Colin Percival has an in-depth look at some security issues with Jungle Disk.
A lot of people, including me, have recommended Jungle Disk to people because the cloud-based backup system encrypts your files before it uploads them to Amazon’s S3 service (as opposed to something like Dropbox which encrypts them after the files are uploaded — meaning Dropbox has the key to unencrypt your files if it wants to or is ordered to by a legal authority).
The problem that Percival points out is that they way Jungle Disk handles that client-side encryption is weak to the point where there is a clear path for attackers to follow in either corrupting data stored on the system; replacing data stored on the system; or, ultimately, cracking your password and having free reign.
I used Jungle Disk for a couple years to as a cloud-based backup of my main data hard drive. I used a 20 character passphrase. As Percival notes in a handy chart he provides, a 10 character strong password would take about 95,000 years to crack using current techniques and a $1,000 off-the-shelf laptop. A $10,000 GPU-based password cracking box is going to reduce that to 95 years, and the CIA’s going to be able to put together a system that could rip through that in 2 years. Given that, why not emulate Alfred E. Neuman — what, me worry? But, as Percival writes,
Now, maybe you don’t have any data stored which Joe Cracker would be willing to spend 10 hours decrypting. Maybe you trust Amazon and Rackspace’s internal procedures and security measures to ensure that nobody — either breaking in from outside, or working for those companies — will have access to your “encrypted” data. Depending on who you are and what data you have stored (your credit card numbers? bank statements? how about last year’s income tax return, complete with your national tax ID number?) you might be justified in such trust. But I would say that this is profoundly missing the point: With good cryptography, you wouldn’t need to trust them.
Anyone who doubts the level of computing resources that an individual or small group of people would be willing to throw at a complex problem based entirely on speculation about the value of doing so need only take a look at some of the crazy rigs and setups being built to do nothing but mine Bitcoins. And, of course, the cost of cracking passwords is only declining with every day that passes.
Again, the best solution with cloud backup or sync in general is for the user to encrypt the files before uploading them. Unfortunately, this increases security but at the cost of convenience (and maximizing convenience is apparently why Jungle Disk has these potential issues in the first place).