A vulnerability was discovered by computer security researchers in early April this year and Microsoft was informed about it. Details of the bug leaked out and led attackers exploit it online. It's still not sure how many Hotmail customers fallen victim of it. Microsoft found a fix and closed the loophole.
Hotmail is said to be the world's largest web-based email service. Microsoft claims that it has more than 300 million users.
Source: http://www.bbc.co.uk/news/technology-17866897