Handle incorrectly UTF-8 encoded query and cookie url

Use a different exception when this situation is detected:
* Sanitiser::Strategy::SanitisingError
This error is not sent to Sentry.
See: alphagov/govuk_app_config#402

Gemfile

@@ -13,6 +13,7 @@ gem "govuk_personalisation"
 gem "govuk_publishing_components"
 gem "nokogiri"
 gem "plek"
+gem "rack-utf8_sanitizer"
 gem "redis"
 gem "sprockets-rails"
 gem "terser"

Gemfile.lock

@@ -449,6 +449,8 @@ GEM
       rack (>= 3.0.0)
     rack-test (2.1.0)
       rack (>= 1.3)
+    rack-utf8_sanitizer (1.9.1)
+      rack (>= 1.0, < 4.0)
     rackup (2.1.0)
       rack (>= 3)
       webrick (~> 1.8)
@@ -613,6 +615,7 @@ DEPENDENCIES
   nokogiri
   plek
   pry
+  rack-utf8_sanitizer
   rails (= 7.2.1.1)
   redis
   rubocop-govuk

config/application.rb

@@ -14,6 +14,7 @@
 require "active_support/time"
 # require "action_cable/engine"
 require "rails/test_unit/railtie"
+require_relative "../lib/sanitiser/strategy"
 
 # Require the gems listed in Gemfile, including any gems
 # you've limited to :test, :development, or :production.
@@ -27,7 +28,7 @@ class Application < Rails::Application
     # Please, add to the `ignore` list any other `lib` subdirectories that do
     # not contain `.rb` files, or that should not be reloaded or eager loaded.
     # Common ones are `templates`, `generators`, or `middleware`, for example.
-    config.autoload_lib(ignore: %w[assets tasks])
+    config.autoload_lib(ignore: %w[assets tasks sanitiser])
 
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration can go into files in config/initializers
@@ -44,5 +45,14 @@ class Application < Rails::Application
 
     # Use temporary directory for page cache if filesystem is read-only
     config.action_controller.page_cache_directory = File.join(ENV["TMPDIR"], "page_cache") if ENV.fetch("USE_TMPDIR_PAGE_CACHE", "false") == "true"
+
+    # Protect from "invalid byte sequence in UTF-8" errors,
+    # when a query or a cookie is a string with incorrect UTF-8 encoding.
+    config.middleware.insert_before(
+      Rack::Head,
+      Rack::UTF8Sanitizer,
+      only: %w[QUERY_STRING],
+      strategy: Sanitiser::Strategy,
+    )
   end
 end

lib/sanitiser/strategy.rb

@@ -0,0 +1,18 @@
+module Sanitiser
+  class Strategy
+    class SanitisingError < StandardError; end
+
+    class << self
+      def call(input, sanitize_null_bytes: false)
+        input
+          .force_encoding(Encoding::ASCII_8BIT)
+          .encode!(Encoding::UTF_8)
+        if sanitize_null_bytes && Rack::UTF8Sanitizer::NULL_BYTE_REGEX.match?(input)
+          raise NullByteInString
+        end
+      rescue StandardError
+        raise SanitisingError, "Non-UTF-8 (or null) character in the query or in the cookie"
+      end
+    end
+  end
+end

test/integration/sanitiser_test.rb

@@ -0,0 +1,84 @@
+require "test_helper"
+
+class SanitiserTest < ActiveSupport::TestCase
+  include Rack::Test::Methods
+
+  def app
+    Rails.application
+  end
+
+  test "with query being correct percent-encoded UTF-8 string does not raise exception" do
+    get "/templates/scheduled_maintenance.html.erb?%41"
+    assert last_response.successful?
+  end
+
+  test "with query being incorrect percent-encoded UTF-8 string raises exception" do
+    assert_raises(Sanitiser::Strategy::SanitisingError) do
+      get "/templates/scheduled_maintenance.html.erb?%AD"
+    end
+  end
+
+  test "with cookie key being correct UTF-8" do
+    header("Cookie", "\x41=value")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+
+  test "with cookie key being incorrect UTF-8" do
+    header("Cookie", "\xAD=value")
+    assert_raises(ArgumentError, match: "invalid byte sequence in UTF-8") do
+      get "/templates/scheduled_maintenance.html.erb"
+    end
+  end
+
+  test "with cookie value being correct UTF-8" do
+    header("Cookie", "key=\x41")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+
+  test "with cookie value being incorrect UTF-8" do
+    header("Cookie", "key=\xAD")
+    assert_raises(ArgumentError, match: "invalid byte sequence in UTF-8") do
+      get "/templates/scheduled_maintenance.html.erb"
+    end
+  end
+
+  test "with cookie path being correct UTF-8" do
+    header("Cookie", "key=value; Path=/\x41")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+
+  test "with cookie path being incorrect UTF-8" do
+    header("Cookie", "key=value; Path=/\xAD")
+    assert_raises(ArgumentError, match: "invalid byte sequence in UTF-8") do
+      get "/templates/scheduled_maintenance.html.erb"
+    end
+  end
+
+  test "with cookie path being correct percent-encoded UTF-8" do
+    header("Cookie", "key=value; Path=/%41")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+
+  test "with cookie path being incorrect percent-encoded UTF-8" do
+    header("Cookie", "key=value; Path=/%AD")
+    assert_raises(Sanitiser::Strategy::SanitisingError) do
+      get "/templates/scheduled_maintenance.html.erb"
+    end
+  end
+
+  test "with referrer headerbeing correct percent-encoded UTF-8" do
+    header("Referer", "http://example.com/?%41")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+
+  test "with referrer header being incorrect percent-encoded UTF-8" do
+    header("Referer", "http://example.com/?%AD")
+    get "/templates/scheduled_maintenance.html.erb"
+    assert last_response.successful?
+  end
+end