Secure Email Resend Requests

TL;DR

To securely handle email resend requests, use unique tokens that expire quickly. Verify these tokens on every request to prevent unauthorised re-sends and protect against replay attacks.

Step-by-step guide: Securing Email Resend Requests

1. Generate a Unique Token
• When a user requests an email resend, create a unique token. This could be a random string of characters (e.g., using UUIDs).
• Store this token in your database associated with the user’s request. Include an expiry timestamp – typically 15-30 minutes is sufficient.

# Example Python code using uuid
import uuid
import datetime

token = str(uuid.uuid4())
expiry_time = datetime.datetime.now() + datetime.timedelta(minutes=15)
# Store token and expiry_time in your database

2. Send the Token in the Email
• Include the generated token as a query parameter in the resend email link. Do not embed it directly into the body of the email.
• Example: Resend Email
3. Verify the Token on Request
• When a user clicks the resend link, extract the token from the URL query parameters.
• Retrieve the token from your database.
• Check if the token exists and hasn’t expired.
• If the token is valid, send the email. Immediately invalidate (delete) the token after successful resend to prevent reuse.

# Example PHP code for verification
$token = $_GET['token'];
// Retrieve token from database based on $token
if ($token && $expiry_time > date('Y-m-d H:i:s')) {
  // Token is valid - send email and delete the token from the database.
} else {
  // Invalid or expired token - display an error message.
}

4. Prevent Replay Attacks
• Deleting the token after a successful resend is crucial. This prevents someone from using the same link multiple times.
• Consider rate limiting requests per user to further mitigate abuse.
5. Secure Token Generation
• Use cryptographically secure random number generators for token creation. Avoid predictable patterns.
• Ensure the tokens are long enough (at least 32 characters) to make them difficult to guess.
6. HTTPS is Essential
• Always use HTTPS to encrypt communication between the user’s browser and your server, protecting the token from interception during transmission.

Additional Considerations

• Logging: Log all resend requests (successful and failed) for auditing purposes.
• Error Handling: Provide clear error messages to users if a token is invalid or expired. Avoid revealing technical details that could aid attackers.
• cyber security Best Practices: Regularly review your code and dependencies for vulnerabilities.