It looks like Dropbox had a late discovery on scope of a previously reported breach. From their blog yesterday:
If you signed up for Dropbox prior to mid-2012 and haven’t changed your password since, you’ll be prompted to update it the next time you sign in. We’re doing this purely as a preventive measure, and there is no indication that your account has been improperly accessed. We’re sorry for the inconvenience.
[…]Why we’re doing this
Our security teams are always watching out for new threats to our users. As part of these ongoing efforts, we learned about an old set of Dropbox user credentials (email addresses plus hashed and salted passwords) that we believe was obtained in 2012. Our analysis suggests that the credentials relate to an incident we disclosed around that time.
Based on our threat monitoring and the way we secure passwords, we don’t believe that any accounts have been improperly accessed. Still, as one of many precautions, we’re requiring anyone who hasn’t changed their password since mid-2012 to update it the next time they sign in.
h/t, The Business Journals.