Users of three popular social media sites have been warned of a security breach that may have given a hacker access to their email addresses and other information.

Customer service software provider Zendesk, which has more than 20,000 clients, says it’s investigating after its system was breached this week.

“As soon as we learned of the attack, we patched the vulnerability and closed the access that the hacker had,” Zendesk CEO Mikkel Svane wrote in a blog post Thursday.

Svane did not name the affected Zendesk clients, but Wired Magazine and others identified them as Twitter, Pinterest and Tumblr.

“Our ongoing investigation indicates that the hacker had access to the support information that three of our customers store on our system,” Svane wrote. “We believe that the hacker downloaded email addresses of users who contacted those three customers for support, as well as support email subject lines. We notified our affected customers immediately and are working with them to assist in their response.”

Svane apologized to Zendesk’s customers and their users.

“We’re incredibly disappointed that this happened and are committed to doing everything we can to make certain it never happens again,” he wrote.

Pinterest sent out emails to affected users, telling them their names, email addresses and subject lines in messages to the company were “improperly accessed” during the Zendesk security breach.

“We’re really sorry this happened, and we’ll keep working with law enforcement and our vendors to ensure your information is protected,” the email said.