Lab: Exploiting DOM clobbering to enable XSS

1. Go to one of the blog posts and create a comment containing the following anchors:

   <a id=defaultAvatar><a id=defaultAvatar name=avatar href="cid:&quot;onerror=alert(1)//">
2. Return to the blog post and create a second comment containing any random text. The next time the page loads, the alert() is called.

The page for a specific blog post imports the JavaScript file loadCommentsWithDomPurify.js, which contains the following code:

The defaultAvatar object is implemented using this dangerous pattern containing the logical OR operator in conjunction with a global variable. This makes it vulnerable to DOM clobbering.

You can clobber this object using anchor tags. Creating two anchors with the same ID causes them to be grouped in a DOM collection. The name attribute in the second anchor contains the value "avatar", which will clobber the avatar property with the contents of the href attribute.

Notice that the site uses the DOMPurify filter in an attempt to reduce DOM-based vulnerabilities. However, DOMPurify allows you to use the cid: protocol, which does not URL-encode double-quotes. This means you can inject an encoded double-quote that will be decoded at runtime. As a result, the injection described above will cause the defaultAvatar variable to be assigned the clobbered property {avatar: ‘cid:"onerror=alert(1)//’} the next time the page is loaded.

When you make a second post, the browser uses the newly-clobbered global variable, which smuggles the payload in the onerror event handler and triggers the alert().