Twitter has emailed its business clients to tell them that personal information may have been compromised.
Unbeknownst to users, billing information of some clients was stored in the browser's cache, it said.
In an email to its clients, Twitter said it was "possible" others could have accessed personal information.
The personal data includes email addresses, phone numbers and the last four digits of clients' credit card numbers.
The tech company says that there is no evidence that clients' billing information was compromised.
The breach affects businesses which use Twitter's advertising and analytics platforms.
It's not yet clear how many businesses have been affected.
The company said it became aware of the issue on 20 May, and has since fixed the problem.
In an email to affected users, the firm said: "We're very sorry this happened. We recognise and appreciate the trust you place in us, and are committed to earning that trust every day."
It is not believed that non-business Twitter users are affected.
It's not the first time Twitter has been exposed to a data breach.