I think:
- open source is surely more vulnerable in this scenario
- traditional finance is centralizes, thus controlling and mitigating damage is usually faster
- Bitcoin, OTOH, due to its nature makes most damage permanent and/or unstoppable
- traditional finance might be more mature in terms of security than some Bitcoin project (emphasis in *some*, let's not generalize)
Also:
- open source can benefit for multiple people using the same tools and help (see current red team initiative)