Photo sharing app Instagram was hit by a bug last week, which resulted in six million accounts being hacked.
The bug, which as since been fixed, allowed hackers to get users' information such as e-mail addresses and contact details, Instagram's co-founder and chief technology officer Mike Krieger said in a note posted on the site on Sept 1.
It affected both verified and non-verified accounts, The Verge website reported.
Hacked details were posted on a database called Doxagram, which let people search for users' contact details for US$10 (S$13.56).
Doxagram was down late Friday, but celebrities' contact information is still floating around the dark web, according to The Verge report.
Cybersecurity firm RepKnight said it found what is supposedly contact information for celebrities including actress Emma Watson, musician Harry Styles, footballer Neymar Jr and many more.
The bug was allegedly responsible for singer Selena Gomez's account being hacked on Aug 28, and nude photos of her former boyfriend Justin Bieber being posted.
Mr Krieger said in his note that no passwords were revealed in the hack: "We quickly fixed the bug, and have been working with law enforcement on the matter. Although we cannot determine which specific accounts may have been impacted, we believe it was a low percentage of Instagram accounts."
According to The Verge, there are over 700 million active Instagram accounts.
Mr Krieger also encouraged users to be vigilant about the security of their accounts, and to report any unusual activity through the app's reporting tools.
"Protecting the community has been important at Instagram from day one, and we're constantly working to make Instagram a safer place. We are very sorry this happened."