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Comment by KMnO4 | Hacker News Reader

How does it do that? Mozilla spends a great deal of effort to make sure this type of fingerprinting isn’t possible.

indexedDB is the short answer. Whilst Mozilla has made efforts to make it harder, there is still a bunch of things that don't work the same way in private mode, which can provide a best guess as to whether you're in private mode.

indexedDB isn't implemented in Firefox's private mode. [0]

So if you can guess that it is Firefox, and can't use indexedDB, then you can guess that you're in private mode.

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=1639542

Does Chromium do the same thing? The page is able to detect incognito mode in Chromium Edge. The page even uses the browser-specific terminology, private mode for Firefox, incognito mode for Chromium Edge.

No, indexedDB not working is a Firefox-specific thing.

But there are other differences you can look for under Chrome, the simplest of which is that navigator.storage is smaller whilst incognito mode is active.

And under Safari you can just check if localStorage actually stores things.

There is a lot more money on the other side of that arms race.

The lack of fingerprinting information is, in itself, a data point.

That wouldn't be able to distinguish between Private Browsing and having just cleared out browser data, though.

There are features like IndexedDB that are disabled in private browsing but not in a clean install

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