According to this post on the company blog, Dropbox discovered of a security breach after receiving complaints about spam from its users. If you have a Dropbox account, now would be a good time to update your password.
A stolen password was also used to access an employee Dropbox account containing a project document with user email addresses. We believe this improper access is what led to the spam. We’re sorry about this, and have put additional controls in place to help make sure it doesn’t happen again.