> 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/business-logic/high-level-logic-vulnerability.md).
# High-level Logical Vulnerability
High-level Logical Vulnerability notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### High-level Logic Vulnerability
**Description of lab**
This lab does not properly validate the data provided by the user. It is possible to exploit a logical flaw in the purchase process in order to change the final price of the items. / The objective is to buy the \*\*
Identifier provided:
* **User**: wiener
* **Password**: peter
**Analysis of the operation of the basket**
When a product is added to the basket, the request sent looks like this:
```bash
productId=1&redir=PRODUCT&quantity=1
```
The `quantity` parameter is directly controlled by the client and is not properly checked on the server side.
**Operating logical fault**
If the quantity is changed using a negative value, for example:
```bash
productId=1&redir=PRODUCT&quantity=-1
```
The total amount of the basket is decreasing, resulting in a negative price (e.g., \*\* $1337\*\*).
However, when validating the basket, the application blocks this case with the message:
> *Cart total price cannot be less than zero*
**Contouring the restriction**
To exploit the vulnerability:
Add the **target jacket** with a valid quantity:
```bash
productId=1&redir=PRODUCT&quantity=1
```
Then add another product with a negative amount to reduce the total:
```bash
productId=2&redir=PRODUCT&quantity=-100
```
This results in a strongly negative total (e.g. **-$6241.00**).
Adjust the negative amount (e.g. `-17`) so that the final total remains **positive**, but very low (less than $100).
---
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