I think they use CC BY-SA 4.0. There was some criticism when they moved from CC BY-SA 3.0 even when it was considered an improvement since it wasn't clear if st

I think they use CC BY-SA 4.0. There was some criticism when they moved from CC BY-SA 3.0 even when it was considered an improvement since it wasn't clear if stackoverflow needed additional consent to change the license for already existing content.