Hummingbird <= 3.9.1 - Cross-Site Request Forgery

The Hummingbird plugin for WordPress is vulnerable to Cross-Site Request Forgery in versions up to, and including, 3.9.1. This is due to missing or incorrect nonce validation on the on_load and maybe_clear_all_cache functions. This makes it possible for unauthenticated attackers to update settings and clear cache via a forged request granted they can trick a site administrator into performing an action such as clicking on a link.