In a blog posted by customer service application maker Zendesk, officials disclosed that a hack of three of its major customers may put users of those services at risk.
"We've become aware that a hacker accessed our system this week," Zendesk wrote in the blog. "As soon as we learned of the attack, we patched the vulnerability and closed the access that the hacker had. Our ongoing investigation indicates that the hacker had access to the support information that three of our customers store on our system."
Although the blog does not name the three customers, officials later disclosed that they are Twitter, Pinterest, and Tumblr.
"We believe that the hacker downloaded email addresses of users who contacted those three customers for support, as well as support email subject lines," the blog says. "We notified our affected customers immediately and are working with them to assist in their response."
The three social networking providers said they have notified users of the data breach. Twitter said the incident affected a "small percentage" of users.
"Zendesk's breach did not result in the exposure of information such as Twitter account passwords," Twitter wrote in a message to affected customers, according to news reports. "It may, however, have included contact information you provided when submitting a support request such as an email, phone number, or Twitter username."
Pinterest and Tumblr sent similar messages, news reports say.
Neither the blog nor the news reports indicate how the attackers gained access to the data.
Because the compromised data included email addresses, users of the services should be particularly wary of phishing attacks and other fraudulent email exploits, security experts say.
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