A lot of these reports sound like what we saw as well. For those of you that don’t have revslider on the site, realize that once a hacker has access to a standard hosting account they can access every single site under the account. A better setup is to have these isolated but that’s not how it is often configured.
Also, revslider may exist inside a theme directory. You need to either really dig through things or run a search for rev slider (if you have shell access,
find . -name “revslider” -type d
when in the site’s wp-content directory).
That all assumes revslider is really the root cause, but we’ve looked closely through the logs for one site and I’m pretty sure it was in that instance.
In addition to the other files mentioned in this thread, please check your /wp-content/plugins/cached_data/ folder. I haven’t personally seen a huge number of these attacks, so I don’t know how common it is. But we have seen that folder created, with a back door, mass mailing script, and some other junk in it that needs to be removed.