Credential Stuffing Attacks: A Developer's Defense Guide

#tutorial

Credential stuffing is one of the most common attack vectors in 2026. If you're building any system with user authentication, you need to understand and defend against it.

What is Credential Stuffing?

Attackers take username/password combinations leaked from one service and try them against other services. Since users reuse passwords, these attacks have surprisingly high success rates - typically 0.1% to 2% of attempts succeed.

The Attack Flow

Attacker obtains leaked credentials (billions available on dark web)
Uses automated tools to test credentials against target sites
Successful logins are harvested for account takeover
Compromised accounts are monetized or used for further attacks

Defensive Measures for Developers

Rate Limiting

Implement intelligent rate limiting that goes beyond simple IP-based restrictions:

# Example: Multi-factor rate limiting
def check_rate_limit(ip, username, fingerprint):
    # Limit by IP
    if redis.get(f"rate:ip:{ip}") > 10:
        return False
    # Limit by username (prevents distributed attacks)
    if redis.get(f"rate:user:{username}") > 5:
        return False
    return True

Breach Detection Integration

Check if user credentials appear in known breaches. LeakRadar.io provides an API to check credentials against 75+ billion leaked records:

// Check if password has been breached
async function isPasswordBreached(password) {
  const response = await fetch('https://leakradar.io/api/check', {
    method: 'POST',
    body: JSON.stringify({ password })
  });
  return response.json();
}

CAPTCHA After Failed Attempts

Show CAPTCHA after 3-5 failed login attempts for the same username. This stops automated attacks while minimizing friction for legitimate users.

Monitor for Anomalies

Track login patterns and alert on anomalies:

Logins from new locations
Multiple failed attempts followed by success
Unusual login times

Proactive Monitoring

Use services like LeakRadar to monitor if your users' credentials appear in new breaches. When detected, force password resets before attackers can exploit them.

Conclusion

Credential stuffing isn't going away. Build your defenses now, before your users become victims.

What anti-stuffing measures have you implemented? Let me know in the comments!

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