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Lately I was logging into a service (I think it was Google or Facebook but I am not sure) and ticket the box to remember my device.

Then the Site alerted me that this was unlikely to work because I was running in private mode. Which was correct.

I am now unable to reproduce this since I don't remember exactly where it was and a normal login appears not to produce that message (any more).

However Today I had a discussion with a friend whether It is possible to recognise if a device is configured in private mode.

I am aware of several "super cookie" methods that are able to reach across the private mode of some browsers, but I do not want to know about those. I am interested in environment information that can be read and interpreted and give information whether private mode is active or is likely active.

To clearify the meaning of my question I give one example that could be used to solve that problem:

There is a so called "CSS History" hack. I do not know about the current state in browsers, however for a long time it was pretty common and worked like this:

Put a link to another website (for example http://www.google.com on a website), then you will be able to read the visited state css state of the link using javscript.

Now if you do that with a popular site like google, you could say:

Well, you never visisted google.com? Thats unlikely so I will assume you just opened a private browsing window!

So are there other methods, and which, and is there a way to combine them in a way that allows to give an estimate whether the user is currently in private mode or not?

The Surrican
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    Pretty clever idea, but I think that hack has been fixed. – Pekka Jun 19 '13 at 12:47
  • Are you it was the site and not the browser being intelligent enough to notice you checked a checkbox named `rememberme`? – Brad Christie Jun 19 '13 at 12:47
  • @Brad Crhstie i checked the checkbox myself, and at that time did not bother with the message. but now it is bothering me ;) – The Surrican Jun 19 '13 at 12:50
  • Alright there are duplicates. Unfortunately none of them are answered in as much detailed as I hoped. The only technique that was pointed ous seems to be the already mentioned css hack... I would know a little bit more. Maby possilbe plugins that could be exploited (flash, java). maby a small download from a known site could be done. maby the google logo... – The Surrican Jun 19 '13 at 12:52
  • for internet there appears to be a possible indication using SMB links crypto.stanford.edu/~dabo/pubs/papers/privatebrowsing.pdf‎ – The Surrican Jun 19 '13 at 12:56
  • also most browsers dont activate extensions in private mode. so the absence of adblock could already be taken as an indication of private mode. – The Surrican Jun 19 '13 at 12:58

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