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For SMS one-time passcodes, count failed guesses against the subscriber account and authenticator, and keep that count when a user requests another code. NIST does not prescribe one universal cooldown or small attempt limit. Its current guidance requires effective throttling for short codes, sets 100 consecutive failures as an applicable upper bound—not a recommended target—and allows lower limits. It also requires a code to be used only once and out-of-band authentication to finish within 10 minutes. NIST SP 800-63B-4
How many SMS OTP attempts should you allow?
Choose a limit based on your service’s threat model and the risk that a lockout could block a legitimate user. Do not treat 100 as a suggested setting: NIST SP 800-63B-4 describes it as a general upper bound for consecutive failed attempts using a specific authenticator on one subscriber account, and says agencies may impose lower limits. It does not specify one smaller number for every SMS login flow. NIST SP 800-63B-4, Section 3.2.2
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SMS codes are short secrets, so the verifier needs a mechanism that effectively limits consecutive failed guesses on the subscriber account. A counter tied only to an individual code is inadequate if requesting a fresh code clears the previous failures.
Does NIST specify an OTP cooldown?
No fixed cooldown schedule is prescribed. NIST lists a progressively increasing wait as one optional technique: the example is “30 seconds up to an hour” as an account approaches its maximum consecutive-failure allowance. That is an example, not a required interval or a universal recommendation. NIST SP 800-63B-4, Section 3.2.2
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Set and document the delay behavior your service actually uses. A fixed delay is one possible policy; progressive waits are another. In either case, distinguish the wait before another guess from the rate limit on sending another SMS. They address different actions: entering codes and requesting messages.
What should happen when someone requests a new code?
A resend must not grant more guesses. NIST states: “Generating a new authentication secret SHALL NOT reset the failed authentication count.” Keep the account’s consecutive-failure count intact across code resends. Define whether a new code supersedes the previous one, and make sure the redemption rules remain unambiguous. NIST SP 800-63B-4, Sections 3.1.3.1 and 3.2.2
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After successful authentication, NIST says previous failures for authenticators used SHOULD be disregarded and the retry count SHOULD be reset, subject to the applicable assurance-level and session condition. Treat that success behavior separately from resending a code. NIST SP 800-63B-4, Section 3.2.2
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How long should an SMS verification code last?
NIST says out-of-band authentication is invalid if it is not completed within 10 minutes. It also requires the verifier to accept a given secret only once during its validity period, and to generate a secret of at least six decimal digits or equivalent. Expiry and single-use redemption are separate controls: a code that has not expired should still be rejected after it has been successfully used. NIST SP 800-63B-4, Section 3.1.3.1
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What an implementation policy should define
Document each part of the flow rather than treating “OTP cooldown” as a single setting:
- Code validity: how long a generated secret can be used, within NIST’s 10-minute maximum for out-of-band completion.
- Redemption: how successful use makes that secret unusable for another login.
- Failed-code accounting: the account/authenticator scope of the counter and the chosen consecutive-failure limit.
- Resends: how often a code can be issued and whether issuing a replacement invalidates the earlier code; neither policy should reset failed guesses.
- Throttling: what wait, challenge, or temporary restriction applies after failures, and how the wait changes as the limit nears.
- Recovery: how a legitimate user can regain access or use another authenticator when SMS is unavailable or attempts are blocked.
Balance guessing protection with account access
A strict lockout can become a denial-of-service path if someone deliberately triggers failed attempts against another person’s account. NIST lists optional ways to reduce that risk, including bot-detection or mitigation challenges before authentication, progressively longer waits, and risk signals such as IP address, geolocation, request timing, and browser metadata. Such signals can supplement the authentication process; they are not themselves authenticators. NIST SP 800-63B-4, Section 3.2.2
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These controls do not make SMS resistant to phishing or prevent number takeover. NIST classifies PSTN-based out-of-band authentication, including SMS, as restricted. Its guidance says to consider indicators such as device swaps, SIM changes, number porting, or other abnormal behavior before sending a secret. NIST’s FAQ also notes that agencies must verify that the destination is a phone rather than an IP address such as VoIP, among other applicable requirements. The guideline is for government digital authentication; other organizations may use it as a reference, but its agency-specific provisions are not universal law. NIST SP 800-63B-4 · NIST Digital Identity Guidelines FAQ
Because PSTN access can be unavailable or risky, NIST says alternative authenticator types must be available under its guidance and recommends communicating limitations before a PSTN authenticator is bound. SMS should not be described as phishing-resistant; that term belongs to cryptographic authenticators that can resist phishing. NIST Digital Identity Guidelines FAQ
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Make the wait understandable to users
Show whether the user is waiting to request another message or to submit another code, and state when the next action will be available. Avoid an unexplained lockout with no recovery route. NIST’s customer-experience guidance notes that out-of-band authentication depends on access to both the primary and secondary channels, and recommends consistent, contextual messaging and code-transfer features such as copy and paste. It does not set a mandatory resend interval. NIST Customer Experience Considerations
Which version of NIST guidance applies?
SP 800-63B-4 is the current NIST guideline identified for authentication and authenticator management. NIST’s publication record lists July 31, 2025, as its publication date and says it supersedes the previous SP 800-63B. Older pages may refer to the superseded revision, so check the edition when using a standards citation. NIST CSRC SP 800-63B-4 publication record
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