I like "they". It's especially suitable in this case, because we don't even know if it is a single person, or a group, or maybe even an automated attack (alltho

I like "they". It's especially suitable in this case, because we don't even know if it is a single person, or a group, or maybe even an automated attack (allthough that sounds unlikely). "They" leaves it open to the reader to imagine their stereotypical hacker.