Multiple mechanisms within Apple's WebKit browser engine have been identified that can bypass iCloud Private Relay and other browser-level proxy configurations, potentially revealing a user's true IP address and DNS information. The vulnerabilities, reported to Apple, affect Safari on both iOS and macOS, as well as any other iOS/macOS browser or application that relies on WebKit's proxy configuration for IP address concealment.
The issue stems from three specific WebKit features that initiate network requests outside of the engine's standard page loading path. Because iCloud Private Relay and similar proxies are implemented at the WebKit level and only apply to traffic within this normal path, they fail to intercept these specific requests.
One such mechanism is DNS prefetching, a feature designed to speed up browsing by resolving the IP addresses of links on a page before a user clicks them. Researchers found that in WebKit, these DNS lookups can bypass the configured proxy or relay, directly utilizing the system's default DNS stack. This exposes the DNS servers in use and, indirectly, the user's general location, even if the subsequent page load is routed through Private Relay.
Another bypass involves WebAuthn, the underlying standard for passkeys. When a website uses passkeys, WebAuthn may need to fetch a small file from the site's domain to verify the credential. On Apple platforms, this fetch is performed outside WebKit's proxied page-loading path, meaning it is not sent through Safari's proxy or Private Relay. Consequently, a website implementing passkeys can directly contact the user's device, revealing its true IP address despite the user's expectation of privacy.
The third identified vulnerability relates to WebTransport and similar technologies. WebTransport is an API that allows websites to establish low-latency, bidirectional connections to a server. In tests, these WebTransport connections were also initiated outside the proxied WebKit code path, creating an additional route for websites to receive direct traffic from the device, again exposing the user's real IP address instead of the expected relay or proxy IP.
These mechanisms are problematic from a user experience standpoint because they appear to be normal browser behavior and do not require any specialized or malicious tricks from a website to exploit.
While Apple has recently introduced a mechanism for non-WebKit browser engines on iOS, primarily due to regulatory pressures like the EU's Digital Markets Act, no major browser vendor has yet shipped a non-WebKit browser. Most are still in the prototype phase, indicating that the vast majority of iOS browsers continue to rely on the WebKit engine and are thus susceptible to these issues.
System-level VPNs, such as Malwarebytes VPN, are not affected by these WebKit vulnerabilities because they tunnel all of a device's network traffic at the operating system level, rather than relying on browser-specific proxy configurations. The researchers have reported their findings to Apple and anticipate that patches will be released by the fall.
