On the privacy of private browsing – A forensic approach

Most readers should already be familiar with the concept of Private Browsing (also known as the Incognito mode in Google Chrome and Inprivate Browsing in IE). This post is about our newly published paper in “Journal of Information Security and Applications” (Vol. 19, No. 1, 2014), which investigates the privacy issues of Private Browsing among four mainstream browsers: IE, Firefox, Chrome and Safari.

Our work is motivated by an apparent lack of research on the subject, despite that millions of users have been using private browsing to protect their privacy on the daily basis. In USENIX Security’08, Agrawal et al. studied the security of then-newly introduced private browsing feature in modern browsers and discovered several security issues. Their experiments were mainly focused on Firefox (in particular, Firefox V3.6). A year later, Said et al. extended the earlier work into analyzing the computer memory and they found traces of private browsing data in memory and cache after the browser is closed. Recently, in ESORICS’13, Lerner et al. presented a software tool that allows automatic verification of the browser extensions’ compliance with the private mode. The tool was mainly tested on Firefox extensions. Apart from these papers, the subject of private browsing seems to have received little attention from the security community.

In our paper, we conducted a systematic approach to investigate the privacy of private browsing across four main browsers: IE, Firefox, Chrome and Safari, and from various angles: not just in memory, but also in local database and web traffic. Our work constitutes an independent evaluation of the private browsing feature provided by mainstream web browsers. It presents the latest understanding on the security of private browsing as of February, 2014.

Our threat model is defined in terms of the attacker’s goals and capabilities. We divide the attacks into two categories: local and remote attacks. Local attacks mean the attacker has physical access to the user’s computer and has full control over it after the target user has exited the private browsing session (i.e., “after the fact” forensic). Remote attacks assume that the attacker is engaged with the user through HTTP(S) and wants to find out if the user is currently in the private browsing mode. Typically, this happens when the target user is visiting a web site controlled by the attacker. We have assessed different attacks in each category. A summary of all the attacks is presented in the following table. Those marked with * contain new results discovered by our study, while others correspond to attacks that have been previously known but validated again by our study. Full details about the attacks can be found in our paper. All the source codes for extensions and timing attacks are freely available here. We welcome any comments.

Firefox  Chrome  IE  Safari  Information leakage
Domain name system Browsing history
Memory inspection Browsing history, passwords, cookies
File timestamp When private mode was last used
Index.dat* N/A N/A N/A When private mode was last used
SQLite database crash* N/A Minor to serious depending on browsers
SQLite added bookmark* N/A Minor to serious depending on browsers
Extension* Browsing history
Cross-mode Interference* N/A N/A N/A User activities in private mode
Hyperlink attack If the user is in private mode
Timing attack* If the user is in private mode

Acknowledgements: This paper is based on an MSc dissertation titled “Is private browsing private?” by Kiavash Satvat.The authors of the paper are Kiavash Satvat, Matthew Forshaw, Feng Hao and Ehsan Toreini.