Holiday class post calendar <= 7.1 - Unauthenticated Remote Code Execution via 'contents'

The Holiday class post calendar plugin for WordPress is vulnerable to Remote Code Execution in all versions up to, and including, 7.1 via the 'contents' parameter. This is due to a lack of sanitization of user-supplied data when creating a cache file. This makes it possible for unauthenticated attackers to execute code on the server.