OTP Security Best Practices for Developers
Short answer: Secure an OTP flow by generating random codes on the server, keeping them short-lived and single-use, limiting attempts and requests, never logging the code, and giving a fallback channel for delivery. Treat the OTP as one factor, not as proof the person is who they say they are, and watch for abuse such as repeated requests to the same number.
This checklist covers generating, sending, verifying and monitoring OTPs. For the sending API, see OTP SMS API; for registration, see OTP and DLT.
Last updated: 2 November 2026.
Checklist by stage
| Stage | Do | Avoid |
|---|---|---|
| Generate | Use a cryptographically secure random generator on the server | Predictable codes, client-side generation, reusing codes |
| Store | Keep a hash or the code server-side with an expiry; delete after use | Logging OTPs, storing them in plain text for long |
| Send | Use a registered template; include the purpose and a do-not-share note | Putting the OTP in URLs or sending it to unverified channels |
| Expire | Short lifetime, single use, invalidate on resend | Long-lived codes or several valid codes at once |
| Verify | Limit attempts per code and per number; add a delay after failures | Unlimited tries or revealing which part was wrong |
| Request limits | Rate-limit requests per number, IP and account | Letting anyone trigger unlimited sends |
| Fallback | Offer WhatsApp or voice OTP after a delay if SMS is not read | Resending SMS repeatedly in a loop |
| Monitor | Track delivery, verification success and unusual patterns | Ignoring spikes to one number or range |
Common attack patterns to plan for
- Brute force: guessing the code. Limit attempts and lock out after failures.
- SMS bombing: triggering many OTPs to one victim. Rate-limit per number and add a cooldown.
- Traffic inflation: automated requests that make you pay for messages. Add CAPTCHA or device checks to the request step.
- Phishing: users tricked into sharing codes. State clearly in the message that it should not be shared.
- SIM swap: a code goes to an attacker. For high-value actions, add another factor or a check.
Delivery and fallback
Delivery depends on the recipient's network and phone, so plan for misses. Use delivery status to decide when to offer another channel. See Voice OTP, WhatsApp authentication messages and fallback routing, and read statuses in delivery reports.
OTP security questions
How long should an OTP be valid?
Keep it short, commonly a few minutes, and match it to the risk of the action. Shorter is safer if users can receive codes quickly.
How many digits should an OTP have?
Six digits is common. Combine length with attempt limits rather than relying on length alone.
Is SMS OTP secure enough?
It is widely used and fine for many cases, but it is weaker than app-based or hardware factors for high-value actions. Add stronger checks where risk is high.
Should I store OTPs?
Only for verification and only briefly. Store a hash if possible and delete after use or expiry.
What should the OTP message say?
The code, the purpose, the expiry and a note not to share it, all within your registered template.
Ready to unify your business communication?
Tell us what you want to send and on which channels, and our team will help you get set up on your own branded panel.