Hundreds of alleged usernames and passwords for online document-sharing site Dropbox were published on Monday on Pastebin, an anonymous information-sharing website.
The anonymous user, who claims to have hacked close to 7 million accounts, is calling for Bitcoin donations to fund the operation.
"We will keep releasing more to the public as donations come in, show your support," the anonymous Pastebin user said on the site.
Dropbox, however, said it has not been hacked.
"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. All other remaining passwords have been expired as well," a Dropbox spokesman said in an email to Reuters.
Dropbox is a Silicon Valley startup that has proved a hit with consumers and boasts more than 200 million users six years after it was started. It has undergone tremendous growth amid the meteoric rise of cloud, which is expected to continue booming alongside mobile computing.
NSA whistleblower Edward Snowden last week advised those concerned about their privacy to "get rid of Dropbox" and cease using Facebook and Google.
(Reporting by Supriya Kurane in Bangalore; Editing by Gopakumar Warrier)