IP and DNS Leaks in WebKit Affecting Proxy Browsers and iCloud Private Relay

WebKit-based browsers on iOS and macOS can be configured to route all web traffic through proxy servers, which is how Tor browsers on iOS and our own Psylo work. We found three WebKit features — DNS prefetching, WebAuthn Related Origin Requests, and WebTransport — that bypass the configured proxy and send traffic directly from the device, which exposes the user's real network. The same leaks also affect Apple's iCloud Private Relay. All three are fixed in Psylo 1.3.1.