Comment: I would like to go out of my way to thank Paymo.biz for the professionalism that they displayed in promptly responding to vulnerabilities brought to their attention to ensure that their users were protected. Within 24 hours, they had responded to my initial communication and shortly thereafter were sharing proposed protections which were then quickly implemented on production systems. Web application vendors can learn from their example.
Yesterday, at the Blackhat DC security conference, I spoke about the dangers of persistent web browser storage. Part of the talk focused on how emerging web browser storage solutions such as Gears (formerly Google Gears) and the Database Storage functionality included in the emerging HTML 5 specification, could be attacked on sites with existing cross-site scripting (XSS) vulnerabilities. The overall message is that while such technologies have built in controls to protect against attacks such as SQL injection (SQLi), when secure technologies are implemented on insecure sites, protections become meaningless.
Both Gears and HTML 5 Database Storage, permit web applications to store content in local relational databases, which reside on the local file system by leveraging the SQLite database format. This provides powerful functionality as web applications can now be taken offline as was recently done with Gmail. At the same time, it adds a new attack vector as persistent data can now potentially be attacked on the desktop, not just the server. Given that we're dealing with a relational database, is client-side SQL injection (csSQLi) possible? Unfortunately, the answer is yes and it's not simply a theoretical attack, it's very practical thanks to the significant prevalence of XSS vulnerabilities.
While Gears has not yet been widely adopted, I expect this to change in the coming months, especially with the exposure that the technology will receive thanks to recent integration with Gmail. As users recognize the power of being able to take web applications offline, developers are sure to investigate adding Gears to their own applications. It's important to note that this attack has nothing to do with insecurities within the Gears technology itself. As mentioned, the attack is made possible when Gears is implemented on a site with existing XSS vulnerabilities. Unfortunately, XSS is a vulnerability which is far too prevalent on the web today. As such, it is inevitable that we'll see sites vulnerable to csSQLi. I hope that this early example illustrates the risks associated with offline browser storage and the importance of ensuring overall application security before adding this powerful functionality. Don't avoid Gears...embrace it...but do so securely.
A full copy of the slides from my talk entitled A Wolf in Sheep's Clothing - The Dangers of Persistent Web Browser Storage is available here.