That finding comes from recently published research, "Using Smartphones as a Proxy for Forensic Evidence contained in Cloud Storage Services," conducted by University of Glasgow computer science PhD student George Grispos -- backed by computer forensics and e-discovery lecturer Brad Glisson and software engineering lecturer Tim Storer, both also of University of Glasgow -- which was presented at this year's 46th Hawaii International Conference on System Sciences.
The researchers said they'd expected to find "that smartphone devices will retain data from these storage services," but didn't know to what extent any leftover "artifacts" might include recoverable information. So they studied three popular cloud storage service apps running on the iPhone and on an HTC Desire running the Android operating system.
Here's what they found: "Using mobile forensic toolkits, data can be recovered from a smartphone device which has accessed a cloud storage service," they said. "The results from the experiment have shown that it is possible to recover files from the Dropbox, Box and SugarSync services using smartphone devices." In addition, artifacts left by those services' mobile apps in some cases allowed the researchers to gain a "proxy view" of files not stored on the device, but stored by the cloud service.
[ Companies have to protect their assets, but where do they cross the line into overzealous prying? See Monitoring Vs. Spying: Are Employers Going Too Far? ]
The extent to which they could recover files varied based on the operating system studied. "On the HTC Desire, both deleted and available files were recovered. The forensic toolkits recovered 9 files from Dropbox, 15 from Box and 11 from SugarSync," the researchers said. "On the iPhone, depending on application and device manipulation either 5 or 7 files were recovered from Dropbox, 7 or 15 from SugarSync and 5 from Box. No deleted application files were recovered from the iPhone."
Interestingly, the investigators could also use the Box app's file artifacts that they recovered to access copies of files that were no longer stored on devices, but still stored with Box. This required recovering file IDs for Box files that had been accessed, as well as authentication tokens linked to a specific Box user's account. With both pieces of information, the researchers could create a URL which accessed the Box API to download copies of files not present on the device, all without logging into the service. This digital forensic investigation technique worked on both the iPhone and Android devices.
Just to be clear, the researchers confined their study to these -- now superseded -- smartphone apps: "Dropbox (iOS version 1.4.7, Android version 2.1.3), Box (iOS version 2.7.1, Android version 1.6.7) and SugarSync (iOS version 3.0, Android version 3.6)."
What can smartphone users do to obscure any cloud-based files they've viewed? According to the researchers, clearing the cache led to them recovering fewer Dropbox and SugarSync files, but had no effect on the Box files. The researchers also reported that their file recovery success on the Android device depended, predictably, on whether the file had been saved for offline use, and if so, whether or not it had been saved to an external memory card, then deleted and overwritten, at which point it was unrecoverable.