> 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/low-level-logic-flaw.md).
# Low-Level Logic Failure
Low-Level Logic Failure notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Flawed Enforcement of Business Rules
**Description of lab**
This lab contains a \*\* logical defect in the purchasing process\*\*. Due to poor application of trade rules, it is possible to manipulate the calculation of the basket in order to buy a "Lightweight l33t leather jacket" at an unexpected price.
Identifier provided:
* **User:** wiener
* **Password:** peter
**Behavioral analysis**
When adding a product to the basket, the following query is used:
```bash
productId=1&redir=PRODUCT&quantity=99
```
This parameter can be sent **a large number of times** without strict control of the cumulative value.
The query is sent to **Intruder** with a null payload configured in **Continue indefinitely** mode, and a maximum of **1 concurrent request**.
After enough iterations, the **total price of the basket becomes negative**.
**Cause of the problem**
Vulnerability comes from a signed **overflow**. / The application uses a **integer signed on 32 bits**, whose limits are:
* Valeur minimale : `-2 147 483 648`
* Valeur maximale : `2 147 483 647`
When the total exceeds the maximum permitted value, it **recalculates in negative values** due to arithmetical exceedance.
**Inadequate controls**
Even if the application prevents payment when the total is less than 0, **no correct verification is made during intermediate calculations**.
This allows you to adjust the basket until you get a positive but very low final amount.
**Final Calculation Adjustment**
To obtain an amount \*\* above $0 and below $100\*\*, the following calculation is made:
* Cost of jacket: **$1,337**
* Value required to cause overflow: \**14 018 329* \*
* Calculation:
```bash
14 018 329 / 1337 ≈ 10 484 fois
```
Batch submission (`quantity = 99`):
```bash
10,484 / 99 ≈ 105 requests
```
**Final Cart Composition**
To obtain a valid total after overflow:
* **32 123** jackets at $1337
* **21** Baby minding shoes at $60.79
* **Final total:** **$54.63**
---
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