A massive leak could affect millions of people who use Snapchat.

The photo app revealed it was informed about a breach of security “by which one could compile a database of Snapchat usernames and phone numbers."

Sites including GigaOm and TheVerge claim 4.6 million usernames and phone numbers appeared on a downloadable database. The site hosting that database is no longer up and running. It’s not clear who the hackers responsible may have been.

Snapchat says users don't need to provide phone numbers, but it can help your friends locate you. Those numbers are never given to other users, either, and the site doesn't allow people to find that contact information solely by having someone's username.

As for the possibility of publishing a mass set of phone numbers, Snapchat says, "Over the past year we’ve implemented various safeguards to make it more difficult to do. We recently added additional counter-measures and continue to make improvements to combat spam and abuse."