
I wonder if it was hidden in a library they linked to their code?
My understanding is that advertisers will give you library code to link to yours, and it would be easy for them to sneak something nasty when they upgrade the library. The trick I guess would be just having a normal library at first so you build trust and get a bunch of apps using it. Then you pull the trigger and issue them a new rev, and when they publish an updated version of their app that includes your rogue library you make money until sometimes notices and it gets punted.
Meanwhile the same bad guys own another small advertiser with a similar library that's ripe for going rogue next month...