Hackers have gained access to the passwords to a number of Tumblr user's email addresses from early 2013. The email addresses affected were members of the service before Yahoo acquired the company. Yahoo is investigating the issue. "Our analysis gives us no reason to believe that this information was used to access Tumblr accounts," the Tumblr blog explained. "As a precaution, however, we will be requiring affected Tumblr users to set a new password."