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, 2 insertions, 0 deletions

diff --git a/actionview/lib/action_view/helpers.rb b/actionview/lib/action_view/helpers.rb
index 46f20c4277..8cc8013718 100644
--- a/actionview/lib/action_view/helpers.rb
+++ b/actionview/lib/action_view/helpers.rb
@@ -13,6 +13,7 @@ module ActionView #:nodoc:
     autoload :CacheHelper
     autoload :CaptureHelper
     autoload :ControllerHelper
+    autoload :CspHelper
     autoload :CsrfHelper
     autoload :DateHelper
     autoload :DebugHelper
@@ -46,6 +47,7 @@ module ActionView #:nodoc:
     include CacheHelper
     include CaptureHelper
     include ControllerHelper
+    include CspHelper
     include CsrfHelper
     include DateHelper
     include DebugHelper