A new fingerprinting technique called “Adbleed” reveals that VPN users aren’t as anonymous as they think.
While VPNs hide your IP address and encrypt traffic, they can’t conceal which country-specific adblock filter lists are installed in your browser and that’s enough to expose your location.
How Adblockers Create a Privacy Leak
Most adblockers like uBlock Origin, Brave, and AdBlock Plus rely on filter lists to block unwanted content.
The main list, EasyList, contains over 54,000 rules covering English-language ads and major international networks.
However, users typically enable additional country-specific lists that block local advertisers.
German users might have EasyList Germany blocking domains like adnx.de and adition.de. French users enable Liste FR to block ad6.fr and similar French ad networks.
These regional lists exist for Italy, Spain, Brazil, Russia, China, Japan, and many other countries. Most adblockers automatically suggest or enable these lists based on your browser’s locale settings.
The Detection Method
According to Adbleed, the fingerprinting technique exploits timing differences in how browsers handle blocked versus unblocked requests.
When an adblocker blocks a domain, it intercepts the request before it reaches the network, causing an error in under 5 milliseconds.
Without blocking, the browser attempts an actual network connection, which takes 50-500 milliseconds even if the domain doesn’t exist.
A simple JavaScript script probes 30 domains unique to each country’s filter list by attempting to load tiny resources from them.
If 20 or more domains fail instantly (under 30ms), the script concludes that country’s list is active. The technique is entirely client-side and requires no special permissions or cookies.
This fingerprinting vector works through VPNs, Tor Browser, and any proxy service. An attacker learns which country-specific lists you use, strongly correlating with your actual location or native language.
Combined with other fingerprinting signals like timezone, keyboard layout, and screen resolution, this significantly narrows down your identity.
The vulnerability exists because while VPNs change your apparent network location, they don’t modify your browser’s configuration. Your adblock rules remain constant regardless of which VPN server you connect through.
Users face an uncomfortable trade-off. You can disable country-specific filter lists, but this allows more local ads through.
Enabling random country lists might help obscure your real location, or you could abandon adblockers entirely, though that worsens privacy in other ways.
Browser developers should consider applying country-specific rules more selectively, perhaps only on matching first-party domains rather than globally.
Until then, remember that your adblock configuration is part of your digital fingerprint.
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