[fragileone]

It was rumoured to be stolen for a long time, but the original developer of SS13 (Exadv1) said in an interview that no theft took place, and he'd voluntarily given the code to fellow programmers[1].

[1] https://www.youtube.com/watch?v=hP5t1b1TKQg

[skeaker]

This is true, but there still has been drama with the ownership of the source code in the past. The Goon fork that has always been one of the biggest forks but is also closed source has leaked on a few separate occasions, for example.