I have enough security that I was alerted to a PHP backdoor that was uploaded to a WordPress site hosted on one of my servers. It was removed within minutes of being uploaded so no damage was done. No other sites show abnormal file uploads and have been checked for extra files and any files that were recently changed or added. It seems that only one site was affected. I was thinking that maybe the new WP or the theme has a bug that allows someone to upload files.
My main question is how can someone upload a file to a server without FTP or access to WordPress to upload it. I found the first uploaded file in the root of the directory.
I have already scanned and clean the site so now I just would like to figure out how they uploaded the file.