At the institute I did my PhD everyone had to take some role apart from research to keep the infrastructure running. My part was admin for the Linux workstations and supporting the admin of the calculation cluster we had (about 11 machines with 8 cores each... hot shit at the time).

At some point the university had some euros of budget left that had to be spent so the institute decided to buy a shiny new NAS system for the cluster.

I wasn't really involved with the stuff, I was just the replacement admin so everything was handled by the main admin.

A few months on and the cluster starts behaving ... weird. Huge CPU loads, lots of network traffic. No one really knows what's going on. At some point I discover a process on one of the compute nodes that apparently receives commands from an IRC server in the UK... OK code red, we've been hacked.

First thing we needed to find out was how they had broken in, so we looked at the logs of the compute nodes. There was nothing obvious, but the fact that each compute node had its own public IP address and was reachable from all over the world certainly didn't help.
A few hours of poking around not really knowing what I'm looking for, I resort to a TCPDUMP to find whether there is any actor on the network that I might have overlooked. And indeed I found an IP adress that I couldn't match with any of the machines.

Long story short: It was the new NAS box. Our main admin didn't care about the new box, because it was set up by an external company. The guy from the external company didn't care, because he thought he was working on a compute cluster that is sealed off behind some uber-restrictive firewall.

So our shiny new NAS system, filled to the brink with confidential research data, (and also as it turns out a lot of login credentials) was sitting there with its quaint little default config and a DHCP-assigned public IP adress, waiting for the next best rookie hacker to try U:admin/P:admin to take it over.

Looking back this could have gotten a lot worse and we were extremely lucky that these guys either didn't know what they had there or didn't care.