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

@@ -15,6 +15,7 @@ gem "govuk_app_config"
 gem "govuk_document_types"
 gem "govuk_publishing_components"
 gem "plek"
+gem "rack-utf8_sanitizer"
 gem "rails-i18n"
 gem "rails_translation_manager"
 gem "rinku", require: "rails_rinku"

Gemfile.lock

@@ -572,6 +572,8 @@ 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)
@@ -795,6 +797,7 @@ DEPENDENCIES
   pact_broker-client
   plek
   pry-byebug
+  rack-utf8_sanitizer
   rails (= 7.2.1.1)
   rails-controller-testing
   rails-i18n

config/application.rb

@@ -13,6 +13,7 @@
 # require "action_view/railtie"
 # 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.
@@ -117,12 +118,21 @@ 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])
 
     # Configuration for the application, engines, and railties goes here.
     #
     # These settings can be overridden in specific environments using the files
     # in config/environments, which are processed later.
-    #
+
+    # 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,
+      sanitizable_content_types: [],
+      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

spec/requests/sanitiser_spec.rb

@@ -0,0 +1,86 @@
+require "slimmer/test"
+
+RSpec.describe "Sanitiser" do
+  context "with query being correct percent-encoded UTF-8 string does not raise exception" do
+    it "does not raise error" do
+      get "/?%41"
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with query being incorrect percent-encoded UTF-8 string raises exception" do
+    it "raises Sanitiser::Strategy::SanitisingError" do
+      expect { get "/?%AD" }.to raise_error(Sanitiser::Strategy::SanitisingError)
+    end
+  end
+
+  context "with cookie key being correct UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Cookie: "\x41=value" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with cookie key being incorrect UTF-8" do
+    it "raises ArgumentError" do
+      expect { get "/", headers: { Cookie: "\xAD=value" } }
+        .to raise_error(ArgumentError, "invalid byte sequence in UTF-8")
+    end
+  end
+
+  context "with cookie value being correct UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Cookie: "key=\x41" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with cookie value being incorrect UTF-8" do
+    it "raises ArgumentError" do
+      expect { get "/", headers: { Cookie: "key=\xAD" } }
+        .to raise_error(ArgumentError, "invalid byte sequence in UTF-8")
+    end
+  end
+
+  context "with cookie path being correct UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Cookie: "key=value; Path=/\x41" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with cookie path being incorrect UTF-8" do
+    it "raises ArgumentError" do
+      expect { get "/", headers: { Cookie: "key=value; Path=/\xAD" } }
+        .to raise_error(ArgumentError, "invalid byte sequence in UTF-8")
+    end
+  end
+
+  context "with cookie path being correct percent-encoded UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Cookie: "key=value; Path=/%41" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with cookie path being incorrect percent-encoded UTF-8" do
+    it "raises Sanitiser::Strategy::SanitisingError" do
+      expect { get "/", headers: { Cookie: "key=value; Path=/%AD" } }
+        .to raise_error(Sanitiser::Strategy::SanitisingError)
+    end
+  end
+
+  context "with referrer header being correct percent-encoded UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Referer: "http://example.com/?%41" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+
+  context "with referrer header being incorrect percent-encoded UTF-8" do
+    it "does not raise error" do
+      get "/", headers: { Referer: "http://example.com/?%AD" }
+      expect(response).to have_http_status(:ok)
+    end
+  end
+end