What's the most cost-effective way to add phishing-resistant login verification using a step-up authentication system?

The most cost-effective phishing-resistant step-up option is usually passkeys (FIDO2/WebAuthn), especially if you can use them as the second factor only for sensitive actions.

Why this is the best value

• Phishing-resistant: Passkeys are bound to the real site, so they don’t work on lookalike phishing domains.
• Low operational cost: No SMS fees, no OTP infrastructure, fewer support issues than codes/password resets.
• User-friendly: Often just Face ID, Touch ID, Windows Hello, or a device PIN.
• Step-up friendly: You can require them only when risk is higher—e.g. new device, password change, payout, admin action, or unusual location.

Recommended approach

1. Keep your existing primary login for normal access.
2. Add a step-up policy for sensitive events:
   • new login session
   • account recovery
   • changing email/password
   • adding payment methods
   • exporting data
   • privileged admin actions
3. Use WebAuthn/passkeys as the step-up method.
4. Allow multiple authenticators so users aren’t locked out:
   • primary passkey on phone/laptop
   • backup passkey or hardware security key
5. Use risk-based triggers to avoid prompting unnecessarily:
   • unfamiliar device
   • impossible travel
   • high-value action
   • suspicious IP / geolocation

Lowest-cost implementation paths

• If you already use an identity provider (IdP):
  • Enable built-in passkey/WebAuthn support and conditional step-up policies.
  • This is usually the cheapest and fastest path.
• If building yourself:
  • Implement WebAuthn on the server and client.
  • Use it only for step-up events, not necessarily as the sole login method at first.

What to avoid if phishing resistance is the goal

• SMS codes: cheap to deploy, but not phishing-resistant.
• Email OTPs: also weak against phishing and account takeover.
• TOTP apps: better than SMS, but still phishable.

Practical best-practice combo

If you want the best balance of cost, security, and user experience:

• Password or passkey for base login
• Passkey/WebAuthn for step-up on sensitive actions
• Backup recovery method with strict controls
• Session re-authentication based on risk

If you want, I can also give you:

• a cheap implementation architecture
• a vendor comparison
• or a step-up policy example for web or mobile apps.