Oh, it's very easy: <img src="/an_existing_image.jpg?&ad_box_" onerror="alert('You are using adblock')"> How it's made: 1. Look into the most popular filte

Oh, it's very easy: <img src="/an_existing_image.jpg?&ad_box_" onerror="alert('You are using adblock')"> How it's made: 1. Look into the most popular filter's used by adblock extensions: https://easylist-downloads.adblockplus.org/easylist.txt https://easylist-downloads.adblockplus.org/easylist.txt 2. Choose for example a querystring rule from the beginning (from the "General advert blocking filters") 3. Append it to the url of an existing image 4. Use the onerror event to detect if it wasn't loaded