The glitch is basically manipulating host migration and redoing Wave 10 on Gold to get the credits. I'm not going to say how to do it because you can google search that. That's most likely the reason why they decreased the credit reward for Wave 10 by 10%, still not going to stop people from glitching. As for how illegal it is? I heard people would get a three day ban for it; with a perma-ban if you do it again. Others have been doing it still and nothing happened to them.
Some numbers I pulled out of my rear: You can do the Indoctrination Devices on Wave 10 in a "few minutes". So you get like 38K per "few minutes". In the time you do one WGG, you get about 100K credit more from glitching.
*Edited my wording since I felt it was jumbled lol.