What's the worst cyber "attack" you have encountered?

The list of real world cyber attack issues is one that grows daily.

One that affected customers I worked with was called Mirai. It's malware that infected Internet connected devices like security cameras, routers, and more. The affected devices could then be used as part of a botnet to flood targets with traffic and take them offline for a time - known as a DDoS attack. You can read a bit more about it here: https://www.csoonline.com/article/3258748/security/the-mirai-botnet-explained-how-teen-scammers-and-cctv-cameras-almost-brought-down-the-internet.html