The Login trigger executes when a user successfully authenticates against an application on an Auth0 tenant. This also includes post-signup authentication.
Diagram showing the Actions Login Flow.
Actions in this trigger are blocking (synchronous), which means they execute as part of a trigger’s process and prevent the rest of the Auth0 pipeline from running until the Action is complete.

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Triggers

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Login / Post Login

The post-login trigger is a function executed after a user logs in and when a is requested.

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References

  • Event object: Provides contextual information about a single user logging in via Auth0.
  • API object: Provides methods for changing the behavior of the flow.

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Common use cases

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Access control

A post-login Action can be used to provide custom logic used to deny access to a user who is attempting to access an application: 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  if (event.user.email && event.user.email.endsWith("@example.com") && event.client.name === "My SPA") {
    api.access.deny(`Access to ${event.client.name} is not allowed.`);
  }
};

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Allow access only on weekdays for a specific application

If you have an application that you want to make sure is only accessible during weekdays, you can create the following Action: 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
 exports.onExecutePostLogin = async (event, api) => {
  if (event.client.name === "APP_NAME") {
    const d = new Date().getDay();

    if (d === 0 || d === 6) {
      api.access.deny("This app is only available during the week.");
    }
  }
}

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Deny access to anyone calling an API

Let’s say you want to deny access to all users who are calling an API. This means that you need to deny access depending on the  value for your API, which you can find in the API Audience field of your API in Dashboard > Applications > APIs. To do this, you would create the following Action: 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
 exports.onExecutePostLogin = async (event, api) => {
  // In Actions, an API will be referred to as a Resource Server.
  if (event.resource_server && event.resource_server.identifier === "http://todoapi2.api") {
    api.access.deny("end_users_not_allowed");
  }
}

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Add user roles to ID and Access tokens

To add user roles to Auth0-issued tokens, use the event.authorization object along with the api.idToken.setCustomClaim and api.accessToken.setCustomClaim methods: 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  const namespace = 'https://my-app.example.com';
  if (event.authorization) {
    api.idToken.setCustomClaim(`${namespace}/roles`, event.authorization.roles);
    api.accessToken.setCustomClaim(`${namespace}/roles`, event.authorization.roles);
  }
}
  • A custom claim cannot include certain terms, and we strongly recommend using namespaced claim that takes the form of a URI. See our documentation on custom claims for more information.
  • The returned to the requesting application is built and signed at the end of the trigger processing. The final, signed JWT is not accessible in an Action.

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Enrich the user profile

Auth0 provides a system for storing metadata on a User Profile. In order to set user_metadata or app_metadata on a user’s profile during their login, use the api.user.setUserMetadata or api.user.setAppMetadata functions. 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  api.user.setUserMetadata("favorite_color", "blue");
};
Once all post-login actions have been executed, Actions will update the user profile in a single operation. This operation is subject to the “Write Users” rate limit.

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Enforce custom MFA policy

A post-login Action can be used to dynamically require for a user according to your application’s needs. 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  // Require MFA for anyone logging in from North America.
  if (event.request.geoip.continentCode === "NA") {
    api.multifactor.enable("any");
  };
};
An MFA Provider must be configured in order to enable MFA during a login. To learn more, read Multi-Factor Authentication.

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Reduce friction with passkeys

A post-login Action can be used to reduce the friction by dynamically skipping MFA for a user that has authenticated with a passkey. 
/**
* Handler that will be called during the execution of a PostLogin flow.
*
* @param {Event} event - Details about the user and the context in which they are logging in.
* @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
*/
exports.onExecutePostLogin = async (event, api) => {
 // Check if a passkey was used to authenticate
 const skipMFA = event.authentication?.methods.some(
   (method) => method.name === "passkey"
 );

 // If a passkey was used skip MFA
 if (skipMFA) {
   api.multifactor.enable("none");
 }
};
The connection must have passkeys and MFA enabled. To learn more, read Passkeys and Multi-Factor Authentication.

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Redirect the user to an external site

Similar to Redirect Rules, a post-login Action can be used to send the user to an external site. When completed, the user can be redirected back to Auth0 to continue their login flow. In the example below, a Redirect Action is used to prompt the user to provide their favorite color. 
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  // Skip the redirect if the user has already chosen a favorite color.
  if (event.user.user_metadata.favorite_color) {
    return;
  }

  const token = api.redirect.encodeToken({
    secret: event.secrets.MY_SHARED_SECRET,
    payload: {
      email: event.user.email,
    },
  });

  // Send the user to https://my-app.example.com along
  // with a `session_token` query string param.
  api.redirect.sendUserTo("https://my-app.example.com", { 
    query: { session_token: token }
  });
};

/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onContinuePostLogin = async (event, api) => {
  // Once the /continue endpoint has been called, unpack the signed token
  // and store the favorite color as user metadata.

  const payload = api.redirect.validateToken({
    secret: event.secrets.MY_SHARED_SECRET,
  });

  api.user.setUserMetadata("favorite_color", payload.favorite_color);
};
The Actions pipeline will be suspended while the user is redirected. Once the user continues their Auth0 login process, the Actions pipeline will resume where it was suspended. Actions that were executed prior to the Redirect will not be executed again. To learn more about Redirect Actions, read Redirect with Actions.

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Modify scopes on an access token

When modifying the scopes associated with an , ensure you adhere to the best practices surrounding audience specification.
  • Always check for expected audience before adding scopes.
  • Prevent using untrusted input when adding scopes.
/**
 * @param {Event} event - Details about the user and the context in which they are logging in.
 * @param {PostLoginAPI} api - Interface whose methods can be used to change the behavior of the login.
 */
exports.onExecutePostLogin = async (event, api) => {
  if (event.request.query.audience === 'https://example.com/api') {
    api.accessToken.addScope("read:xyz");
  }
};

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Deny access to specific JA3/JA4 fingerprints

The event.security_context object contains the JA3/JA4 fingerprint values for the current transaction. 
exports.onExecutePreUserRegistration = async (event, api) => {
  const clientJa4 = event?.security_context?.ja4;
  console.log('[ACTION]', {clientJa4});
  const badFingerprints = ['t13d1517h2_8daaf6152771_b6f405a00624','t13d1516h2_8daaf6152771_d8a2da3f94cd'];
  if (clientJa4 && badFingerprints.includes(clientJa4)){
    api.access.deny('suspicious_tls_fingerprint', 'Your TLS fingerprint has been flagged as suspicious');
  }
};

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Learn more