Multi-factor authentication is the most effective single thing most apps can do to protect accounts, because it means a stolen password is no longer enough to get in. But not all second factors are equal: an authenticator app or a passkey is far stronger than an SMS code, and where you enforce the check matters more than which factor you pick. For a mobile app, MFA is worth offering, and worth implementing so the second factor is genuinely verified on your server. Here is what MFA is, how the methods compare, and how to add it to a mobile app.
Short answer
Multi-factor authentication (MFA), often two-factor (2FA), requires a second factor beyond the password to sign in, so a stolen password alone does not grant access. Per OWASP, the common factors are something you know (a password), something you have (an authenticator app, a passkey, a security key, or an SMS code), and something you are (a biometric). For a mobile app, prefer stronger second factors, an authenticator app generating time-based codes, push approval, or passkeys, over SMS codes, which are the weakest, and enforce the second-factor check on your server, not the client. MFA is one of the highest-value account protections you can add.
What you should know
- MFA adds a second factor: a stolen password alone is not enough.
- Factor types: something you know, have, or are.
- Not all factors are equal: authenticator apps and passkeys beat SMS.
- Enforce it server-side: the second-factor check must be on the backend.
- Offer recovery: recovery codes so users are not locked out.
What is MFA, and what are the factor types?
It is requiring more than one independent factor to authenticate. Authentication factors fall into three categories: something you know, like a password; something you have, like a phone running an authenticator app, a passkey, a hardware security key, or a device receiving an SMS code; and something you are, like a fingerprint or face. MFA combines factors from different categories so that compromising one is not enough, the classic case being a password plus a code from an authenticator app. The strength of MFA depends on the second factor's resistance to attack: a factor that cannot be phished or intercepted, like a passkey or a hardware key, is much stronger than one that can, like an SMS code. So MFA is not a single thing; it is a combination, and the choice of second factor determines how much protection it actually adds.
How do the MFA methods compare?
By how resistant the second factor is to attack. The table compares common methods.
| Method | Strength |
|---|---|
| SMS one-time code | Weakest; vulnerable to SIM swap and interception |
| Authenticator app (time-based code) | Strong; codes are generated on-device, not sent |
| Push approval | Strong; beware approval fatigue and prompt bombing |
| Hardware security key or passkey | Strongest; phishing-resistant public-key auth |
| Email code | Weak to moderate; only as secure as the email account |
The pattern is that factors delivered over a channel that can be intercepted or socially engineered, SMS especially, are the weakest, while factors based on a secret that stays on the device or on public-key cryptography are the strongest. SMS MFA is still much better than no MFA, but if you can offer an authenticator app or passkeys, those resist the attacks SMS is prone to. Push approval is strong but watch for users approving prompts reflexively.
How do you add MFA to a mobile app?
Offer a strong second factor, verify it server-side, and provide recovery. Let users enable MFA, and prefer methods that resist phishing and interception: an authenticator app generating time-based codes, push approval, or passkeys, while treating SMS as a fallback rather than the default. Generate and verify the second factor on your server, so the check cannot be bypassed by a manipulated client, and store any shared secret, such as a TOTP seed, securely on the backend, never exposed to the client. Rate-limit verification attempts to prevent brute-forcing the code, and provide recovery codes or another recovery path so a user who loses their second factor is not permanently locked out. The principle, as with all authentication, is that the client presents the factor but the server is the authority that verifies it, and the strength of your MFA is set by the second factor you offer.
What to watch out for
The first trap is making SMS your only or default second factor, when it is the weakest and prone to SIM swap; offer an authenticator app or passkeys. The second is verifying the second factor on the client, which a manipulated client can bypass; enforce it server-side. The third is omitting recovery, which locks users out, or omitting rate limiting, which lets codes be brute-forced. MFA is largely a server and design matter, but a pre-submission scan such as PTKD.com (https://ptkd.com) reads your app against OWASP MASVS and checks how it stores secrets and tokens, which complements MFA by confirming the credentials around it are handled securely. The verification logic lives on your server.
What to take away
- MFA requires a second factor beyond the password, so a stolen password alone does not grant access, making it one of the highest-value account protections.
- Factors are something you know, have, or are, and the strength depends on the second factor: authenticator apps, push, and passkeys beat SMS, which is the weakest.
- Offer a strong second factor, verify it on your server, rate-limit attempts, and provide recovery codes so users are not locked out.
- Use a pre-submission scan such as PTKD.com to confirm the secrets and tokens around your MFA are stored securely.
