Update the WordPress SEO Plugin by Squirrly SEO plugin to the latest available version (at least 12.1.11).
Rafie Muhammad (Patchstack) discovered and reported this Arbitrary File Upload vulnerability in WordPress SEO Plugin by Squirrly SEO 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 12.1.11.