Strange patterns in index.php and wp-config.php

Hi

A lot of our Customers running older Versions of WordPress (4.7.x and 4.8.x) are having modified Files, especially index.php and wp-config.php. In the first Line of that Files, a pattern similar to the following one can be found:

<?php $ehvmxbm = ‘% x27Y%6<.msv`ftsbqA7>q%6< x7fw6* ….. (about 6000 Bytes more)

What kind of Malware is that? And can it be fixed by upgrading to 4.9.4. Or is a complete Reinstall advised?

Thanks for all help.