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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.
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.
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.
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