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To use the Embedded APIs in Regular Web Applications, make sure you enable the Passwordless OTP grant at Auth0 Dashboard > Applications > Applications in your application’s settings under Advanced Settings > Grant Types. Passwordless authentication for Regular Web Applications consists of two steps:
  1. Capture the user identifier in your application (the user’s email or phone number) and invoke the /passwordless/start endpoint to initiate the passwordless flow. The user will get an email, an SMS with a one-time-use code, or a magic link.
  2. If you did not send a magic link, prompt the user for the one-time-use code, and call the /oauth/token endpoint to get authentication tokens.
Note that when using magic links, you don’t need to call /oauth/token. The user will click the magic link and be redirected to the application’s callback URL. Below, we list a few code snippets that can be used to call these API endpoints for different scenarios. Send a one-time-use code via email 
curl --request POST \
  --url 'https://{yourDomain}/passwordless/start' \
  --header 'content-type: application/json' \
  --data '{"client_id": "{yourClientId}", "client_secret": "{yourClientSecret}", "connection": "email", "email": "{userEmail}","send": "code"}'
Send a magic link via email You need to specify send: link. 
curl --request POST \
  --url 'https://{yourDomain}/passwordless/start' \
  --header 'content-type: application/json' \
  --data '{"client_id": "{yourClientId}", "client_secret": "{yourClientSecret}", "connection": "email", "email": "{userEmail}","send": "link"}'
Send a one-time-use password via SMS 
curl --request POST \
  --url 'https://{yourDomain}/passwordless/start' \
  --header 'content-type: application/json' \
  --data '{"client_id": "{yourClientId}", "client_secret": "{yourClientSecret}", "connection": "sms", "phone_number": "{userPhoneNumber}","send": "code"}'
Authenticate an SMS user 
curl --request POST \
  --url 'https://{yourDomain}/oauth/token' \
  --header 'content-type: application/json' \
  --data '{"grant_type": "http://auth0.com/oauth/grant-type/passwordless/otp", "client_id": "{yourClientId}", "client_secret": "YOUR_CLIENT_SECRET", "username": "USER_PHONE_NUMBER", "otp": "code", "realm": "sms", "audience": "your-api-audience","scope": "openid profile email"}'
Authenticate an Email user 
curl --request POST \
  --url 'https://{yourDomain}/oauth/token' \
  --header 'content-type: application/json' \
  --data '{"grant_type": "http://auth0.com/oauth/grant-type/passwordless/otp", "client_id": "{yourClientId}", "client_secret": "{yourClientSecret}", "username": "{userPhoneNumber}", "otp": "code", "realm": "email", "audience": "your-api-audience", "scope": "openid profile email"}'
Authenticate a user through a magic link When you send a magic link, you don’t need to call an API to authenticate the user. Users will click the link and be redirected to the callback URL.

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Setting the auth0-forwarded-for header for rate-limit purposes

The /passwordless/start endpoint has a rate limit of 50 requests per hour per IP. If you call the API from the server-side, your backend’s IP may easily hit these rate limits. To learn how to address this issue, read the Rate Limiting in Passwordless Endpoints section of Using Passwordless APIs.

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Customize MFA

Customizable MFA with the Resource Owner Password Grant, Embedded, or Refresh Token flows is in Early Access. To learn more, read Product Release Stages. To participate in the early access, contact Auth0 Support.
Customize with embedded flows. Use the MFA API to allow users to enroll and challenge with factors of their choice that are supported by your application. When using Lock for Web, the oauth/token endpoint returns the mfa_required error and includes the mfa_token you need to use the MFA API and mfa_requirements parameter with a list of authenticators your application currently supports: 
{
  "error": "mfa_required",
  "error_description": "Multifactor authentication required",
  "mfa_token": "Fe26...Ha",
  "mfa_requirements": {
    "challenge": [
      { "type": "otp" },
      { "type": "push-notification" },
      { "type": "phone" },
      { "type": "recovery-code" }
      { "type": "email"} //can only work with challenge
    ]
  }
}
Use the mfa_token to call the mfa/authenticator endpoint to list all factors the user has enrolled and match the same type your application supports. You also need to obtain the matching authenticator_type to issue challenges: 
[
  {
    "type": "recovery-code",
    "id": "recovery-code|dev_qpOkGUOxBpw6R16t",
    "authenticator_type": "recovery-code",
    "active": true
  },
  {
    "type": "otp",
    "id": "totp|dev_6NWz8awwC8brh2dN",
    "authenticator_type": "otp",
    "active": true
  }
]
Proceed to enforce the MFA challenge by calling the request/mfa/challenge endpoint. Further customize your MFA flow with Auth0 Actions. To learn more, read Actions Triggers: post-challenge - API Object.