Note: This blog post was written before the new Chromium-based Microsoft Edge was announced. As a consequence, it mostly discusses the behavior of the Legacy Microsoft Edge browser. The new Chromium-based Edge behaves largely the same way as Google Chrome.
All leading browsers offer a “Private Mode” and they all behave in the same general ways.
While in Private mode, browsers typically ignore any previously cached resources and cookies. Similarly, the Private mode browser does not preserve any cached resources beyond the end of the browser session. These features help prevent a revisited website from trivially identifying a returning user (e.g. if the user’s identity were cached in a cookie or JSON file on the client) and help prevent “traces” that might be seen by a later user of the device.
In Firefox’s and Chrome’s Private modes, a memory-backed cache container is used for the HTTP cache, and its memory is simply freed when the browser session ends. Unfortunately, WinINET never implemented a memory cache, so in Internet Explorer InPrivate sessions, data is cached in a special WinINET cache partition on disk which is “cleaned up” when the InPrivate session ends.
Because this cleanup process may be unreliable, in 2017, Edge Legacy made a change to simply disable the cache while running InPrivate, a design decision with significant impact on the browser’s network utilization and performance. For instance, consider the scenario of loading an image gallery that shows one large picture per page and clicking “Next” ten times:
Another interesting quirk of Edge Legacy’s InPrivate implementation is that the browser will not download FavIcons while InPrivate. Surprisingly (and likely accidentally), the suppression of FavIcon downloads also occurs in any non-InPrivate windows so long as any InPrivate window is open on the system.
Web Platform Storage
Akin to the HTTP caching and cookie behaviors, browsers running in Private mode must restrict access to HTTP storage (e.g. HTML5 localStorage, ServiceWorker/CacheAPI, IndexedDB) to help prevent association/identification of the user and to avoid leaving traces behind locally. In some browsers and scenarios, storage mechanisms are simply set to an “ephemeral partition” while in others the DOM APIs providing access to storage are simply configured to return “Access Denied” errors.
You can explore the behavior of various storage mechanisms by loading this test page in Private mode and comparing to the behavior in non-Private mode.
Within IE and Edge Legacy’s InPrivate mode, localStorage uses an in-memory store that behaves exactly like the sessionStorage feature. This means that InPrivate’s storage is (incorrectly) not shared between tabs, even tabs in the same browser instance.
Beyond the typical Web Storage scenarios, browser’s Private Modes should also undertake efforts to prevent association of users’ Private instance traffic with non-Private instance traffic. Impacted features here include anything that has a component that behaves “like a cookie” including TLS Session Tickets, TLS Resumption, HSTS directives, TCP Fast Open, Token Binding, ChannelID, and the like.
In Private mode, a browser’s AutoComplete features should be set to manual-fill mode to prevent a “NameTag” vulnerability, whereby a site can simply read an auto-filled username field to identify a returning user.
On Windows, most browsers support silent and automatic authentication using the current user’s Windows login credentials and either the NTLM and Kerberos schemes. Typically, browsers are only willing to automatically authenticate to sites on “the Intranet“. Some browsers behave differently when in Private mode, preventing silent authentication and forcing the user to manually enter or confirm an authentication request.
In Firefox Private Mode and Edge Legacy’s InPrivate, the browser will not automatically respond to a HTTP/401 challenge for Negotiate/NTLM credentials.
In Chrome Incognito, Brave Incognito, and IE InPrivate, the browser will automatically respond to a HTTP/401 challenge for Negotiate/NTLM credentials even in Private mode.
- In Edge Legacy and the new Chromium-based Edge, the security manager returns MustPrompt when queried for URLACTION_CREDENTIALS_USE.
- Unfortunately Edge Legacy’s Kiosk mode runs InPrivate, meaning you cannot easily use Kiosk mode to implement a display that projects a dashboard or other authenticated data on your Intranet.
- For Firefox to support automatic authentication at all, the
network.negotiate-auth.allow-non-fqdn and/or network.automatic-ntlm-auth.allow-non-fqdn preferences must be adjusted.
Detection of Privacy Modes
While browsers generally do not try to advertise to websites that they are running inside Private modes, it is relatively easy for a website to feature-detect this mode and behave differently. For instance, some websites like the Boston Globe block visitors in Private Mode (forcing login) because they want to avoid circumvention of their “Non-logged-in users may only view three free articles per month” paywall logic.
- Stack Overflow topic
- Mozilla indexedDB issue
- Inferring Incognito from web-accessible extension surface
Signaling Privacy Mode
Update: Perhaps surprisingly, the new Microsoft Edge sends a deliberate signal to the user’s default search engine when it is loaded in InPrivate mode. The HTTPS request header
PreferAnonymous: 1 is sent on requests to the server to allow it to avoid caching any data related to the user’s use of the search engine.
This header is sent only to the search engine, and not to other sites.
Browsers based on Chromium often also have a Guest Browsing mode, which has a superset of Private Mode behavior.
When you “Browse as Guest”, the browser session treats itself as running in Private Mode, but unlike a normal Private Mode session, the session also starts with an empty user profile. That means the browser will not
- show history entries in the address box or History page
- list your bookmarks
- offer to fill in your account information or passwords
- offer to fill in autocomplete information
When you close all Guest Profile windows, your history and other state generated in the session is deleted, just like in regular Private mode.
Using a Guest Profile helps prevent you from accidentally leaking information to sites (e.g. inadvertently triggering an autofill).
Advanced Private Modes
The Brave Browser offers a “Private Window with Tor” feature that routes traffic over the Tor anonymizing network; for many users this might be a more practical choice than the highly privacy-preserving Tor Browser Bundle, which offers additional options like built-in NoScript support to help protect privacy.