A five years old was able to brake the password system for the Xbox One.
And how he do it?
The exploit was simple; if you enter the wrong password when attempting to access someone else’s Xbox Live account, you’ll be taken to a second password verification screen. However, if you just enter spaces on this screen, the system will think that you entered the correct password and will let you in. Von Hassel figured this out purely by accident as he fooled around with the Xbox One randomly. Just in case you are thinking about logging in on your friends account and spending all of his money on DLC, hold your horses, because Microsoft has already fixed the exploit.
More details below in the original news article.