The Tool Desk
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1. Expire every challenge on the server
Give each issued code a server-enforced expiration time. Reject it after that deadline even if the app still displays the code-entry screen or a client sends an earlier challenge identifier. The server, not the phone’s clock or local state, decides whether a challenge is live.
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There is no universal expiry duration established by the OWASP guidance. Choose one according to the delivery method, user experience, and risk of the protected account or action. Store enough server-side state to evaluate expiry consistently across API instances.
2. Consume a code after successful verification
A code that has authenticated a user must not authorize another login or action. Make verification and consumption one atomic operation: if two requests submit the same valid code concurrently, no more than one may succeed. A check followed by a separate update can leave a race in which both requests pass before either marks the code as used.
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For app-generated TOTP, enforce replay protection too. A time-based code can remain mathematically valid during its accepted time window; track successful use so the same code cannot authenticate twice within that window.
3. Bound guesses and code issuance
Limit both verification attempts and code sends. They are separate abuse paths: guessing targets an account, while repeated sends can harass a user and create messaging costs. Use independent limits for dimensions such as the target account and network source or device. An IP-only limit can be bypassed by spreading attempts across sources; an account-only lockout can be abused to deny a victim access.
Rank #2
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Choose limits without exposing them
- Set thresholds and cooldowns according to account risk, normal traffic, delivery-provider constraints, and denial-of-service consequences. OWASP does not prescribe one universal attempt count or resend interval.
- Use throttling that controls bursts, monitor allowed and blocked traffic, and return HTTP 429 when a request is rejected for rate limiting.
- Keep public errors generic. Do not disclose whether an account exists, which limit fired, or how many attempts remain. The OWASP Authentication Cheat Sheet recommends generic authentication errors; apply the same principle to OTP issuance and verification.
4. Bind verification to server-side challenge state
Track the challenge lifecycle and sequence on the server. Bind each code to the intended account and purpose—for example, login versus a sensitive account change—and reject expired, reused, or out-of-order transitions. Do not let a client-provided flag mark a challenge complete, switch the requested purpose, or disable a check.
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Rank #3
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5. Protect OTPs in transit, storage, and logs
- Require HTTPS for API traffic carrying codes or authentication state.
- Do not write OTP values to application, analytics, crash, or support logs. Treat them as authentication secrets, not ordinary telemetry.
- Avoid retaining codes in plaintext longer than needed to verify them. OWASP notes that hashing can reduce accidental disclosure, but a short numeric OTP has a small search space and its hash can be brute-forced after a database exposure. Hashing does not replace short expiry, strict attempt limits, or access controls.
These handling recommendations are consistent with the OWASP Multifactor Authentication Cheat Sheet’s OTP guidance.
6. Secure delivery, factor changes, and recovery
SMS and voice delivery add risks outside the API: codes may be intercepted through carrier attacks, phished from users, visible on the same device receiving them, or abused to generate unwanted messages. Rate-limit sends and, where available, monitor phone-number or SIM-change signals. Treat those signals as risk inputs, not proof that an account is safe or compromised.
Rank #4
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- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
Require reauthentication before replacing a phone number or other factor, and notify the user when an authentication factor changes. Design account recovery to uphold the authentication policy rather than silently bypassing it; otherwise, recovery becomes the easier route into the account.
7. Provide a stronger option for higher-risk use
Typed OTPs are not phishing-resistant: a user can be tricked into entering a real code into a fraudulent site or sharing it with an attacker. For accounts or actions that need stronger protection, consider passkeys or other FIDO2/WebAuthn cryptographic authenticators. A physical security key is another possible authenticator, but adopting one does not fix weak OTP issuance, verification, or recovery APIs.
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Define fallback behavior explicitly. Do not quietly downgrade a failed stronger authentication ceremony to SMS. NIST SP 800-63B-4 describes assurance-level requirements; for example, at AAL2 at least one authenticator must be replay-resistant and verifiers must offer at least one phishing-resistant option. That is standards context, not a claim that every consumer login must target AAL2.
Quick Recap
How SMS OTP, TOTP, and FIDO2/WebAuthn differ
| Dimension | SMS OTP | TOTP app | FIDO2/WebAuthn |
|---|---|---|---|
| Phishing resistance | Not phishing-resistant; a user can be induced to disclose a code. | Not phishing-resistant; a user can be induced to disclose a code. | Designed to resist phishing by binding authentication to the relying party’s origin. |
| Replay resistance | Requires server-enforced expiry, single-use consumption, and attempt controls. | Requires verification controls that prevent a successfully used time-based code from being accepted again. | Uses cryptographic challenge-response; verifier challenge handling must still be correct. |
| Carrier or device compromise | Exposed to carrier takeover or interception, as well as access to messages on the receiving device. | Avoids SMS delivery exposure, but compromise or loss of the device or authenticator can affect access. | Depends on the authenticator and recovery setup; it avoids sharing a typed OTP with a phishing site. |
| User friction | Requires access to the phone and receipt of a message or call. | Requires opening an authenticator and entering its current code. | Often uses a platform prompt or security key interaction rather than typing a code. |
| Delivery dependency | Depends on messaging or voice delivery and carrier reachability. | No per-login SMS delivery is required; the authenticator generates codes locally. | No SMS delivery is required for the authentication ceremony. |
| Recovery complexity | Can depend on restoring phone-number control; number changes need careful verification. | Users need a secure plan for authenticator loss or device replacement. | Users need a secure way to recover or replace authenticators without creating a weaker bypass. |
| Deployment burden | Requires delivery integration, send controls, and operational handling of delivery failures. | Requires enrollment and server-side secret protection, time-window validation, and replay controls. | Requires WebAuthn-compatible client and server implementation, enrollment, and recovery flows. |
| Cost considerations | Message or voice delivery can create per-send costs and abuse exposure. | Avoids per-login message delivery costs, while still requiring implementation and support. | May require platform support or physical keys depending on the deployment; no universal cost applies. |
Operational checks before release
- Verify that expired, consumed, wrong-purpose, and out-of-sequence challenges fail on the server.
- Exercise concurrent submissions of the same valid code and confirm that at most one succeeds.
- Test verification and resend limits independently, including requests distributed across accounts and network sources.
- Check that errors do not reveal account existence, challenge validity, remaining attempts, or the limit dimension that triggered throttling.
- Review logs, analytics, and recovery flows for OTP disclosure or routes that bypass the intended authentication policy.
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