Directory traversal vulnerability in actionpack/lib/action_dispatch/middleware/static.rb in Action Pack in Ruby on Rails 3.x before 3.2.20, 4.0.x before 4.0.11, 4.1.x before 4.1.7, and 4.2.x before 4.2.0.beta3, when serve_static_assets is enabled, allows remote attackers to determine the existence of files outside the application root via a /..%2F sequence.