Dropbox is asking users who signed up before mid-2012 to change their passwords if they haven\u2019t done so since then.The cloud storage service said it was asking users to change their passwords as a preventive measure, and not because there is any indication that their accounts were improperly accessed.Dropbox said it was taking the measure because its security teams learned about an old set of Dropbox user credentials, consisting of email addresses and hashed and salted passwords, which it believes were obtained in 2012 and could be linked to an incident the company reported around the time.In July 2012, Dropbox said its investigation found that usernames and passwords recently stolen from other websites were used to sign in to a small number of of Dropbox accounts. It said it had contacted the users affected to help them protect their accounts.The move by Dropbox comes in the wake of several breaches including that of LinkedIn in 2012, which has led to concerns that email addresses and passwords used by people across accounts could be used to compromise other services.Starting in 2007 with a consumer focus, Dropbox, which allows users to store, access and share\u00a0 files easily from a variety of devices, launched in 2013 its Dropbox Business service, its entry into the business market. The company claims over 500 million registered users, with over 200,000 businesses and organizations using Dropbox Business.Users who signed in before 2012 and haven't changed their password since then will be prompted by Dropbox to change it the next time they sign in. Users will have to set a stronger password with the help of a meter provided by Dropbox that measures its strength.\u201cIf you don\u2019t receive a prompt, you don\u2019t need to do anything. However, for any of you who\u2019ve used your Dropbox password on other sites, we recommend you change it on Dropbox and other services,\u201d wrote Patrick Heim, head of trust and security at Dropbox, in a blog post Thursday. Dropbox is also recommending that users use two-factor authentication when resetting their passwords.On Twitter, a number of users pasted copies of emails they received from Dropbox about the password change.