An illuminated version of Dropbox's logo hangs in the company's San Francisco office.

Dropbox is asking users who signed up before mid-2012 to change their passwords if they haven t 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.

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