Hackers obtained access to a set of Tumblr user email addresses with salted and hashed passwords from early 2013, the Yahoo-owned microblogging site Tumblr announced Thursday.

The set of emails predates Yahoo's acquisition of Tumblr in May 2013. The company said it recently learned of the security breach, and it has been thoroughly investigated.

"Our analysis gives us no reason to believe that this information was used to access Tumblr accounts," the Tumblr staff wrote on its blog. "As a precaution, however, we will be requiring affected Tumblr users to set a new password."