Session management is one of the most critical security controls in modern web applications — especially in fintech platforms handling sensitive financial and KYC data.

In this article, I’ll walk through a real-world session behavior analysis where an access token remained valid even after logout. This write-up focuses on methodology, testing approach, and lessons learned.

To evaluate whether logout properly invalidates previously issued access tokens for authenticated API endpoints.

Target endpoint tested:

/account/user-info

🔎 Understanding the Threat Model

When a user logs out, the expected behavior is:

• All active access tokens associated with that session should be invalidated.

• Any further request using that token should return 401 Unauthorized.

If this doesn’t happen, the system may allow continued access to sensitive user data.

🧪 Step-by-Step Testing Methodology

Step 1: Capture an Access Token

• After logging into the application normally:

  • Open Developer Tools (Network tab).

  • Locate any authenticated API request.

  • Extract the Authorization: Bearer <token> header.
    
    

Example:

Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

Step 2: Perform Logout

• Log out using the application’s official logout functionality.

• Then:

  • Clear browser storage (cookies, localStorage, sessionStorage)

  • Close the browser completely

This simulates a normal user ending their session.

Step 3: Replay the Old Token

Using curl, Postman, or Burp Repeater, send a request manually:

curl -i "https://target-api.com/account/user-info" \

  -H "Authorization: Bearer <OLD_TOKEN>" \

  -H "Accept: application/json"

Step 4: Analyze the Response

If the response returns:

HTTP/1.1 200 OK

Along with full authenticated user data —
this indicates that the token remains valid even after logout.

📊 Observed Behavior

• A previously issued access token continued to return 200 OK

• Sensitive user profile data was accessible

• Newly generated tokens were invalidated correctly upon logout

• Behavior was inconsistent between old and new tokens

🧠 Why This Matters

In financial systems, access tokens typically grant access to:

• Email address

• Phone number

• Residential address

• KYC and tax-related data

• Risk profile and investment preferences

• Account status information
  
  

If a token persists beyond logout, users may assume they are safe when they are not.

⚙️ Possible Technical Explanation

This behavior often occurs when:

• The system uses stateless JWTs

• Logout only clears client-side storage

• The backend does not maintain a token blacklist

• Access tokens are allowed to expire naturally
  
  

In such architectures, logout does not actively revoke tokens — it only removes them from the browser.

🔐 Security Lessons Learned

During session testing, always validate:

1. Token expiration (exp claim in JWT)

2. Behavior after password change

3. Behavior after 2FA reset

4. Behavior after account lock

5. Cross-device token reuse

6. IP binding validation
   
   

Session security must be evaluated holistically — not just based on logout behavior.

📌 Key Takeaway

• Session management vulnerabilities are not always about broken code — sometimes they are about architectural design decisions.

• Understanding how tokens are generated, validated, and invalidated is critical when assessing authentication security.

• Even when a finding is not classified as a vulnerability, the methodology and learning experience are valuable.

🚀 Final Thoughts

Security research is not just about winning bounties. It’s about understanding systems deeply. Every session test improves your threat modeling skills. Every rejection sharpens your impact analysis. Keep testing. Keep learning. Keep improving.