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Introduction:
Broken Access Control remains a top-tier vulnerability, consistently ranking in the OWASP Top 10. A recent bug bounty case demonstrates a critical flaw where deactivated administrative accounts retain their API-level privileges, allowing them to perform unauthorized actions long after being stripped of access. This exposes a fundamental weakness in how applications handle authorization state.
Learning Objectives:
- Understand the mechanics of Broken Access Control vulnerabilities in API contexts.
- Learn to identify and test for authorization flaws using manual and automated techniques.
- Implement robust server-side validation to prevent privilege escalation via deactivated accounts.
You Should Know:
- Intercepting and Replaying State-Changing Requests with Burp Suite
`POST /api/v1/users/update HTTP/1.1
Host: target.com
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9…
Content-Type: application/json
{“user_id”: 456, “email”: “[email protected]”}`
Step‑by‑step guide:
- Intercept Traffic: Configure your browser to use a proxy like Burp Suite. Log into an admin account and perform a sensitive action, such as modifying another user’s email address.
- Capture the Request: Burp Suite’s Proxy > Intercept tab will capture the HTTP request. Forward the request to complete the action normally.
- Send to Repeater: Right-click the intercepted request in the Proxy history and select “Send to Repeater.”
- Simulate Deactivation: Have another administrator (or use a separate account) deactivate the admin account you used to make the request.
- Replay the Request: In the Burp Repeater tab, click “Send.” Observe the response. A successful (200 OK) response, despite the account being deactivated, confirms the Broken Access Control vulnerability.
-
Automated Testing for Insecure Direct Object References (IDOR) with Nuclei
`nuclei -u https://target.com -t /path/to/idor-templates.yaml -H “Authorization: Bearer“`
Step‑by‑step guide:
- Template Creation: Create a YAML template (
idor-templates.yaml) that defines the test cases. It should include the previously captured request and target different object IDs (e.g., user_id, account_id). - Run Nuclei: Execute the command in your terminal. Nuclei will automatically replay the requests with various IDs, testing for unauthorized access.
- Analyze Output: Nuclei will report which requests returned successful responses, indicating potential IDOR vulnerabilities. This automates the process of testing access controls across a wide range of objects.
3. Server-Side Session Validation in Node.js
// Middleware to validate user status on every request
<h2 style="color: yellow;">const validateActiveUser = (req, res, next) => {</h2>
const user = getUserFromDB(req.user.id); // Fetch fresh user state from DB
<h2 style="color: yellow;">if (!user || !user.isActive) {</h2>
<h2 style="color: yellow;">return res.status(401).json({ error: 'Account deactivated' });</h2>
}
<h2 style="color: yellow;">next();</h2>
<h2 style="color: yellow;">};</h2>
<h2 style="color: yellow;">// Apply to all API routes</h2>
<h2 style="color: yellow;">app.use('/api/', validateActiveUser);
Step‑by‑step guide:
- Implement Middleware: This code snippet creates an Express.js middleware function.
- Fresh State Check: Instead of relying solely on the JWT token’s validity, the middleware queries the database on every request to check the `isActive` flag associated with the user ID.
- Apply Globally: The `app.use()` function applies this validation check to every request starting with
/api/. This ensures that no API endpoint can be accessed by a deactivated user, effectively mitigating the vulnerability. -
Linux Auditd Rules for Monitoring User State Changes
`sudo auditctl -w /etc/passwd -p wa -k user_state_change
sudo auditctl -w /etc/shadow -p wa -k user_state_change
sudo auditctl -w /path/to/app/db/schema.sql -p wa -k user_state_change`
Step‑by‑step guide:
- Install Auditd: Ensure the `auditd` service is installed and running (
sudo systemctl status auditd). - Add Rules: These commands add rules to watch (
-w) the specified files (/etc/passwd,/etc/shadow, application database) for write or attribute changes (-p wa). - Tag Events: The `-k` flag tags the events with a key for easy searching.
- Search Logs: Use `ausearch -k user_state_change` to review logs for any modifications to user accounts or critical application data, helping to track admin actions and potential misuse.
5. Windows PowerShell: Querying Active User Sessions
`Get-CimInstance -ClassName Win32_LoggedOnUser | Select Antecedent Dependent | Format-List`
Step‑by‑step guide:
- Open PowerShell: Launch Windows PowerShell with administrative privileges.
