I've been looking at the stories about this and one thing I haven't been able to find: how was he able to modify the data in Disney's system after they fired hi

I've been looking at the stories about this and one thing I haven't been able to find: how was he able to modify the data in Disney's system after they fired him? Stole someone else's credentials, left a back door they didn't catch, or did they fail to switch him off?