(CNN) -- Customer support tool Zendesk announced in a blog post Friday that it had been hacked earlier this week. The hack affected users of three services that use Zendesk: Pinterest, Tumblr and Twitter.
"We believe that the hacker downloaded email addresses of users who contacted those three customers for support, as well as support email subject lines," Zendesk said in the post. It did not name the three customers publicly, but subsequent e-mails from Pinterest, Tumblr and Twitter to their customers confirmed the breach.
"We recently learned that the vendor we use to answer support requests and other emails (Zendesk) experienced a security breach," said Pinterest in an email. The pinboard-style photosharing social network site said it was only sending the e-mail to people who had used Zendesk and were possibly compromised.
It urged users to take measures to keep their accounts secure by not sharing their passwords and using strong passwords.
Tumblr told its customers to review any e-mail conversations they had with Zendesk to make sure there was no account information that could be used in phishing attacks.
CNN's Heather Kelly contributed to this report.