- Execute Command: This command uses the `Get-CimInstance` cmdlet to query the `Win32_LoggedOnUser` class, which provides details about the relationships between users and sessions on the system.
- Analyze Output: The output will list all currently logged-on users and their associated sessions. This is crucial for forensic analysis to identify active or persistent sessions belonging to accounts that have recently been deactivated, allowing for manual session termination.
6. Forcing JWT Token Invalidation on User Deactivation
// Upon user deactivation, add token to a deny-list (Redis)denylist:${token}
<h2 style="color: yellow;">const redis = require('redis');</h2>
<h2 style="color: yellow;">const client = redis.createClient();</h2>
<h2 style="color: yellow;">async function invalidateToken(userId, token) {</h2>
// Set token in Redis with an expiration time (matching JWT expiry)
await client.setEx(, 3600, 'invalid'); // Expire in 1 hourdenylist:${token}
}
<h2 style="color: yellow;">// Additional validation middleware</h2>
<h2 style="color: yellow;">const checkToken = (req, res, next) => {</h2>
<h2 style="color: yellow;">const token = req.header('Authorization').replace('Bearer ', '');</h2>
<h2 style="color: yellow;">client.get(, (err, reply) => {</h2>
<h2 style="color: yellow;">if (reply) {</h2>
<h2 style="color: yellow;">return res.status(401).send({ error: 'Token invalidated' });</h2>
}
<h2 style="color: yellow;">next();</h2>
<h2 style="color: yellow;">});</h2>
<h2 style="color: yellow;">};
Step‑by‑step guide:
- Setup Redis: Ensure a Redis server is running and the `redis` npm package is installed.
- Invalidation Function: The `invalidateToken` function is called immediately when a user is deactivated. It stores the user’s active token in Redis with a time-to-live (TTL) equal to the token’s remaining validity period.
- Validation Middleware: The `checkToken` middleware runs before every request. It checks if the presented token exists in the Redis deny-list. If it does, access is denied immediately, even if the token’s signature is technically still valid.
7. Exploiting and Mitigating JWT Vulnerabilities with jwt_tool
`python3 jwt_tool.py
Step‑by‑step guide:
- Get jwt_tool: Clone the toolkit from GitHub (`git clone https://github.com/ticarpi/jwt_tool`).
- Capture a JWT: Intercept a valid API request to obtain a JWT token from the `Authorization` header.
- Run Exploit: This command attempts a key confusion attack (
-X k), using a provided public key (-pk) to sign a modified token. It injects (-I) a new value (-pv) into the `email` claim (-hc). - Mitigation: To prevent this, ensure your server explicitly specifies the expected algorithm (e.g.,
RS256) when verifying JWTs and does not fall back to a `none` algorithm or accept tokens signed with unexpected keys.
What Undercode Say:
- Statelessness is a Double-Edged Sword: Relying on stateless tokens (JWTs) for authorization is efficient but dangerous. The server must not trust the token blindly and must validate every critical action against a fresh, authoritative source of truth (the database).
- Authorization != Authentication: A deactivated user is still authenticated (their token is valid) but should no longer be authorized. Most vulnerabilities occur because developers check the former but neglect the latter on a per-request basis.
The core issue is a logical flaw in authorization checks. The application authenticates the user’s JWT token correctly but fails to perform a subsequent, real-time check against the user’s current status in the database before executing a privileged action. This creates a dangerous time window where a deactivated account remains functionally active at the API level. Mitigation requires a shift from purely token-based trust to a hybrid model where critical permissions are re-validated against the system’s live state for every single request.
Prediction:
This specific vulnerability pattern will become increasingly prevalent as applications continue to adopt microservices architectures and stateless APIs. The complexity of distributed systems makes consistent authorization checks more difficult to implement. We predict a rise in automated scanning tools specifically designed to hunt for these “zombie account” vulnerabilities by replaying requests with altered user states. Furthermore, regulatory frameworks like GDPR, which mandate the right to erasure, will begin to include stricter guidelines on immediate access revocation, turning these bugs into not just security risks but also compliance failures.
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Ankitrathva Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
