WebKit flaw leaks real IPs in proxy browsers and iCloud Private Relay

Security researchers Talal Haj Bakry and Tommy Mysk from Mysk have uncovered a critical privacy flaw in Apple's WebKit engine. Their investigation, prompted by a user bug report, revealed that three WebKit features bypass the proxy configurations set via WKWebsiteDataStore.proxyConfigurations (available since iOS 17). DNS prefetching resolves hostnames through the device's normal DNS path, revealing the user's real DNS servers (iOS 26.0). More severely, WebAuthn Related Origin Requests (iOS 18.0) and WebTransport (iOS 26.4) send direct HTTP/3 connections or credential validation traffic from the device, exposing its real IP address. These features operate outside the standard page-loading process, so even Apple's iCloud Private Relay—designed with a two-hop proxy—is vulnerable.

The issue affects every WebKit-based proxy browser on iOS, including Tor browsers and Onion Browser, because Apple's App Store policy forces all iOS browsers to use WebKit. VPNs are not impacted, as they tunnel all device traffic at the system level. Mysk has notified the Tor Project and Onion Browser developers. The fix is already live in Psylo 1.3.1, which blocks dns-prefetch hints and disables WebTransport and WebAuthn by default, with per-silo toggles for users who need them. The proof-of-concept detection site is leaks.psylo.app. This highlights a systemic gap in WebKit's proxy model, raising questions about the privacy guarantees of Apple's own relay service and third-party privacy browsers.