A dark window and a discreet symbol do not turn a browser into an invisibility cloak. Private browsing is primarily designed to limit the traces left in the browser on the device being used. It can be useful on a shared computer, but its name does not answer an essential question very well: private from whom?
The ICI programme ‘Digital life, private life’, published on 14 November with a CNIL expert, looks at this use. Browser documentation helps distinguish three situations: what the next person using the computer will see, what a website knows when you sign in, and what can be observed on the network.
A separate session, not a general wipe
Google explains that Chrome opens a separate session in private mode. Cookies and data needed for websites to work exist during this session; they are deleted when it ends. Chrome does not then retain a history of the pages visited.
You have to close all private windows to end the session. Private mode does not delete downloaded files or saved bookmarks.
A document saved in the Downloads folder must therefore be managed separately, like any other file on the computer.
Signing in still means identifying yourself
Mozilla points out that a website recognises someone who signs in to their account, even in a private window. Entering the username for an email or social media account gives that service precisely that identity. Separating cookies from ordinary browsing does nothing to undo this deliberate sign-in.
In Firefox, this mode can also be used to access a second account without signing out of the first, or to start afresh with a session that has none of the previous session’s cookies. These are practical uses; they are not a guarantee of anonymity.
The browser adds protections against known trackers and makes some device identification techniques more difficult. This ‘fingerprint’ can combine characteristics such as the screen, fonts or hardware. Limiting these signals and blocking scripts improves protection, but does not justify concluding that no website will be able to recognise the user. Nor does the presence of adverts, by itself, show that private mode has failed.
On Mac, Safari also keeps traces separate across devices
The Safari documentation for Mac offers another useful distinction: pages from private windows are not stored in iCloud to appear among the open tabs on other Apple devices. AutoFill information and pages visited are not saved during this browsing session.
Apple also specifies that downloaded files remain on the Mac, even if they do not appear in the downloads list. An open window still lets you return to previous pages using the back button. Leaving private mode in one window does not close the others.
Safari also offers protections against tracking and fingerprinting, which may affect how some websites work. These details apply to Safari on Mac: menus and gestures should not be assumed to work in exactly the same way on a phone.
The network is another matter
Google warns that the websites visited and network administrators — an employer, school or internet service provider — may still be able to observe activity. Private mode does not promise to hide it from them; that does not mean they can see all content.
So it is best to be clear about what you need before using it. Preventing a session from remaining open for the next user, separating two accounts and concealing your activity from the network are three different aims. Private browsing addresses some of the first kinds of use. It does not mean all three can be dealt with using a single button.
Illustrative photograph: using a laptop. Photo: Shixart1985 — Wikimedia Commons, CC BY 2.0.
