Glossary
WebRTC leak
When a browser behind a proxy reveals its real IP through WebRTC, the technology behind calls and video in the browser. WebRTC collects the address over UDP, which most proxies don't carry, so the request goes around the proxy.
Also called: WebRTC IP leak, STUN leak.
WebRTC asks a STUN server for your public address so that peers can connect directly. RFC 8828, the standard on WebRTC IP handling, says that behind a proxy "WebRTC's STUN checks will bypass the proxy and reveal the public IP address of the client." A site's script can read that address and see it next to the proxy's.
Antidetects fix it per profile. The usual modes replace the public address with the proxy's IP (called Altered, Replace, Based on IP or Masked), switch WebRTC off, or block only the UDP traffic that doesn't go through the proxy. Most vendors recommend replacing, because a browser with WebRTC switched off stands out: Check Point lists "WebRTC suppression patterns" among the signals platforms should look for.
Since Chrome 76 in 2019, Chrome hides local network addresses behind random names ending in .local, so the leak that matters now is the public IP. Test every new profile on a WebRTC leak test with its real proxy.
Related terms: Proxy, DNS leak, Rotating proxy, Browser profile, Antidetect browser.