Apple WebKit privacy leaks impact Tor, Psylo, and iCloud Private Relay

Researchers have discovered three WebKit features that can bypass application-level proxy settings on iOS and macOS, potentially exposing users' real IP addresses or DNS servers.

The leaks affect proxy-based browsers, including iOS Tor browsers and Psylo, as well as Apple’s iCloud Private Relay.

The investigation began after a Psylo user reported DNS leaks on certain websites. Developers Bakry and Mysk traced the behavior to DNS prefetching and subsequently discovered two additional proxy bypasses involving WebAuthn Related Origin Requests and WebTransport.

All three features operate outside the network path configured through WKWebsiteDataStore.proxyConfigurations, an API introduced in iOS 17 and macOS 14. Proxy browsers use this interface to route WebKit traffic through remote servers so websites see the proxy’s IP address rather than the device’s actual network.

Security Bulletin: WebKit's handling of passkeys can cause iCloud Private Relay to leak your device's real IP address. The same behaviour affects all iOS browsers that rely on proxy connections to hide the device's IP, including Psylo, Onion Browser, and other proxy or Tor… pic.twitter.com/BvCW3aYzHV

— Psylo: Privacy Browser & Proxy (@psylo_app) August 4, 2026

Apple requires all browsers distributed through the iOS App Store to use WebKit. As a result, browsers relying on WebKit’s application-level proxy configuration may be affected regardless of their branding or privacy features. Apple’s iCloud Private Relay is also exposed because the affected requests bypass Safari’s normal relayed traffic path. System-level VPNs are not affected, as they tunnel network traffic from the entire device.

The first issue involves dns-prefetch, an HTML hint that allows websites to resolve domains before they are needed. WebKit sends these lookups through the device’s regular DNS connection instead of the configured proxy. A website could embed a unique hostname for each visitor and monitor its authoritative DNS server to identify the user’s actual DNS provider or network.

This behavior appeared on iPhones with iOS 26.0, released in September 2025. It also explains why the original leak occurred only on certain websites: the request is triggered only when a page includes a DNS-prefetch hint.

The second leak affects WebAuthn Related Origin Requests, a passkey feature available since iOS 18.0. When a website attempts to use a passkey associated with another approved domain, the operating system retrieves a validation file from https:///.well-known/webauthn.

That request is made by Apple’s credential service rather than the browser’s proxied network stack. A site can trigger the fetch without visible user interaction by using conditional WebAuthn mediation, allowing the destination server to observe the device’s real IP address.

The third issue concerns WebTransport, a low-latency protocol based primarily on HTTP/3 and QUIC. Calling new WebTransport(url) causes WebKit to establish a direct connection without applying the browser session’s proxy settings. WebTransport became publicly available in iOS 26.4 in March 2026.

Onion Browser users running its “Silver” security level are reportedly protected from the WebTransport leak because that mode enables WebKit Lockdown Mode, which disables the feature.

The researchers disclosed the findings to the Tor Project and Onion Browser’s developers and published a proof-of-concept test at leaks.psylo.app.

Psylo 1.3.1 mitigates the problems by blocking DNS-prefetch hints and disabling WebAuthn and WebTransport by default. Users can re-enable the latter two features for individual browsing silos when required.