Twitter said on Tuesday email addresses and phone numbers uploaded by users to meet its security requirements may have been 'inadvertently' used for advertising purposes.
The micro-blogging site said the issue was rectified as of Sept. 17, without disclosing how many users were impacted.
"This was an error and we apologise," the company said in a blog post.
Social media companies, including Twitter and Facebook, have faced heat from users and regulators globally on how their platforms handle user data.
Twitter said when advertisers uploaded their marketing lists, it may have matched people on the platform to their list based on the email or phone number provided by account holders.
The company said the information was not shared with third parties, but was made available internally to allow users to receive targeted advertising.
(Reporting by Munsif Vengattil in Bengaluru; Editing by Anil D'Silva)