tl;dr (excerpts from Sucuri's blog post): "The issue lies in the way WP-Super-Cache would display information stored in cache file’s key..." "....the $details

tl;dr (excerpts from Sucuri's blog post): "The issue lies in the way WP-Super-Cache would display information stored in cache file’s key..." "....the $details[ ‘key’ ] is directly appended to the page’s content, without being sanitized first..."