersionHeaders, CachedResponses
+  include AllowBrowser, Authentication, Authorization, BlockBannedRequests, SetCurrentRequest, SetPlatform, TrackedRoomVisit, VersionHeaders, CachedResponses, InlineScriptHashes
   include Turbo::Streams::Broadcasts, Turbo::Streams::StreamName
 end
diff --git a/app/controllers/concerns/inline_script_hashes.rb b/app/controllers/concerns/inline_script_hashes.rb
new file mode 100644
index 0000000..c0fde41
--- /dev/null
+++ b/app/controllers/concerns/inline_script_hashes.rb
@@ -0,0 +1,33 @@
+# Allows the scripts the importmap helper emits: the inline importmap and its
+# entry-point module by content hash, and the origins its URLs point to. The
+# hashes change only when the importmap does (a deploy), so cached HTML stays
+# valid, unlike a per-request nonce.
+module InlineScriptHashes
+  extend ActiveSupport::Concern
+
+  ENTRY_POINT_SCRIPT = %(import "application")
+
+  included do
+    content_security_policy do |policy|
+      if sources = policy.directives["script-src"]
+        importmap = Rails.application.importmap.to_json(resolver: helpers)
+        policy.directives["script-src"] = sources | script_origins(importmap) | script_hashes(importmap, ENTRY_POINT_SCRIPT)
+      end
+    end
+  end
+
+  private
+    # The importmap is resolved by Rails' asset helpers, so its URLs already
+    # carry any configured asset host, however it's written: with or without a
+    # scheme, protocol-relative, a %d set, or a proc.
+    def script_origins(importmap)
+      JSON.parse(importmap)["imports"].values.filter_map do |url|
+        origin = URI.join(request.base_url, url).origin
+        origin unless origin == request.base_url
+      end.uniq
+    end
+
+    def script_hashes(*scripts)
+      scripts.map { |script| "'sha256-#{Digest::SHA256.base64digest(script)}'" }
+    end
+end
diff --git a/config/initializers/content_security_policy.rb b/config/initializers/content_security_policy.rb
index 54f47cf..e343526 100644
--- a/config/initializers/content_security_policy.rb
+++ b/config/initializers/content_security_policy.rb
@@ -1,25 +1,18 @@
-# Be sure to restart your server when you modify this file.
-
-# Define an application-wide content security policy.
-# 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
+# Enforcing baseline policy. Scripts run only from this origin; the two inline
+# scripts the importmap helper emits are allowed by hash (InlineScriptHashes),
+# so there is no per-request nonce and rendered HTML stays cacheable.
 #
-#   # Generate session nonces for permitted importmap and inline scripts
-#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
-#   config.content_security_policy_nonce_directives = %w(script-src)
+# Nothing else is restricted: images, media, fonts, styles (including an
+# account's custom styles) and connections behave as before.
 #
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+# DISABLE_CONTENT_SECURITY_POLICY=true is the escape hatch for an install whose
+# proxy or customization injects scripts the policy would block.
+unless ENV["DISABLE_CONTENT_SECURITY_POLICY"] == "true"
+  Rails.application.configure do
+    config.content_security_policy do |policy|
+      policy.script_src :self
+      policy.object_src :none
+      policy.base_uri   :self
+    end
+  end
+end
diff --git a/test/controllers/cached_responses_test.rb b/test/controllers/cached_responses_test.rb
index 8968aa0..fa420f4 100644
--- a/test/controllers/cached_responses_test.rb
+++ b/test/controllers/cached_responses_test.rb
@@ -26,6 +26,18 @@ class CachedResponsesTest < ActionDispatch::IntegrationTest
     ActionController::Base.perform_caching = @previous_caching
   end
 
+  test "a reused page carries the same enforcing policy, with no nonce" do
+    get room_messages_url(@room)
+    first = response.headers["Content-Security-Policy"]
+    assert_match "script-src 'self' 'sha256-", first
+    assert_no_match "nonce-", first
+
+    MessagesController.any_instance.expects(:find_paged_messages).never
+    get room_messages_url(@room)
+    assert_response :success
+    assert_equal first, response.headers["Content-Security-Policy"]
+  end
+
   test "all four read actions reuse completed pages" do
     urls = [ room_url(@room), room_messages_url(@room), user_sidebar_url(:me), searches_url(q: "hello") ]
     urls.each do |url|
diff --git a/test/integration/content_security_policy_test.rb b/test/integration/content_security_policy_test.rb
new file mode 100644
index 0000000..7e78d38
--- /dev/null
+++ b/test/integration/content_security_policy_test.rb
@@ -0,0 +1,78 @@
+require "test_helper"
+
+class ContentSecurityPolicyTest < ActionDispatch::IntegrationTest
+  PAGE_PATH = "/"
+
+  setup { sign_in :david }
+
+  test "enforcing policy allows exactly the inline scripts the page carries" do
+    get root_url
+    follow_redirect! while response.redirect?
