Are you sure your hosting service is secure?
Some injections have been recently develepod to exploit some security holes in hosting services.
That is totally true, but I think we should first think around the posibility that the user had his own DB/scripts exploited, as long as he reports he has changed the hosting and the problem still persists.
But, in order to discard that, could you say us:
-If you are on a shared hosting, a VPS or a dedicated server?
-If you have any other server side software that is out-of-date, and therefore it cold be exploited, causing the problem?
-If you know any other users in your hosting service that could have the same problem of yours?
Please, note that if you are on a free hosting you could have migrated from a hoster to any of their resellers, or perhaps on the reverse way....
BTW, remember to review your CHMOD for all your files when you get rid of this nasty thing. It is the first common error among non-profesional developers (in which I must include myself, I am just an amateur....).
