Description
WordPress Plugin Appointments is prone to a vulnerability that lets remote attackers inject and execute arbitrary code because the application fails to sanitize user-supplied input before being passed to the unserialize() PHP function. Attackers can possibly exploit this issue to execute arbitrary PHP code within the context of the affected webserver process. WordPress Plugin Appointments version 2.2.1 is vulnerable; prior versions may also be affected.
Remediation
Update to plugin version 2.2.2 or latest
References
https://www.wordfence.com/blog/2017/10/3-zero-day-plugin-vulnerabilities-exploited-wild/
https://plugins.svn.wordpress.org/appointments/trunk/changelog.txt