A couple weeks ago, Dropbox announced that it invalidated some old "shared links" users used to share confidential documents, like tax returns [1] . The real question here is of course, how did these tax returns get exposed in the first place. 

Dropbox usually requires a username and a password to access documents, and even offers a two-factor solution as an option. But regardless, the user may allow a document to be access by others, who just know the "secret" link.

As we all know, the problem is "secret" links easily leak. But as users rely more on cloud services to share files, and passwords for each shared file are way too hard to set up, this is going to happen more and more. Dropbox isn't the only such service that offers this feature. In a recent discussion with some banks, the problem came up in that more and more customers attempt to share documents with the bank for things like mortgage applications. While the banks do refuse to accept documents that way, the pressure exists and I am sure other businesses with less regulatory pressure, will happily participate.

For a moment, lets assume the cloud service works "as designed" and your username and password is strong enough. Cloud services can be quite useful as a cheap "offsite backup", for example to keep a list of serial numbers of your possessions in case of a burglary or catastrophic event like a fire. But as soon as you start sharing documents, you run the risk of others not taking care of them as well as you would. May it be that their passwords are no good, or maybe they will let the "secret link" you gave them wander. 

Confidential personal, financial or medical information should probably not go into your cloud account. And if they do, encrypt before uploading them. 

Here are a couple of steps to de-cloud your life:

- setup an "ownCloud" server. It works very much like Dropbox with mobile clients available for Android and iOS. But you will have to run the server. I suggest you make it accessible via a VPN connection only. Sharepoint may be a similar solution for Windows folks.

- run your own mail server: This can be a real pain and even large companies move mail services to cloud providers (only to regret it later ...?). But pretty much all cloud mail providers will store your data in the clear, and in many ways they have to. Systems to provide real end-to-end encryption for cloud/web-based e-mail are still experimental at this point.

- Offsite backup at a friends/relatives house. With wide spread use of high speed home network connections, it is possible to setup a decent offsite backup system by "co-locating" a simple NAS somewhere. The disks on the NAS can be encrypted and the connection can use a VPN again.

- For Apple users, make local backups of your devices instead of using iCloud. iCloud stores backups unencrypted and all it takes for an attacker to retrieve a backup is your iCloud username/password.

Any other tips to de-cloud?

[1] https://blog.dropbox.com/2014/05/web-vulnerability-affecting-shared-links/

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Johannes B. Ullrich, Ph.D.
SANS Technology Institute
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