[88e282102ae2e5b]

There are plenty of other motivations. It could be:

1) a novice hacker looking for experience 2) an automated bot that scanned IPs for some vulnerability and acted on its own when it found one 3) someone looking to extort the owner for money 4) someone with a personal vendetta against the owner 5) someone looking to secretly plant bitcoin mining software on the servers (who got caught) 6) someone looking to alter their own score to help them get some sort of math job

I could probably go on for a while, but you get the point. It's hard to ascribe intent to this sort of thing without more information.

[qopp]

5) The Bitcoin mining thing is a bit of a historic artifact, today you could barely achieve a cent per month on even the very fastest of cpus

[TeMPOraL]

Or 7) someone who did it for teh lulz. While rare, this also happens. But I agree; it's hard to guess motivations behind the attack without more info.

[brownbat]

8) Waterholing targeting a user or group that frequents the site?

I like #2 though, or as I'd put it: flotsam of the internet, like how slammer ended up at davis besse.

[S4M]

As Project Euler is completely free and has a positive impact on the programming community, it would be very sad if the reasons for the hacking were 3) or 4).

[fibo]

I would discard 1) cause a real hacker, even if novice, would enter without damaging or stopping the service. He/She would maybe report to the owner the vulnerability or even would keep it secret.