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  3. Race Condition in Plan Coupon Limits
MediumVDP25 min

Race Condition in Plan Coupon Limits

By @gorka

Plataforma de cupones donde cada plan (Free/Pro/Premium) tiene un limite de cupones canjeables. Race condition permite superar el limite enviando peticiones concurrentes para diferentes cupones

227 views11 completedUpdated Aug 2026
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Hunters who solved it· 8

VA1
@vazquezdv1820059 hours ago
kaneki2
@kaneki11 days ago
flyingwhales3
@flyingwhales12 days ago
r4mattra
@r4mattra13 days ago
AL
@alndrwla23 days ago
AL
@alex.burja.200029 days ago
0xyo
@0xyoJun 2026
pl4nkton
@pl4nktonJun 2026

Objectives

1
Entender el sistema de planes y limites
2
Identificar que el check de limite tiene delay
3
Enviar peticiones concurrentes para diferentes cupones
4
Superar el limite del plan Free (2 cupones)

Information

Difficulty
Medium
Duration
25 min
Bounty
VDP
Completed
11
Creator
gorka@gorka
Updated
Aug 2026

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Reproduce it and find the bug yourself

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Tools

Burp Suite

Prerequisites

  • Race conditions
  • Peticiones concurrentes

Tags

Race Condition

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BBLABS.ESLab Solved
MediumVDP
// achievement_unlocked

Race Condition in Plan Coupon Limits

Race Condition
Aug 2026
gorka
solved_by@gorkaMember since Mar 2026
bblabs.es// real bug bounty practice

Community writeups

Despliegue

# Opcion 1: autodeploy
./autodeploy.sh

# Opcion 2: manual
docker compose up --build

# Acceder
https://localhost:1000

Compatible con macOS, Linux y Windows (Docker Desktop / WSL).

Para parar:

./autodeploy.sh destroy
# o: docker compose down -v

Contenido del Lab

La aplicacion

DealZone es una plataforma de cupones con planes de suscripcion que limitan la cantidad de cupones canjeables.

La vulnerabilidad

POST /api/coupons/:id/claim verifica el total de cupones canjeados contra el limite del plan con un delay antes de completar el canjeo. Peticiones concurrentes para diferentes cupones pasan todas el check.

Cadena de ataque

1. Login como alice (Free, limite 2)
2. Listar cupones disponibles (12 cupones)
3. Enviar 12 peticiones concurrentes (una por cupon)
4. Todas pasan el check del limite
5. Resultado: 12 cupones canjeados con plan Free

Solucion interactiva

Accede a /writeup dentro de la aplicacion para una guia paso a paso con botones interactivos.

Flag

(mas cupones canjeados que el limite del plan)

Remediacion

  1. Transacciones atomicas con bloqueo de fila
  2. Incremento atomico del contador de cupones
  3. Verificacion post-commit del limite

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