Dropbox has suffered an alleged leak of hundreds of usernames and passwords, data which was then published on info-sharing site Pastebin.
Hacking group Anonymous said it had accessed the details of nearly 7 million accounts.
Dropbox has said that it has not been hacked.
A Dropbox spokesperson told Reuters: "These usernames and passwords were unfortunately stolen from other services and used in attempts to log in to Dropbox accounts. We’d previously detected these attacks and the vast majority of the passwords posted have been expired for some time now."
The third party service hack is similar to this week’s Snapchat data breach, where hackers accessed nearly 200,000 images by breaking in to photo-saving service Snapsaved.com.