📋 Quick Facts

  • Vulnerability Type: Broken Access Control (IDOR-adjacent)
  • Affected Feature: "Active Hiring" premium filter
  • Attack Complexity: Low
  • Impact: Unauthorized access to premium features and data
  • CVSS Category: Authorization/Access Control
  • Bounty Amount: Not disclosed
  • Platform Type: Job Search/Recruitment Platform

🎯 Researcher Attribution

This vulnerability was disclosed through public bug bounty channels. While the original researcher's identity remains anonymous, their work highlights a critical oversight in feature-level access control that affects many SaaS platforms offering tiered subscription models.

🔍 The Story: When Premium Isn't So Premium

Picture this: You're browsing a popular job search platform, looking for opportunities. You notice a shiny "Active Hiring" filter badge with a small lock icon and "Premium Only" label. This feature promises to show you companies actively recruiting right now — a game-changer for job seekers willing to pay the subscription fee.

But what if that paywall was just an illusion?

The researcher discovered that while the UI dutifully hid the premium filter behind a paywall, the backend API told a different story. The application trusted the client-side enforcement completely, assuming that if the button was hidden, no one would ever try to access the underlying functionality.

Spoiler alert: That assumption was wrong.

🧠 Understanding Broken Access Control

Before diving into the technical details, let's understand what went wrong here.

Access control is the security mechanism that determines who can access what resources in an application. Think of it like a bouncer at an exclusive club — their job is to check IDs and ensure only authorized guests get in.

Broken Access Control occurs when these checks are missing, improperly implemented, or can be bypassed. It's like having a bouncer who only guards the front door while leaving the side entrance wide open.

In 2021, OWASP (Open Web Application Security Project) moved Broken Access Control from #5 to the #1 spot in their Top 10 Web Application Security Risks. This wasn't arbitrary — 94% of applications tested showed some form of access control issue, with an average incidence rate of 3.81%.

The Client-Side Trust Fallacy

Many developers make a critical mistake: They implement authorization checks in the UI but forget to enforce them on the backend. This is like putting a lock on your front door but leaving the windows open.

In this case:

  • ✅ UI Layer: Premium badge displayed, filter button disabled for free users
  • ❌ API Layer: No validation of user subscription status when processing filter requests

🔬 Technical Analysis

Let's break down how this vulnerability likely worked from a technical perspective.

Normal Premium User Flow

For legitimate premium subscribers, the request flow looks like this:

1. User clicks "Active Hiring" filter
2. Frontend sends API request: GET /api/jobs?filter=active_hiring
3. Backend checks user subscription status
4. Returns filtered results if authorized

The Vulnerability: Bypassing the Paywall

The researcher likely discovered the vulnerability through one of these methods:

Method 1: HTTP Proxy Interception

Using tools like Burp Suite or OWASP ZAP:

  • Created a free account and browsed job listings
  • Intercepted API requests for standard filters
  • Noticed the filter parameter structure: ?filter=recent or ?filter=remote
  • Modified the request to ?filter=active_hiring (guessed or discovered through JavaScript analysis)
  • Backend processed the request without checking subscription status
  • Premium results returned to unprivileged user

Method 2: JavaScript Analysis

  • Examined the frontend JavaScript bundles
  • Found references to premium filter endpoints
  • Located the API call structure in disabled UI components
  • Replicated the request manually via browser console or curl

Method 3: Parameter Fuzzing

  • Identified the filter parameter
  • Fuzzed common premium feature names
  • Discovered active_hiring as a valid but unprotected value

The Technical Root Cause

The vulnerability stemmed from missing server-side authorization checks. Here's what the vulnerable code might have looked like:

// VULNERABLE CODE EXAMPLE
app.get('/api/jobs', async (req, res) => {
  const { filter } = req.query;
  
  // No check for user subscription status!
  const jobs = await database.getJobsByFilter(filter);
  
  return res.json(jobs);
});

The correct implementation should include authorization:

// SECURE CODE EXAMPLE
app.get('/api/jobs', async (req, res) => {
  const { filter } = req.query;
  const user = req.user; // From authentication middleware
  
  // Check if filter requires premium access
  const premiumFilters = ['active_hiring', 'salary_insights', 'company_insights'];
  
  if (premiumFilters.includes(filter) && !user.hasPremiumSubscription) {
    return res.status(403).json({
      error: 'This feature requires a premium subscription'
    });
  }
  
  const jobs = await database.getJobsByFilter(filter);
  return res.json(jobs);
});

💥 Impact Assessment

Business Impact

Direct Revenue Loss

  • Free users accessing premium features reduce subscription conversion
  • Existing premium users may downgrade if they discover the bypass
  • Potential class-action risk from paying customers

Data Exposure

  • Premium filters often aggregate proprietary data (hiring trends, salary insights)
  • Competitors could scrape premium intelligence without paying
  • Market research value leaked to non-subscribers

Trust Damage

  • Premium subscribers paying for features available for free
  • Platform credibility questioned
  • Negative press coverage potential

Security Impact

CVSS 3.1 Estimate: 6.5 (Medium)

  • Attack Vector: Network (N)
  • Attack Complexity: Low (L)
  • Privileges Required: Low (L) — requires basic account
  • User Interaction: None (N)
  • Confidentiality Impact: High (H) — premium data exposed
  • Integrity Impact: None (N)
  • Availability Impact: None (N)

User Impact

For job seekers: Unauthorized access to premium hiring intelligence

For recruiters: Potential exposure of hiring status and trends

For the platform: Loss of competitive advantage and subscription revenue

🎓 Key Takeaways for Bug Hunters

1. Always Test Behind the UI

Don't assume that just because a feature is hidden or disabled in the UI, it's properly protected on the backend. Premium features are goldmines for access control bugs.

