The usernames and phone numbers for 4.6 million Snapchat accounts have been downloaded by hackers, who temporarily posted the data online.
A website called SnapchatDB released the data but censored the last two digits of the phone numbers.
It has since been taken offline but a cached version is still available.
The hack comes days after an Australian firm, Gibson Security, warned of vulnerabilities in Snapchat's app which it said could be exploited by hackers.
Gibson Security said it was not involved in the hack: "We know nothing about SnapchatDB, but it was a matter of time till something like that happened," the firm tweeted.
The hackers behind the website that published the data said they had exploited the security flaw highlighted by Gibson Security.
"We used a modified version of gibsonsec's exploit/method," they were quoted as saying by tech blog, Tech Crunch.
Snapchat has grown in popularity as an app that allows people to share pictures, safe in the knowledge they delete themselves after being viewed.
It has a feature called Find Friends, which allows users to upload their address book contacts to help find friends who are also using the service.
In its report published on 25 December, Gibson Security warned that a vulnerability on the Snapchat app could be used to reveal the phone numbers of users.
The firm said it had first warned Snapchat about this four months ago, adding that "nothing had been really been improved upon".
Gibson claimed that it had been able to crunch through ten thousand phone numbers of Snapchat users "in approximately 7 minutes on a gigabit line on a virtual server".
In response to the Gibson report, Snapchat acknowledged a potential vulnerability but said it had taken measures to protect user data.
"Theoretically, if someone were able to upload a huge set of phone numbers, like every number in an area code, or every possible number in the US, they could create a database of the results and match usernames to phone numbers that way," it said in a blogpost last week.
"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."
However, the hackers behind the SnapchatDB, the site that published the phone numbers, said the measures were not strong enough.
"Even now the exploit persists. It is still possible to scrape this data on a large scale," they claimed.
"Their latest changes are still not too hard to circumvent."