IPv6 leak
An IPv6 leak occurs when a VPN or proxy tunnels only IPv4 traffic, so websites can still reach you over your real IPv6 address.
What an IPv6 leak is
Most proxies, and a surprising number of VPN setups, carry IPv4 traffic only. If your device also has a working IPv6 connection from your ISP, the operating system will happily use it for any site that publishes an AAAA record. Those connections go straight out through your real provider, outside the tunnel. The site sees your genuine IPv6 address, which maps to your real country and network, while the IPv4 side of the same session shows the proxy. That contradiction is an IPv6 leak.
Modern browsers prefer IPv6 when both are available, so this is not an edge case. A large share of major websites are reachable over IPv6, and a leak happens on the first one you open.
Why it matters
For anyone checking a proxy or antidetect setup, an IPv6 leak undoes everything else. A site does not need WebRTC tricks or DNS timing; it simply sees two different addresses in two different countries and can connect them through cookies or the login itself. Our anonymity score deducts 15 points when an IPv6 address is reachable from your browser and geolocates to a different country than your IPv4 address.
For privacy users the exposure is the same: the IPv6 side of your traffic is unencrypted and attributable to your ISP account.
How to check
The IPv6 test asks your browser to fetch a resource from a host that has only an IPv6 address. If the request succeeds, the test reports the address it came from and where that address is located. If it fails, your browser cannot reach the IPv6 internet at all, which is the safe outcome for a tunneled setup. The IP leak test runs this with the WebRTC and DNS checks in one pass.
How to fix it
The simplest fix is to disable IPv6 on the network adapter while the proxy is in use, or at the router if you want it off for every device. If your VPN provider supports IPv6 inside the tunnel, enable that instead so both protocols exit from the same place. Antidetect browsers that let you choose "IPv4 only" for a profile are doing the same thing at the application level. Re-run the test afterward: a result of "no IPv6 connectivity" means nothing can leak over IPv6.
A common misconception
An IPv6 address alone is not a leak. The problem is only when IPv6 is reachable and ends up in a different place than the address you intend to show.