The patch changes a single argument in XMLSerializer::serializeToString: the ResolveURLs mode passed to serializeFragment is changed from ResolveURLs::No to ResolveURLs::NoExcludingURLsForPrivacy. WebKit supports ‘masked’ URLs (the webkit-masked-url:// scheme) that Safari web extensions use to hide the true resource URL of content they inject into a page, so that the page’s own script cannot observe which extension is present or what real URL a resource has. The serialization layer therefore distinguishes several URL-handling modes: ResolveURLs::No leaves attribute URLs exactly as-is (which, for a masked attribute, means emitting the underlying real URL text rather than the masked placeholder), while ResolveURLs::NoExcludingURLsForPrivacy additionally suppresses/masks privacy-sensitive URLs during serialization.

The invariant that was violated is that DOM-serialization APIs reachable from page script must never reveal a privacy-masked URL’s true value. HTML serialization (innerHTML) already honored masking — the pre-existing test asserts document.body.innerHTML contains ‘<iframe src=“webkit-masked-url://hidden/">’ — but XMLSerializer.serializeToString used ResolveURLs::No and thus did not apply the privacy exclusion, so calling (new XMLSerializer()).serializeToString(document.body) leaked the real URL behind masked resources.

The fix restores the invariant by making the XML serializer use the privacy-aware mode, so the added test now expects the masked iframe to serialize as ‘webkit-masked-url://hidden/’ while a genuinely non-masked iframe still serializes as its real ‘http://apple.com/baz.html' URL. Because the vulnerable behavior of ResolveURLs::No vs NoExcludingURLsForPrivacy lives in the shared serializeFragment/markup-serialization code (not shown in this diff), the precise masking logic there is inferred, but the change of enum value and the new expectation string establish that XMLSerializer was previously emitting unmasked URLs.

This is a pure information-disclosure/privacy bug, not a memory-safety primitive: it leaks the true URL behind extension-masked resources to page script via XMLSerializer, enabling detection and fingerprinting of Safari web extensions and thereby user tracking/de-anonymization. There is no memory corruption, code execution, or crash, and no escalation path toward RCE; the leaked data is limited to URLs the extension intended to hide. The disclosure occurs within the WebContent process to ordinary page JavaScript (no sandbox or cross-process boundary is broken), with real-world impact being loss of the anti-tracking guarantee the masking mechanism exists to provide.