Update the WordPress Autoptimize plugin to the latest available version (at least <= 2.7.7).
Nguyen Van Khanh discovered and reported this Arbitrary File Upload vulnerability in WordPress Autoptimize Plugin. This could allow a malicious actor to upload any type of file to your website. This can include backdoors which are then executed to gain further access to your website. This vulnerability has been fixed in version 2.7.7.