How to Build a Secure SMS OTP Login System With Cooldowns and Attempt Limits

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A technical guide outlines best practices for implementing passwordless phone login using SMS one-time passwords, recommending developers treat the process as a state machine rather than simple send-and-check handlers. The approach involves issuing a random six-digit code, storing only a hashed digest, expiring it quickly, and allowing only one verification attempt per code. Four separate controls must be tracked independently: OTP expiry, minimum cooldown between sends, a per-window send quota, and a failed-attempt counter. Crucially, requesting a new code should rotate the existing secret without resetting the failure budget, closing a loophole that could allow unlimited guessing. The guide suggests example defaults of a 10-minute OTP lifetime, 60-second cooldown, five sends per hour, and five failed attempts, while emphasizing these values should be tuned against real-world abuse scenarios.

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