Hi, this exploit happens because of other scripts on your server and it simply targets TP out.php since its used on MANY sites with MANY traffic hence attack makes sense.
To solve this issue ask your host to secure your server, update all scripts to latest versions, remove unneeded scripts and make sure all is 500% clean before doing UPDATE of tp, you can update TP by going into tools/updater, and it will replace out.php with original one.