This is a Gatekeeper/quarantine-bypass logic bug on macOS in the Web Archive loading path that leads to universal cross-site scripting (UXSS). A .webarchive main resource can declare an arbitrary origin and carry script, so opening one runs content with the authority of spoofed origins; macOS normally protects users from such downloaded content via the com.apple.quarantine extended attribute and Gatekeeper’s “unidentified developer” approval prompt. The violated invariant is that a quarantined, not-yet-user-approved local .webarchive must not be silently loaded and executed. This CVE is a follow-up to the web-archive isolation work (CVE-2024-40857): that patch made WebPageProxy::receivedNavigationActionPolicyDecision() swap in a non-persistent data store for substitute or file:// .webarchive Use decisions, but it did not check the file’s quarantine state, so a webarchive delivered to disk and reached via a file:// navigation (for example a link click, as the added test exercises) would load and execute despite being quarantined and unapproved.

The fix adds, under PLATFORM(MAC), a guard inside that same branch: for a non-substitute (file://) web archive, if isQuarantinedAndNotUserApproved(webarchiveURL.fileSystemPath()) is true, it logs, builds a cancellation ResourceError, reports didFailProvisionalNavigationWithError, and returns before any data-store swap or load — cancelling the navigation exactly as Gatekeeper intends. The accompanying layout test loads a page that programmatically clicks a link to a quarantined .webarchive and asserts the load is cancelled, and mac.py is changed to set the com.apple.quarantine xattr on the test resource so the quarantine path is actually exercised. Note: isQuarantinedAndNotUserApproved(), cancelledError(), and the exact ResourceError plumbing live outside the shown diff, so their internals are inferred from the call site rather than shown by the patch.

This is a same-origin-policy bypass (UXSS) gated on a quarantine/Gatekeeper check, not a memory-safety primitive: the diff contains no allocation or bounds logic and cannot be groomed toward memory corruption. Exploited pre-patch, a quarantined .webarchive that the user was never prompted to approve executes script as spoofed origins, a confidentiality/integrity break in the WebContent process and browsing profile rather than a route to native code execution. Because it requires getting a quarantined archive onto disk and a file:// navigation to it, real-world exploitation depends on delivery and some user interaction, consistent with the medium rating.