Prevent "ArgumentError: invalid %-encoding" for bad requests from spiders

Gemfile

@@ -32,6 +32,7 @@ gem 'newrelic_rpm'
 gem 'virtus',               '~> 1.0.1'
 gem 'warden',               '~> 1.2.3'
 gem 'pundit',               '~> 0.2.3'
+gem 'rack-robustness',      '~> 1.1.0'
 
 group :development do
   gem 'quiet_assets',   '~> 1.0.1'

Gemfile.lock

@@ -219,6 +219,7 @@ GEM
     rack (1.5.2)
     rack-protection (1.5.0)
       rack
+    rack-robustness (1.1.0)
     rack-test (0.6.2)
       rack (>= 1.0)
     rails (4.0.4)
@@ -382,6 +383,7 @@ DEPENDENCIES
   pry-rails (~> 0.3.2)
   pundit (~> 0.2.3)
   quiet_assets (~> 1.0.1)
+  rack-robustness (~> 1.1.0)
   rails (= 4.0.4)
   rake (~> 10.0.4)
   rb-inotify (~> 0.9.0)

config/application.rb

@@ -33,5 +33,13 @@ module Asciinema
     end
 
     config.i18n.enforce_available_locales = true
+
+    config.middleware.use ::Rack::Robustness do |g|
+      g.no_catch_all
+      g.on(ArgumentError) { |ex| 400 }
+      g.content_type 'text/plain'
+      g.body{ |ex| ex.message }
+      g.ensure(true) { |ex| env['rack.errors'].write(ex.message) }
+    end
   end
 end