Let's say there's a program ("ABC") which is licensed under GPL. There's a similar program ("CAD") on the market, which I believe is a modified ABC. It is therefore in violation of the GPL.
Am I allowed to take and use (or reverse engineer) CAD's source code without the author's consent, or do I need to sue CAD's author first for violating GPL?
Looks related: What are the potential legal consequences for a company that has GPL violations? The answer to that question makes me suspect the answer to this question is "yes", but I'm not sure, since I can potentially be wrong about CAD.