From the article. Doesn’t state what data but he did steal customers data, over the course of a year, before Apple stopped him which is a bit of a key thing, he got away with it for a year before Apple caught him... I wouldn’t call that the best security you can have. According to the article he boasted about his hacks all the time and was ‘well known’ in the hacking community.
http://www.theage.com.au/national/victoria/melbourne-teen-hacked-into-apple-s-secure-computer-network-court-told-20180816-p4zxwu.html
The teen, who cannot be named for legal reasons, broke into Apple’s mainframe from his suburban home on multiple occasions over a year because he was such a fan of the company, according to his lawyer.
The Children’s Court heard on Thursday that he had downloaded 90gb of secure files and accessed customer accounts.
The teen’s defence lawyer said his client had become so well known in the international hacking community that even mentioning the case in detail could expose him to risk.
He then used Whatsapp to communicate his offending to others.
The ongoing access continued until Apple eventually detected his presence and he was blocked.