Abusing locality in shared web hosting

The increasing popularity of the World Wide Web has made more and more individuals and companies to identify the need of acquiring a Web presence. The most common way of acquiring such a presence is through Web hosting companies and the most popular hosting solution is shared Web hosting. In this paper we investigate the workings of shared Web hosting and we point out the potential lack of session isolation between domains hosted on the same physical server. We present two novel server-side attacks against session storage which target the logic of a Web application instead of specific logged-in users. Due to the lack of isolation, an attacker with a domain under his control can force arbitrary sessions to co-located Web applications as well as inspect and edit the contents of their existing active sessions. Using these techniques, an attacker can circumvent authentication mechanisms, elevate his privileges, steal private information and conduct attacks that would be otherwise impossible. Finally, we test the applicability of our attacks against common open-source software and evaluate their effectiveness in the presence of generic server-side countermeasures.