Where does elp.php come from?

Where does elp.php come from?

From yesterday on, all my sites (one and same hosting) has an elp.php-file in the root-folder of every domain.

This file is publicly open and does show a list of all files, with possibility to edit all files (even config.php etc...) I am sure this was a hack... but how to find where it came from? I deleted all of them, but how to force them forbidden? Or avoid them?

1. I have changed .htaccess to forbid "elp.php" in every domain.
2. I removed them all
3. ... ????

Google didnt find my things, except an amount of /Index-list etc.