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MediumVDP35 min

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Client-Side Role Bypass + Missing Server RBAC

By @gorka

Plataforma de ranking de hackers donde el rol del usuario se controla desde el cliente. Modificando la respuesta del servidor con Burp Suite (Match & Replace) se puede escalar a admin y cambiar la contrasena del super-hacker

243 views19 completedUpdated Aug 2026
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Hunters who solved it· 8

kaneki1
@kaneki10 days ago
flyingwhales2
@flyingwhales12 days ago
r4mattra3
@r4mattra13 days ago
AL
@alex.burja.200029 days ago
0xyo
@0xyoJul 2026
gorka
@gorkaJun 2026
pl4nkton
@pl4nktonJun 2026
AT
@atxMay 2026

Objectives

1
Registrarse y observar que la respuesta contiene role: user
2
Usar Burp Match & Replace para cambiar role: user por role: admin en las respuestas
3
Acceder al panel de administracion que ahora aparece en la interfaz
4
Descubrir que los endpoints de admin no validan el rol en el servidor
5
Cambiar la contrasena del usuario thebesthacker para obtener la flag

Information

Platform
HackerOne
Difficulty
Medium
Duration
35 min
Bounty
VDP
Completed
19
Creator
gorka@gorka

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BBLABS.ESLab Solved
MediumVDP
// achievement_unlocked

Client-Side Role Bypass + Missing Server RBAC

Business Logic
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
http://localhost:1700

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

Para parar:

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

Contenido del Lab

La aplicacion

CyberRank es una plataforma de ranking de hackers con leaderboard, panel de administracion y un super-hacker panel secreto.

La vulnerabilidad

El servidor envia role: user en las respuestas de autenticacion y el frontend muestra/oculta secciones basandose en ese valor. Sin embargo, los endpoints de admin (/api/admin/*) no verifican el rol del usuario en el servidor, confiando unicamente en el cliente.

Cadena de ataque

1. Registrarse → role: user
2. Burp → Match & Replace: role.*user → role.*admin
3. Refrescar → Panel admin visible
4. GET /api/admin/users → Lista usuarios (funciona sin ser admin!)
5. Cambiar contrasena de thebesthacker
6. Login como thebesthacker → Flag

Solucion interactiva

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

Remediacion

  1. Validar roles en el SERVIDOR, nunca confiar en el cliente
  2. Implementar middleware de autorizacion en cada endpoint admin
  3. No exponer el rol en las respuestas del API si no es necesario
  4. Principio de menor privilegio en cada endpoint
Updated
Aug 2026

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Tools

Burp SuiteNavegador

Prerequisites

  • Uso basico de Burp Suite (Match & Replace)
  • Entender la diferencia entre validacion client-side y server-side

Tags

Business Logic