Shoes, bags and other .html adverts

Hello everyone.

I am facing a lot of hacks these days with WordPress websites (any version), domains are filled with .html scam files, adverts of nike shoes, DC bags, watches. For example:

000-nike-airmax-ladies.html
001-converse-highcut-snlp.html
001-rakuten-adidas-sneakers-men.html
001-studded-converse.html
002-adidas-stance.html
003-converse-snelkers-bllck.html
003-nb-574-list-price.html
003-nb-gray-navy.html

Files everywhere, in the root of domain, in wp-admin,wp-includes,wp-content folders. Checked Httpd logs, FTP logs, nothing suspicious. Maybe someone faced the same issue? Please share your experience.

With all respect
Wozner