Description
WordPress Plugin Special Text Boxes is prone to a vulnerability that lets attackers upload arbitrary files because the application fails to properly sanitize user-supplied input. An attacker can exploit this vulnerability to upload arbitrary code and run it in the context of the webserver process. This may facilitate unauthorized access or privilege escalation; other attacks are also possible. WordPress Plugin Special Text Boxes version 5.1.90 is vulnerable; prior versions may also be affected.
Remediation
Update to plugin version 5.2.91 or latest
References
https://wordpress.org/support/topic/wp-special-textboxes-was-exploited-with-backdoor
https://wordpress.org/support/topic/backdoor-site-hack-how-to-fix