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I recently visited (an official) website in search for an episode of a TV-show. The site popped an information dialog stating that I had to disable AdBlock to see the content. Is this at all possible or is it just an educated guess? Does Firefox advertise what addons I have installed? If so: how and why does it work that way?

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3 Answers 3

up vote 37 down vote accepted

Yes, a website can detect that Adblock Plus is installed by referencing a Javascript file known to be blocked by Adblock Plus (because it matches a default rule) and check if this Javascript code has run, by checking a variable for instance.

See an explanation here: How To Detect Adblock Plus

And more details at Wikipedia: Adblock - Detection.

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Thanks for the quick and clarifying response. –  Anders Feb 3 '10 at 8:10

It's entirely possible, it can be done with a bit of javascript. You can see a proof of concept here that shows how to detect multiple different adblock programs, which also uses javascript for detection. It isn't really the fact that Firefox "advertises" your installed add-ons, it's the method that most adblock programs use to function. Most will simply hide elements using the CSS visibility property and filter certain URLs. These URLs can be faked in an attempt to fool the filter, and it is an ongoing battle with regards to blocking detection.

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Also interesting answer. Will look at poc to get more information. –  Anders Feb 3 '10 at 8:12
    
It seems to have been fixed. I clicked on the first link and the detecting script link was broken. –  user39559 Sep 23 '10 at 16:33

Without using any trickery, most browsers advertise installed plugins (like Flash), but not add-ons (like Adblock, which is detectable by other means like Snark answered). See Panopticlick, and help their investigation...

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