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two-factor-authentication-best-practices

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by better-auth · part of better-auth/skills

Configure TOTP authenticator apps, send OTP codes via email/SMS, manage backup codes, handle trusted devices, and implement 2FA sign-in flows using Better Auth's twoFactor plugin. Use when users need MFA, multi-factor authentication, authenticator setup, or login security with Better Auth.

🔥🔥🔥🔥✓ VerifiedFreeQuick setup
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🧩 One of 5 skills in the better-auth/skills package — works on its own, and pairs well with its siblings.

This is the playbook your agent receives when the skill activates — you don't need to read it to use the skill, but it's here to audit before installing.

Enabling 2FA for Users

Requires password verification. Returns TOTP URI (for QR code) and backup codes.

const enable2FA = async (password: string) => {
  const { data, error } = await authClient.twoFactor.enable({
    password,
  });

  if (data) {
    // data.totpURI — generate a QR code from this
    // data.backupCodes — display to user
  }
};

twoFactorEnabled is not set to true until first TOTP verification succeeds. Override with skipVerificationOnEnable: true (not recommended).

TOTP (Authenticator App)

Displaying the QR Code

import QRCode from "react-qr-code";

const TotpSetup = ({ totpURI }: { totpURI: string }) => {
  return <QRCode value={totpURI} />;
};

Verifying TOTP Codes

Accepts codes from one period before/after current time:

const verifyTotp = async (code: string) => {
  const { data, error } = await authClient.twoFactor.verifyTotp({
    code,
    trustDevice: true,
  });
};

TOTP Configuration Options

twoFactor({
  totpOptions: {
    digits: 6, // 6 or 8 digits (default: 6)
    period: 30, // Code validity period in seconds (default: 30)
  },
});

OTP (Email/SMS)

Configuring OTP Delivery

import { betterAuth } from "better-auth";
import { twoFactor } from "better-auth/plugins";
import { sendEmail } from "./email";

export const auth = betterAuth({
  plugins: [
    twoFactor({
      otpOptions: {
        sendOTP: async ({ user, otp }, ctx) => {
          await sendEmail({
            to: user.email,
            subject: "Your verification code",
            text: `Your code is: ${otp}`,
          });
        },
        period: 5, // Code validity in minutes (default: 3)
        digits: 6, // Number of digits (default: 6)
        allowedAttempts: 5, // Max verification attempts (default: 5)
      },
    }),
  ],
});

Sending and Verifying OTP

Send: authClient.twoFactor.sendOtp(). Verify: authClient.twoFactor.verifyOtp({ code, trustDevice: true }).

OTP Storage Security

Configure how OTP codes are stored in the database:

twoFactor({
  otpOptions: {
    storeOTP: "encrypted", // Options: "plain", "encrypted", "hashed"
  },
});

For custom encryption:

twoFactor({
  otpOptions: {
    storeOTP: {
      encrypt: async (token) => myEncrypt(token),
      decrypt: async (token) => myDecrypt(token),
    },
  },
});

Backup Codes

Generated automatically when 2FA is enabled. Each code is single-use.

Displaying Backup Codes

const BackupCodes = ({ codes }: { codes: string[] }) => {
  return (
    <div>
      <p>Save these codes in a secure location:</p>
      <ul>
        {codes.map((code, i) => (
          <li key={i}>{code}</li>
        ))}
      </ul>
    </div>
  );
};

Regenerating Backup Codes

Invalidates all previous codes:

const regenerateBackupCodes = async (password: string) => {
  const { data, error } = await authClient.twoFactor.generateBackupCodes({
    password,
  });
  // data.backupCodes contains the new codes
};

Using Backup Codes for Recovery

const verifyBackupCode = async (code: string) => {
  const { data, error } = await authClient.twoFactor.verifyBackupCode({
    code,
    trustDevice: true,
  });
};

Backup Code Configuration

twoFactor({
  backupCodeOptions: {
    amount: 10, // Number of codes to generate (default: 10)
    length: 10, // Length of each code (default: 10)
    storeBackupCodes: "encrypted", // Options: "plain", "encrypted"
  },
});

Handling 2FA During Sign-In

Response includes twoFactorRedirect: true when 2FA is required:

Sign-In Flow

  1. Call signIn.email({ email, password })
  2. Check context.data.twoFactorRedirect in onSuccess
  3. If true, redirect to /2fa verification page
  4. Verify via TOTP, OTP, or backup code
  5. Session cookie is created on successful verification
const signIn = async (email: string, password: string) => {
  const { data, error } = await authClient.signIn.email(
    { email, password },
    {
      onSuccess(context) {
        if (context.data.twoFactorRedirect) {
          window.location.href = "/2fa";
        }
      },
    }
  );
};

Server-side: check "twoFactorRedirect" in response when using auth.api.signInEmail.

Trusted Devices

Pass trustDevice: true when verifying. Default trust duration: 30 days (trustDeviceMaxAge). Refreshes on each sign-in.

Security Considerations

Session Management

Flow: credentials → session removed → temporary 2FA cookie (10 min default) → verify → session created.

twoFactor({
  twoFactorCookieMaxAge: 600, // 10 minutes in seconds (default)
});

Rate Limiting

Built-in: 3 requests per 10 seconds for all 2FA endpoints. OTP has additional attempt limiting:

twoFactor({
  otpOptions: {
    allowedAttempts: 5, // Max attempts per OTP code (default: 5)
  },
});

Encryption at Rest

TOTP secrets: encrypted with auth secret. Backup codes: encrypted by default. OTP: configurable ("plain", "encrypted", "hashed"). Uses constant-time comparison for verification.

2FA can only be enabled for credential (email/password) accounts.

Disabling 2FA

Requires password confirmation. Revokes trusted device records:

const disable2FA = async (password: string) => {
  const { data, error } = await authClient.twoFactor.disable({
    password,
  });
};