Websites Can Silently Detect Your Browser Extensions Without Permission

Web pages can identify which browser extensions a user has installed through at least three distinct techniques, all exploiting features built into browsers by design. The first method targets extensions that accept messages from web pages via the chrome.runtime.sendMessage API, particularly those using wildcard permissions in their manifest. The second and more widespread technique loads extension assets — such as icons — from predictable URLs, requiring no cooperation from the extension itself. A third approach detects DOM changes made by extensions like ad blockers or password managers by planting decoy elements and observing how they are altered. Extension developers are identified as best positioned to close these gaps, though most remain unaware their published extensions are detectable.