It is a
common misconception that private browsing modes can protect users from being
tracked by other websites or their
Internet service provider (ISP). Private browsers on
iOS, not created by
Apple, must adhere to specific standards and regulations to be available on its platform for
iPhone and
iPad. Specifically, these browsers are required to employ the
WebKit framework for rendering web pages. Consequently, third-party browsers cannot use their own rendering engines and must depend on Apple's framework instead. This constraint impacts the range of privacy features that these browsers can provide. This is one of the reasons why some browsers have partly addressed this shortcoming by offering additional privacy features that can be automatically enabled when using private browsing mode, such as Firefox's "Tracking Protection" feature to control use of web trackers (which has since been rolled into a larger "content blocking" function extended outside of private browsing mode), and
Opera offering an in-house
VPN service embedded within the browser. This research was later extended to include the Chrome and Safari browsers. The gathered data proved that the browsers' private mode implementations cannot fully hide users' browsing activities and that browsers in private mode leave traces of activities in caching structures and files related to the
paging process of the
operating system. Another independent security analysis, performed by a group of researchers at
Newcastle University, reported a range of potential security vulnerabilities in the implementation of the private modes across Chrome, Firefox, Internet Explorer, and Safari, including that: •
Browser extensions could still record history if they were active in private mode. Although Chrome and Firefox have since required extensions to be enabled on an opt-in basis for their private browsing modes, an installed extension in the normal mode could learn the user's activities in the private mode by measuring the usage of shared computing resources. • Data erasure by the browser alone is found to be insufficient. For example, the records of visited websites during the private session can be retained in memory for a long time even after the private session is closed. In addition, the visited website records are usually kept by the operating system in the local
DNS cache. Furthermore, the modified timestamps of certain profile files saved on the disk may reveal if the private mode was previously turned on and when it was turned on. •
Software bugs present in some browsers were found to seriously degrade the security of the private mode. For example, in some earlier versions of Safari, the browser retained private browsing history records if the browser program was not closed normally (e.g., as a result of a crash), or if the user acted to add a
bookmark within the private mode. • Depending on whether the session is in the private or the normal mode, web browsers typically exhibit different
user interfaces and traffic characteristics. This allows a remote website to tell if the user is currently in the private mode: for example, by checking the color of the hyperlinks or measuring the time of writing cookies. Bugs and security vulnerabilities in extensions themselves may also leak personally identifiable data from private mode. Implementations of the
HTML5 FileSystem API can be used to detect users in private mode. In Google Chrome, the FileSystem API was not available in Incognito mode prior to version 76. To
prevent circumvention of
paywall policies and evasion of
web tracking scripts used to monetize traffic, a number of websites — including
The New York Times — have used such behavior to block access to users in private browsing mode, and requiring them to subscribe or log in. Chrome 76 allows the FileSystem API to be used in Incognito mode; explaining the change, Google argued that the ability to detect the use of Incognito mode infringes on users' privacy. However, it was later discovered that the disk space quotas for the API differed between normal and Incognito modes, providing another means by which to detect Incognito users. Despite statements otherwise by Google, this has not yet been patched. Scripts have also been developed to detect private browsing mode on other browsers, such as Firefox.
Associated lawsuit In December 2023, Google settled a $5 billion consumer privacy lawsuit that alleged that its practices allowed it to track users in private browsing mode in various browsers. == See also ==