Sometimes various browsers experience WebRTC Leaks which may cause the real IP address leak, even when using a VPN.
However you can easily disable the WebRTC leaks.
Mozilla Firefox: Type about:config in the address bar. Scroll down to media.peerconnection.enabled, double click to set it to false.
Google Chrome: Install this plug-in and it will prevent any WebRTC leaks.
You can read more about WebRTC leaks in our blog post.