Hackers have stolen over 60 million account details for online cloud storage platform Dropbox. Although the accounts were stolen during a previously disclosed breach, and Dropbox says it has already forced password resets, it was not known how many users had been affected, and only now is the true extent of the hack coming to light.
Motherboard obtained a selection of files containing email addresses and hashed passwords for the Dropbox users through sources in the database trading community. In all, the four files total in at around 5GB, and contain details on 68,680,741 accounts. The data is legitimate, according to a senior Dropbox employee.
Earlier this week, Dropbox announced it was forcing password resets for a number of users after discovering a set of account details linked to a 2012 breach. The company did not publish an exact figure on the number of resets, and said it had taken the move proactively.
"Our security teams are always watching out for new threats to our users. As part of these ongoing efforts, we learned about an old set of Dropbox user credentials (email addresses plus hashed and salted passwords) that we believe were obtained in 2012. Our analysis suggests that the credentials relate to an incident we disclosed around that time," the company wrote.
These 60 million user accounts are related to the same data breach incident. Motherboard was provided the full set by breach notification service Leakbase, and found many real users in the dataset who had signed up to Dropbox in around 2012 or earlier.
"We've confirmed that the proactive password reset we completed last week covered all potentially impacted users," said Patrick Heim, Head of Trust and Security for Dropbox. "We initiated this reset as a precautionary measure, so that the old passwords from prior to mid-2012 can't be used to improperly access Dropbox accounts. We still encourage users to reset passwords on other services if they suspect they may have reused their Dropbox password."
A spokesperson told Motherboard that Dropbox has seen no evidence of malicious access of these accounts.
Nearly 32 million of the passwords are secured with the strong hashing function bcrypt, meaning it is unlikely that hackers will be able to obtain many of the users' actual passwords. The rest of the passwords are hashed with what appears to be SHA-1, another, aging algorithm. These hashes seem to have also used a salt; that is, a random string added to the password hashing process to strengthen them.
Dropbox has changed its password hashing practices several times since 2012, in order to keep passwords secure.
The Dropbox dump does not appear to be listed on any of the major dark web marketplaces where such data is often sold: the value of data dumps typically diminishes when passwords have been adequately secured. One hacker told Motherboard he or she was already in possession of the data though.
This is just the latest so-called "mega-breach" to be revealed. This summer, hundreds of millions of records from sites such as LinkedIn, MySpace, Tumblr, and VK.com from years-old data breaches were sold and traded amongst hackers.
Update 31/8/2016: Added extra comment from Dropbox, and clarified a sentence on sourcing.