Researchers at the University of Glasgow have found that cloud storage apps that say they send files to the cloud also leave retrievable versions of files on the devices.
The files aren't there for all to see, but do represent a resource useful to forensic investigators or those willing to look hard – for whatever purpose.
The extent to which data remains on phones is detailed in a paper titled Using Smartphones as a Proxy for Forensic Evidence contained in Cloud Storage Services, delivered at the 46th Hawaii International Conference on System Sciences and authored by George Grispos, William Bradley Glisson and Tim Storer.
The paper explains the authors performed a hard reset on an iPhone 3G running iOS 3 and an HTC Desire running Android 2.1. Those phones were then equipped with 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). 20 files - JPGs, .DOCS, .PDFs, .MP3s and .MP4s - were created on each device. Some of the files were opened or altered and some left alone. The phones were then “manipulated in one of the following ways”:
- Active power state - the smartphone was not powered down and the application's cache was not cleared;
- Cache cleared - the applications cache was cleared;
- Powered off - the smartphone was powered down; and
- Cache cleared and powered off
Next, the phones were “processed to create a forensic dump of its internal memory.” Which is when the researchers found lots of files in lots of places.
The exact results are detailed here (PDF), but the long and short of the study is that the HTC Desire's SD card and the iPhone's main storage both yielded files users would reasonably expect to have been vaporised and condensed in the cloud services mentioned above, rather than remaining on the handsets.
Both phones even yielded the unique file ID number for items uploaded to Box, an authentication token for that service and a URL. Together, “This information can be merged to reconstruct a URL, which will result in the file associated with the ZBOXID being downloaded.”
The paper offers the following conclusion:
“The results from this research have shown that smartphone devices which access cloud storage services can potentially contain a proxy view of the data stored in a cloud storage service. The recovery of data from these devices can in some scenarios provide access to further data stored in a cloud storage account. From the client perspective, it can potentially provide a partial view of the data without access to the data provider. The recovery of this evidence is dependent on two factors. First, the cloud storage application has been used to view the files in the cloud. Second, the user has not attempted to clear the cache of recently viewed files.”
The paper also suggests more research is needed, because the whole point of cloud storage is access from multiple devices and security of those devices is therefore very important.
More research also seems prudent because the authors used superseded versions of the apps. Dropbox on iOS, for example, is now at version 2.1.3, many updates beyond the version 1.4.7 used for this analysis.
The authors' long-term plan is to get that additional research done and to eventually “propose a set of security measures for both cloud providers and smartphone users to mitigate the potential risk of data leakage.” ®