IP and DNS Leaks in WebKit Affecting Proxy Browsers and Apple iCloud Private Relay | Mysk Blog – In-Depth Cybersecurity & Mobile App Privacy Research
IP and DNS Leaks in WebKit Affecting Proxy Browsers and Apple iCloud Private Relay
2026-08-04by
Talal Haj Bakry<br>and
Tommy Mysk
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.
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Summary#
Our proof-of-concept website leaks.psylo.app that detects the leaks
WebKit-based browsers on iOS and macOS can be built to route all web traffic through proxy servers. This is how all proxy browsers work on iOS, including iOS Tor browsers and our own browser, Psylo. Every network connection a web page makes is supposed to flow through the configured proxy, so websites only ever see the proxy’s IP address. We found three WebKit features that bypass the proxy configuration and send traffic directly from the device instead:
DNS prefetching resolves hostnames through the device’s normal DNS path, which reveals the user’s real DNS servers instead of the proxy’s. Available since iOS 26.0.
WebAuthn Related Origin Requests make the operating system’s credential service fetch a validation file directly from the device. This exposes the device’s real IP address. Available since iOS 18.0.
WebTransport opens a direct HTTP/3 connection and bypasses the proxy, which also exposes the device’s real IP address. Available since iOS 26.4.
These leaks also impact Apple’s iCloud Private Relay. It must be noted that VPNs are not affected, since they tunnel the device’s entire network traffic at the system level.
We’ve reached out to the Tor Project and the developers of Onion Browser on iOS about these issues.
To test the leaks, you can visit our proof-of-concept website at leaks.psylo.app.
Fixed in Psylo 1.3.1: Psylo now blocks dns-prefetch hints and disables WebTransport and WebAuthn by default. For websites that genuinely need these features, each one can be re-enabled through per-silo toggles. This explicit opt-in keeps the privacy trade-offs in the user’s hands. See Mitigations Introduced in Psylo 1.3.1 for details.
Background#
Proxy Configuration on iOS and macOS#
Introduced in iOS 17 and macOS 14, WKWebsiteDataStore.proxyConfigurations allows WebKit-based browsers route all of their own web traffic through proxy servers at the application level. This API is the foundation of proxy browsers on iOS: every network connection a web page makes is supposed to flow through the configured proxy, so websites only ever see the proxy’s IP address.
DNS Leak Reported by a Psylo User#
This investigation started with a bug report from a Psylo user who noticed DNS leaks when visiting only certain websites, and we immediately started looking into it. Psylo routes all traffic from each silo through the Mysk Private Proxy Network (or the user’s own configured custom proxy), so DNS queries should all originate from the proxy server and never from the device. It also seemed odd that this only affected some websites, and not all.
As we dug deeper, we found the source of the DNS leaks, plus two more leaks that actually reveal the device’s real IP address. All three leaks live in WebKit, where they bypass the proxy settings provided by WKWebsiteDataStore.proxyConfigurations. Since Apple’s App Store policy requires every iOS browser to use WebKit, any iOS browser that relies on this API for proxying is affected, including all iOS Tor browsers and Psylo. These leaks are also present in Apple’s iCloud Private Relay. VPNs, on the other hand, are not affected by these issues, since the device’s entire network traffic is tunneled through the VPN at the system level.
iCloud Private Relay#
iCloud Private Relay is Apple’s privacy feature for iCloud+ subscribers. When enabled, it proxies Safari’s<br>(and only Safari’s) web traffic and DNS queries through a two-hop relay, designed so that no single party, not even Apple, can see both who you are and which sites you visit. As it turns out, all three leaks described in this article occur outside WebKit’s standard page-loading process, meaning Private Relay is susceptible to the same leaks.
1. DNS Prefetching#
DNS prefetching lets...