From Remote IDs to Free Rides: Unauthorized Subscription Extensions in [Redacted Student App]

🧠 TL;DR

A single endpoint let me stack premium subscription time without paying a cent. By exploiting predictable IDs, verbose validation messages, and zero rate-limiting, I extended premium access by decades — no injections, no stolen creds, just pure business logic abuse.

🕵️ Background

The [Redacted Student App] offers subscription perks when users register purchased remotes. The API that handled these registrations turned out to be a treasure chest:

• Only used client-side format validation
• Leaked format requirements in errors
• Accepted predictable alphanumeric codes
• No CAPTCHA or rate-limiting

Initial login flow — standard user account for testing.

🔎 Discovery Steps

✅ Step 1: Legit Flow

Logged in and registered a device via the UI. Observed a predictable POST request:

POST /student/remote/register
{
  "remote_id": "ABC1234F"
}

Normal subscription UI flow before the vulnerability.

🚫 Step 2: Invalid Test

Sent an invalid payload to test validation behavior:

{
  "remote_id": "NOTVALID"
}

Response:

"Remote Code is not a valid ID. IDs are 8 characters long and contain only A-F and 0-9."

The app just leaked its entire regex validation logic in an error response — effectively telling me how to craft valid codes.

Validation error revealing format pattern and input rules.

🔍 Step 3: Leaked Valid Codes

Searching public resources revealed multiple PDFs and articles containing test or example remote IDs. These became the base for generating valid code candidates.

• Official documentation samples
• Support forum posts with “example” IDs
• Cached logs on GitHub issues

📈 Step 4: Brute-Forcing Begins

Wrote a small Python script to combine leaked codes and generated IDs matching the exposed regex. The endpoint had:

• No CAPTCHA
• No rate limit
• No IP block or account throttling

Result: dozens of 200 OK responses. Each valid hit extended the subscription.

Scripted brute-forcing using leaked and generated codes.

🎁 Step 5: Unlimited Subscriptions

Each valid code added 4 years of premium access. After stacking a few dozen, the app proudly displayed:

“Subscription valid until 2073.”

Subscription extended decades beyond intended duration.

🚨 Impact Snapshot

• Unlimited premium activation ($50/code)
• Potential resale or black-market abuse
• Massive revenue loss risk
• Exploitable by low-skill attackers

🧰 Root Cause

• Business logic trusted user-supplied identifiers
• Error messages revealed internal validation logic
• Codes weren’t unique or identity-bound
• No brute-force or replay defenses

🛠️ Recommendations

🧠 Final Thought

This wasn’t a “loud” exploit — no injections, no crashes, no backdoors. Just a quiet business logic flaw that handed out real monetary value to anyone paying attention.

Sometimes, the most devastating vulnerabilities are the ones that simply trust too much.