"A stolen password was used to access an employee Dropbox account containing a project document with user email addresses," Aditya Agarwal, Dropbox’s VP of Engineering, wrote on Tuesday.
"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."
He also mentioned that their investigation also discovered that some Dropbox accounts were compromised through the use of usernames and passwords recently stolen from other websites, pointing out the need for using different passwords for every online account.
Among the security improvements Dropbox is planning to add to make sure things like this don't happen again is optional two-factor authentication, a new page that lets users examine all active logins to their account, and the forced password change for users who have not changed their passwords in a while or are using a common one.
He also announced new automated mechanisms to help identify suspicious activity, and says that more of these will be added over time.