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.

4 files changed, 94 insertions, 0 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index cd419b68f7..98bf9c944b 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,5 +1,33 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   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.
+
+    *Andrew White*
+
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*
diff --git a/actionpack/lib/action_controller/metal/content_security_policy.rb b/actionpack/lib/action_controller/metal/content_security_policy.rb
index 48a7109bea..95f2f3242d 100644
--- a/actionpack/lib/action_controller/metal/content_security_policy.rb
+++ b/actionpack/lib/action_controller/metal/content_security_policy.rb
@@ -5,6 +5,14 @@ module ActionController #:nodoc:
     # TODO: Documentation
     extend ActiveSupport::Concern
 
+    include AbstractController::Helpers
+    include AbstractController::Callbacks
+
+    included do
+      helper_method :content_security_policy?
+      helper_method :content_security_policy_nonce
+    end
+
     module ClassMethods
       def content_security_policy(**options, &block)
         before_action(options) do
@@ -22,5 +30,15 @@ module ActionController #:nodoc:
         end
       end
     end
+
+    private
+
+      def content_security_policy?
+        request.content_security_policy
+      end
+
+      def content_security_policy_nonce
+        request.content_security_policy_nonce
+      end
   end
 end
diff --git a/actionpack/lib/action_dispatch/http/content_security_policy.rb b/actionpack/lib/action_dispatch/http/content_security_policy.rb
index ffac3b8d99..a3407c9698 100644
--- a/actionpack/lib/action_dispatch/http/content_security_policy.rb
+++ b/actionpack/lib/action_dispatch/http/content_security_policy.rb
@@ -21,6 +21,12 @@ module ActionDispatch #:nodoc:
         return response if policy_present?(headers)
 
         if policy = request.content_security_policy
+          if policy.directives["script-src"]
+            if nonce = request.content_security_policy_nonce
+              policy.directives["script-src"] << "'nonce-#{nonce}'"
+            end
+          end
+
           headers[header_name(request)] = policy.build(request.controller_instance)
         end
 
@@ -51,6 +57,8 @@ module ActionDispatch #:nodoc:
     module Request
       POLICY = "action_dispatch.content_security_policy".freeze
       POLICY_REPORT_ONLY = "action_dispatch.content_security_policy_report_only".freeze
+      NONCE_GENERATOR = "action_dispatch.content_security_policy_nonce_generator".freeze
+      NONCE = "action_dispatch.content_security_policy_nonce".freeze
 
       def content_security_policy
         get_header(POLICY)
@@ -67,6 +75,30 @@ module ActionDispatch #:nodoc:
       def content_security_policy_report_only=(value)
         set_header(POLICY_REPORT_ONLY, value)
       end
+
+      def content_security_policy_nonce_generator
+        get_header(NONCE_GENERATOR)
+      end
+
+      def content_security_policy_nonce_generator=(generator)
+        set_header(NONCE_GENERATOR, generator)
+      end
+
+      def content_security_policy_nonce
+        if content_security_policy_nonce_generator
+          if nonce = get_header(NONCE)
+            nonce
+          else
+            set_header(NONCE, generate_content_security_policy_nonce)
+          end
+        end
+      end
+
+      private
+
+        def generate_content_security_policy_nonce
+          content_security_policy_nonce_generator.call(self)
+        end
     end
 
     MAPPINGS = {
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