Three WebKit Features Leak Real IP and DNS, Breaking iOS Proxy Browsers and Private Relay

Researchers at Mysk found that three WebKit features silently sidestep the app-level proxy configuration (WKWebsiteDataStore.proxyConfigurations) that every iOS proxy browser depends on, sending traffic directly from the device. DNS prefetching (via a dns-prefetch link tag) resolves hostnames through the device’s normal DNS path, exposing the user’s real resolvers; WebAuthn Related Origin Requests make the OS credential service fetch a .well-known/webauthn validation file directly — even with no user interaction under conditional mediation — revealing the real IP; and WebTransport opens a QUIC/HTTP/3 connection outside the proxied path, again leaking the device’s IP. Because Apple’s App Store policy forces all iOS browsers onto WebKit, every proxy browser built on this API is affected, including iOS Tor browsers, Onion Browser, and Mysk’s own Psylo.

The leaks also defeat iCloud Private Relay. All three operations happen outside WebKit’s standard page-loading pipeline, so Safari’s two-hop relay never sees them and the destination server observes the device’s real network. Full-tunnel VPNs are unaffected, since they wrap all traffic at the system level. The features became exploitable at different points: WebAuthn since iOS 18.0, DNS prefetching since iOS 26.0, and WebTransport since iOS 26.4 — meaning the exposure widened as recent iOS releases enabled each capability by default.

The finding illustrates a structural weakness in Apple’s app-level proxying model: any OS service or network primitive that bypasses the browser’s own stack becomes an anonymity leak that individual browser vendors can’t fully close. Psylo 1.3.1 mitigates by blocking dns-prefetch hints and disabling WebTransport and WebAuthn by default, with per-silo opt-in toggles, and Onion Browser’s Lockdown-based ‘Silver’ level already neutralizes WebTransport. Mysk has notified the Tor Project and Onion Browser and published a proof-of-concept at leaks.psylo.app.