Designing Secure Password Reset & OTP Verification: Timing Attacks, Rate Limits & State

[ Attacker ] ---> (Automated 1000 OTPs/sec) ---> [ Vulnerable Reset API ] ---> 💥 Account Takeover!

+--------------+               +---------------+               +---------------+
| 1. Request   | -> Token ->   | 2. Verification| -> Session -> | 3. Reset      |
| Email / Phone|    Hashed     | 6-Digit OTP   |    One-Time   | New Password  |
+--------------+               +---------------+               +---------------+
       |                               |                               |
       v                               v                               v
  Rate Limit:                     Attempts <= 5,                  Hash with
  Max 3 / hour                    TTL = 5 mins                    Argon2id

import * as crypto from 'crypto';

export function secureCompare(a: string, b: string): boolean {
  const bufA = Buffer.from(a);
  const bufB = Buffer.from(b);

  if (bufA.length !== bufB.length) {
    // Perform dummy comparison to equalize timing
    crypto.timingSafeEqual(bufA, bufA);
    return false;
  }

  return crypto.timingSafeEqual(bufA, bufB);
}

@Injectable()
export class PasswordResetService {
  async initiateReset(email: string) {
    const user = await this.usersService.findByEmail(email);
    // Generic response regardless of whether user exists to prevent email enumeration
    const genericResponse = { message: 'If the email exists, a verification code has been dispatched.' };

    if (!user) return genericResponse;

    const rateKey = `rate:otp:${user.id}`;
    const requestsCount = await this.redis.incr(rateKey);
    if (requestsCount === 1) await this.redis.expire(rateKey, 3600); // 1 hour window
    if (requestsCount > 3) throw new BadRequestException('Too many reset attempts. Please try again later.');

    // Generate cryptographically strong 6-digit OTP
    const rawOtp = crypto.randomInt(100000, 999999).toString();
    const hashedOtp = crypto.createHash('sha256').update(rawOtp).digest('hex');

    await this.redis.set(`reset:otp:${user.id}`, hashedOtp, 'EX', 300); // 5 minutes TTL
    await this.notificationQueue.sendOtpEmail(user.email, rawOtp);

    return genericResponse;
  }
}