One of the frequent causes of vulnerabilities being disclosed in WordPress plugins these days involves in part functions that are made available through WordPress’ AJAX functionality that don’t have the proper restrictions on who can access them. In many instances this involves functions that are made available to anyone logged in, despite only being intended for high level users. In the other instances, there is the more serious problem of these function being made available to anyone, including those that are not logged in. Seeing as many websites don’t allow untrusted individuals access to an account, a vulnerable function being accessible to those not logged in greatly increases the chances of it being exploited.
One common cause of security issues with WordPress plugins that we continue to see happening is a failure to properly check on whether a user should be able to use admin AJAX functions they are sending requests to. Since the wp_ajax_ hook makes the AJAX function accessible to any logged in user, without checking their capabilities even a Subscriber level users can access functions meant only for Administrators.