Email Validation Service Attacks

TL;DR

Attackers are abusing your email validation service to check if usernames exist on your platform without needing an account. This can lead to data harvesting and targeted attacks. We’ll cover how to identify this, limit the abuse, and improve security.

Understanding the Problem

Your email validation service likely works by checking if an email address is deliverable or registered. Attackers exploit this by submitting large lists of potential usernames (e.g., [email protected], [email protected]) to see which ones respond positively. A positive response confirms the username exists, even without a password.

Step-by-step Solution

1. Monitor Your Logs: The first step is identifying if an attack is happening.
   • Look for unusually high volumes of validation requests from single IP addresses or ranges.
   • Check for patterns – are attackers trying many variations of a common username?
   • Examine the timestamps; attacks often happen outside normal business hours.
2. Implement Rate Limiting: This is your primary defense.
   • Limit the number of validation requests allowed per IP address within a specific timeframe (e.g., 10 requests per minute). This prevents attackers from rapidly checking large lists.
   • Consider different rate limits for authenticated users versus unauthenticated requests.
   • Example using Nginx:

   limit_req_zone $binary_remote_addr zone=mylimit:10m rate=10r/m;

   server { ... limit_req zone=mylimit burst=20 nodelay; ... }

3. Use CAPTCHAs for Unauthenticated Requests: If someone is submitting many requests without logging in, make them solve a CAPTCHA.
   • This significantly slows down automated attacks.
   • Implement CAPTCHAs before the validation request is processed.
4. Introduce Delays: Add a small random delay to each validation response, even for legitimate requests.
   • This makes it harder for attackers to determine which usernames exist quickly.
   • A delay of 1-2 seconds can be effective without noticeably impacting user experience.
5. Obfuscate Responses: Don’t return a simple ‘true’ or ‘false’.
   • Instead, use generic responses like “Email format is valid” regardless of whether the email exists. This prevents attackers from directly identifying existing usernames.
   • Be careful not to leak information that could still be useful to an attacker (e.g., error messages).
6. Consider a Honeypot: Create fake email addresses and monitor if they are being validated.
   • This can help you identify attackers and their techniques.
7. Review Your Validation Logic: Ensure your service doesn’t reveal unnecessary information.
   • Avoid returning specific error codes that indicate username existence.
   • If possible, only validate email format, not actual registration status.
8. Implement Web Application Firewall (WAF) Rules: A WAF can block malicious requests based on known attack patterns.
   • Configure rules to detect and block suspicious activity related to email validation.

Further Considerations

Regularly review your logs, update your security measures, and stay informed about new attack techniques in the cyber security landscape.