Add support for automatic nonce generation for Rails UJS

Because the UJS library creates a script tag to process responses it
normally requires the script-src attribute of the content security
policy to include 'unsafe-inline'.

To work around this we generate a per-request nonce value that is
embedded in a meta tag in a similar fashion to how CSRF protection
embeds its token in a meta tag. The UJS library can then read the
nonce value and set it on the dynamically generated script tag to
enable it to execute without needing 'unsafe-inline' enabled.

Nonce generation isn't 100% safe - if your script tag is including
user generated content in someway then it may be possible to exploit
an XSS vulnerability which can take advantage of the nonce. It is
however an improvement on a blanket permission for inline scripts.

It is also possible to use the nonce within your own script tags by
using `nonce: true` to set the nonce value on the tag, e.g

    <%= javascript_tag nonce: true do %>
      alert('Hello, World!');
    <% end %>

Fixes #31689.

1 files changed, 16 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/content_security_policy_test.rb b/actionpack/test/dispatch/content_security_policy_test.rb
index 5184e4f960..b88f90190a 100644
--- a/actionpack/test/dispatch/content_security_policy_test.rb
+++ b/actionpack/test/dispatch/content_security_policy_test.rb
@@ -253,6 +253,11 @@ class ContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
       p.report_uri "/violations"
     end
 
+    content_security_policy only: :script_src do |p|
+      p.default_src false
+      p.script_src :self
+    end
+
     content_security_policy_report_only only: :report_only
 
     def index
@@ -271,6 +276,10 @@ class ContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
       head :ok
     end
 
+    def script_src
+      head :ok
+    end
+
     private
       def condition?
         params[:condition] == "true"
@@ -284,6 +293,7 @@ class ContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
       get "/inline", to: "policy#inline"
       get "/conditional", to: "policy#conditional"
       get "/report-only", to: "policy#report_only"
+      get "/script-src", to: "policy#script_src"
     end
   end
 
@@ -298,6 +308,7 @@ class ContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
 
     def call(env)
       env["action_dispatch.content_security_policy"] = POLICY
+      env["action_dispatch.content_security_policy_nonce_generator"] = proc { "iyhD0Yc0W+c=" }
       env["action_dispatch.content_security_policy_report_only"] = false
       env["action_dispatch.show_exceptions"] = false
 
@@ -337,6 +348,11 @@ class ContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
     assert_policy "default-src 'self'; report-uri /violations", report_only: true
   end
 
+  def test_adds_nonce_to_script_src_content_security_policy
+    get "/script-src"
+    assert_policy "script-src 'self' 'nonce-iyhD0Yc0W+c='"
+  end
+
   private
 
     def env_config