+
+    policy = response.headers["Content-Security-Policy"]
+    assert policy, "expected an enforcing Content-Security-Policy header"
+    assert_nil response.headers["Content-Security-Policy-Report-Only"]
+    assert_match "object-src 'none'", policy
+    assert_match "base-uri 'self'", policy
+    assert_no_match "unsafe-inline", policy
+    assert_no_match "nonce-", policy
+
+    script_src = policy[/script-src ([^;]+)/, 1].split
+    inline = Nokogiri::HTML5(response.body).css("script").reject { |s| s["src"] || s["type"] == "text/template" }
+    assert_operator inline.size, :>=, 2
+    inline.each do |script|
+      assert_includes script_src, "'sha256-#{Digest::SHA256.base64digest(script.text)}'", "inline #{script["type"]} script not allowed by hash"
+    end
+  end
+
+  test "scripts may load from a configured asset host, however it's written" do
+    {
+      "https://cdn.example.test" => %r{\Ahttps://cdn\.example\.test\z},
+      "cdn.example.test" => %r{\Ahttp://cdn\.example\.test\z},
+      "//cdn.example.test" => %r{\Ahttp://cdn\.example\.test\z},
+      "https://assets%d.example.test" => %r{\Ahttps://assets[0-3]\.example\.test\z},
+      ->(source) { "https://one.example.test" } => %r{\Ahttps://one\.example\.test\z},
+      ->(source, request) { "https://#{request.host}.example.test" } => %r{\Ahttps://www\.example\.com\.example\.test\z}
+    }.each do |asset_host, expected_origin|
+      with_asset_host(asset_host) do
+        get PAGE_PATH
+        follow_redirect! while response.redirect?
+        assert_response :success
+
+        script_src = response.headers["Content-Security-Policy"][/script-src ([^;]+)/, 1].split
+        page = Nokogiri::HTML5(response.body)
+        urls = JSON.parse(page.at_css("script[type=importmap]").text)["imports"].values +
+          page.css("link[rel=modulepreload]").map { it["href"] } + page.css("script[src]").map { it["src"] }
+        origins = urls.map { URI.join(request.base_url, it).origin }.uniq - [ request.base_url ]
+
+        assert_not_empty origins, asset_host.inspect
+        origins.each do |origin|
+          assert_match expected_origin, origin, asset_host.inspect
+          assert_includes script_src, origin, asset_host.inspect
+        end
+        assert_includes script_src, "'sha256-#{Digest::SHA256.base64digest(page.at_css("script[type=importmap]").text)}'", asset_host.inspect
+      end
+    end
+  end
+
+  test "the escape hatch boots without a policy" do
+    output = IO.popen({ "DISABLE_CONTENT_SECURITY_POLICY" => "true" },
+      [ "bin/rails", "runner", "print Rails.application.config.content_security_policy.inspect" ], chdir: Rails.root, err: File::NULL, &:read)
+
+    assert_equal "nil", output
+  end
+
+  test "uploaded files are guarded on their way out" do
+    assert_includes Rails.application.middleware.map(&:klass), UploadedScriptGuard
+  end
+
+  private
+    def with_asset_host(asset_host)
+      previous, ApplicationController.asset_host = ApplicationController.asset_host, asset_host
+      Rails.application.importmap.send(:clear_cache)
+      yield
+    ensure
+      ApplicationController.asset_host = previous
+      Rails.application.importmap.send(:clear_cache)
+    end
+end