Pro tip: Use your browser's developer tools to find disabled buttons, then search for their associated API calls in JavaScript files.

2. Enumerate Feature Flags and Parameters

Look for patterns in API parameters:

  • ?premium=true/false
  • ?feature=premium_filter
  • ?tier=basic|premium|enterprise
  • ?filter=active_hiring

Try toggling these values even when using a free account.

3. Compare Free vs. Premium Accounts

If possible, create both account types and compare:

  • API endpoints available
  • Request/response structures
  • Headers and cookies
  • Parameter validation

Look for differences — or suspicious similarities.

4. Check for Horizontal Privilege Escalation Too

While this bug was vertical (free → premium), also test:

  • Can Premium User A access Premium User B's exclusive data?
  • Are user-specific filters properly isolated?

5. Document Business Logic Thoroughly

When reporting, explain:

  • The business value of the premium feature
  • Exact steps to reproduce
  • Potential revenue impact
  • Proof of concept (screenshots/videos)

This helps prioritization and can lead to higher bounties.

🛡️ Prevention Tips for Developers

1. Enforce Authorization on Every Endpoint

Golden Rule: Never trust the client. Every API endpoint must independently verify authorization.

// Use middleware for consistent checks
const requiresPremium = (req, res, next) => {
  if (!req.user.isPremium) {
    return res.status(403).json({ error: 'Premium subscription required' });
  }
  next();
};
app.get('/api/premium/active-hiring', requiresPremium, getActiveHiring);

2. Implement Feature Flags Server-Side

Don't just check subscription status — use a centralized authorization service:

const canAccessFeature = (user, feature) => {
  const featureMap = {
    'active_hiring': ['premium', 'enterprise'],
    'salary_insights': ['enterprise'],
    'basic_search': ['free', 'premium', 'enterprise']
  };
  
  return featureMap[feature]?.includes(user.subscriptionTier) ?? false;
};

3. Separate Premium and Free Endpoints

Consider using distinct API paths:

  • /api/jobs - public endpoint with basic features
  • /api/premium/jobs - requires subscription check via middleware

This makes it architecturally clear which endpoints need protection.

4. Regular Security Audits

  • Review authorization logic for all paid features quarterly
  • Test with different privilege levels during QA
  • Include access control tests in automated security testing
  • Conduct periodic penetration tests

5. Defense in Depth

Layer your protections:

  • UI-level hiding (UX improvement, not security)
  • API gateway authorization checks
  • Service-level permission validation
  • Database-level row security (where applicable)

6. Logging and Monitoring

Track premium feature access:

  • Log all requests to premium endpoints
  • Alert on free users accessing premium features
  • Monitor unusual access patterns

📚 Resources for Learning More

OWASP Resources

  • OWASP Top 10 — A01:2021 Broken Access Control
  • OWASP Testing Guide — Testing for Authorization
  • OWASP ASVS — V4: Access Control Verification Requirements

Bug Bounty Platforms

  • HackerOne — Access Control vulnerability reports
  • Bugcrowd — IDOR and authorization disclosures
  • YesWeHack — Premium feature bypass case studies

Practice Labs

  • PortSwigger Web Security Academy — Access Control Labs
  • OWASP WebGoat — Access Control Flaws module
  • HackTheBox — Web challenges focusing on authorization

Books

  • "Web Application Security" by Andrew Hoffman
  • "The Web Application Hacker's Handbook" by Dafydd Stuttard
  • "Bug Bounty Bootcamp" by Vickie Li

⏱️ Timeline

  • Discovery Date: Unknown
  • Reported Date: Unknown
  • Vendor Response: Unknown
  • Fix Deployed: Unknown
  • Public Disclosure: 2026–02–10
  • Bounty Awarded: Not disclosed

Note: Complete timeline details were not available in the original disclosure.

🙏 Acknowledgments

Thanks to the anonymous researcher who discovered and responsibly disclosed this vulnerability. Your work helps make the internet more secure for everyone.

A special acknowledgment to the bug bounty platform that facilitated this disclosure, demonstrating their commitment to security through responsible vulnerability handling.

To the platform affected: Thank you for taking security seriously and working to protect your users' data and business interests.

This writeup is for educational purposes only. Always practice responsible disclosure and obtain proper authorization before testing applications. Unauthorized access to computer systems is illegal under laws like the CFAA (US), Computer Misuse Act (UK), and similar legislation worldwide.

Stay curious, stay ethical, and happy hunting! 🔐