MFA recovery codes: generating, storing and consuming them

What it is

NIST SP 800-63B revision 3 (cited) treats a printed list of one-time codes as a look-up secret authenticator: the issuer generates the secrets with an approved random bit generator, each secret must carry at least 20 bits of entropy, a given secret may be used successfully only once, and the verifier stores them in a form resistant to offline attack. Secrets below 112 bits are salted and hashed with a key derivation function, and secrets below 64 bits need rate limiting on failed attempts. The OWASP MFA cheat sheet (cited) lists a set of single-use recovery codes handed out at MFA setup as one of the ways to keep users from being locked out, alongside enrolling several factor types and support-verified identity.

Why it matters

Lost phones and wiped devices are routine, and the recovery path is the part of MFA an attacker attacks: a weak recovery flow makes the second factor decorative. Codes that are short, reusable, stored in clear or accepted without throttling turn "something you have" into a guessable password.

How to apply

• Issue a small fixed set of codes at enrolment, each comfortably above the 20 bits NIST requires (ten characters from a 36-symbol alphabet give about 51 bits, by arithmetic), grouped for readability; show them once, with a copy and download control, and require the user to confirm they stored them.
• Store a salted hash per code; mark a code consumed on success and never accept it again; count failed attempts per account and lock or delay as for passwords.
• Treat a code as a full second factor, not a password replacement: it is accepted only after the first factor.
• After a code is used, tell the user through an out-of-band channel, and prompt them to re-enrol a factor and regenerate the set; regeneration invalidates all old codes and itself requires re-authentication with an existing factor, as the cheat sheet asks for any factor change.
• Log issue, use and regeneration events in the audit log.

Pitfalls

Emailing the codes. Letting a support agent "reset MFA" on a phone call without a rigorous identity check, which bypasses everything above. Displaying remaining codes in the settings page to anyone with a live session. Codes generated from a non-cryptographic random source.