Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|improve this question
up vote 40 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.

share|improve this answer

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.

share|improve this answer
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...

share|improve this answer

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .