Ask HN: Most interesting cyber attack that you faced in your career?

I spent most of today alleviating a cyber attack on one of the sites of our clients, which was attacked by a Chinese botnet to send spam emails. It was nothing extraordinarily clever and probably a result of some automated crawler looking for vulnerabilities.

But it was a very interesting thing to work on and resolve eventually. It got me thinking about other people's experiences with cyber attacks. Some interesting stories and experiences to share?

What was the most interesting cyber attack that you dealt with in your career?