SAN FRANCISCO, Sept. 1 -- Online file storage and sharing service Dropbox accounts were compromised last month after hackers exploited a flaw involving Lenovo's account authentication system, with files accessed in some cases.

Dropbox said hackers viewed and downloaded files from roughly 5,000 accounts during unauthorised access between August 4 and August 21, international newswires Reuters and Bloomberg reported today.

The company said the affected accounts were not protected by multi-factor authentication and were linked to Lenovo IDs, which can be used to access Dropbox accounts.

Hackers exploited an issue with Lenovo's email verification process to create Lenovo IDs using the email addresses of Dropbox users who had not signed up ...