Privacy · Plain-English answer
A WebRTC leak is when your browser reveals your real IP address to websites despite a VPN being connected. WebRTC, the technology behind in-browser video calls, can discover your local and public addresses directly, bypassing the tunnel. Any website can trigger the check with a little JavaScript.
WebRTC exists so browsers can make direct peer-to-peer connections for video calls and file sharing. To connect two peers behind home routers, it uses a discovery process called ICE, which asks STUN servers to report the addresses your machine can be reached on. Depending on browser and platform, that process can reveal your real public IP even while your normal traffic goes through the VPN.
No call needs to be in progress. A web page can run the discovery silently in the background.
Test it by connecting your VPN and visiting an IP checker such as /tools/my-ip/. If your real address appears anywhere alongside the VPN address, WebRTC is leaking.
Modern VPN implementations mitigate this by routing all traffic at the system level, so WebRTC's requests also travel through the tunnel and discover only the VPN address. You can additionally disable or restrict WebRTC in some browsers, or use extensions that block it, at the cost of breaking in-browser video calls.
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