IP and DNS Leaks in WebKit Affecting Proxy Browsers and iCloud Private Relay

WebKit provides an API that lets iOS and macOS browsers route their own traffic through a proxy, a model used by Tor browsers and other privacy-focused apps. The expectation is that every network request a page makes will travel through that proxy, masking the device’s IP and DNS. Researchers discovered that three built-in WebKit capabilities can circumvent this proxy configuration, sending requests straight from the host. This behavior contradicts the intended isolation of proxy traffic and creates a privacy gap.

The first capability, DNS prefetch, resolves hostnames ahead of time via a standard DNS lookup, which follows the device’s normal resolver path instead of the proxy. The second, WebAuthn related origin requests, triggers the OS credential service to fetch validation data directly, revealing the device’s IP address. The third, WebTransport, opens a direct HTTP/3 channel that also bypasses the proxy. Each of these features has been present since specific iOS releases, meaning the vulnerability has existed for multiple OS versions.

Because the leaks occur outside the normal page-loading pipeline, Apple’s iCloud Private Relay, designed to hide both user identity and visited sites, suffers the same exposure. In contrast, VPN solutions encapsulate all network traffic at the system level, so they remain unaffected by these WebKit bypasses. This distinction is important for users choosing between proxy-based privacy tools and full-device VPNs.

The immediate mitigation comes from an update to the Psylo browser, which now blocks DNS prefetch hints and disables WebTransport and WebAuthn by default. Developers of other WebKit-based browsers will need to incorporate similar safeguards, either by turning off the features or by providing explicit user consent toggles. Updating to the patched version incurs the usual software rollout effort, but it also requires testing any sites that legitimately depend on these capabilities.

For any application that relies on WKWebsiteDataStore.proxyConfigurations, the discovery signals a need to audit WebKit usage for hidden network paths. Until the underlying WebKit implementation is corrected, browsers must treat these three features as potential leak vectors and either disable them or ensure they respect proxy settings. Failure to do so will continue to expose real IP and DNS information, undermining the privacy guarantees that proxy browsers aim to provide.