mix phx.gen.auth

The mix phx.gen.auth command generates a flexible, pre-built authentication system into your Phoenix app. This simple generator allows you to quickly move past the task of adding authentication to your codebase and stay focused on the real-world problem your application is trying to solve.

Getting started

Let's start by running the following command from the root of our app (or apps/my_app_web in an umbrella app):

$ mix phx.gen.auth Accounts User users

This creates an Accounts context with an Accounts.User schema module. The final argument is the plural version of the schema module which is used for generating database table names and route helpers. The mix phx.gen.auth generator is similar to mix phx.gen.html except it does not accept a list of additional fields to add to the schema and it generates many more context functions.

Since this generator installed additional dependencies in mix.exs, let's fetch those:

$ mix deps.get

Now we need to verify the database connection details for the development and test environments in config/ so the migrator and tests can run properly. Then run the following to create the database:

$ mix ecto.setup

Let's run the tests to make sure our new authentication system works as expected.

$ mix test

And finally, let's start our Phoenix server and try it out.

$ mix phx.server

Developer responsibilities

Since Phoenix generates this code into your application instead of building these modules into Phoenix itself, you now have complete freedom to modify the authentication system so it works best with your use case. The one caveat with using a generated authentication system is it will not be updated after it's been generated. Therefore as improvements are made to the output of mix phx.gen.auth, it becomes your responsibility to determine if these changes need to be ported into your application. Security-related and other important improvements will be explicitly and clearly marked in the CHANGELOG.md file and upgrade notes.

Generated code

The following are notes about the generated authentication system.

Password hashing

The password hashing mechanism defaults to bcrypt for Unix systems and pbkdf2 for Windows systems. Both systems use the Comeonin interface.

Forbidding access

The generated code ships with an authentication module with a handful of plugs that fetch the current user, require authentication and so on. For instance, in an app named Demo which had mix phx.gen.auth Accounts User users run on it, you will find a module named DemoWeb.UserAuth with plugs such as:

  • fetch_current_user - fetches the current user information if available
  • require_authenticated_user - must be invoked after fetch_current_user and requires that a current user exists and is authenticated
  • redirect_if_user_is_authenticated - used for the few pages that must not be available to authenticated users


The generated functionality ships with an account confirmation mechanism, where users have to confirm their account, typically by email. However, the generated code does not forbid users from using the application if their accounts have not yet been confirmed. You can add this functionality by customizing the require_authenticated_user in the Auth module to check for the confirmed_at field (and any other property you desire).


The generated code is not integrated with any system to send SMSs or emails for confirming accounts, resetting passwords, etc. Instead it simply logs a message to the terminal. It is your responsibility to integrate with the proper system after generation.

Tracking sessions

All sessions and tokens are tracked in a separate table. This allows you to track how many sessions are active for each account. You could even expose this information to users if desired.

Note that whenever the password changes (either via reset password or directly), all tokens are deleted and the user has to log in again on all devices.

User Enumeration attacks

A user enumeration attack allows an attacker to enumerate all emails registered in the application. For example, if trying to log in with a registered email and a wrong password returns a different error than trying to log in with an email that was never registered, an attacker could use this discrepency to find out which emails have accounts.

The generated authentication code protects against enumeration attacks on all endpoints, except in the registration and update email forms. If your application is really sensitive to enumeration attacks, you need to implement your own registration workflow, which tends to be very different from the workflow for most applications.

Case sensitiveness

The email lookup is made to be case insensitive. Case insensitive lookups are the default in MySQL and MSSQL. In SQLite3 we use COLLATE NOCASE in the column definition to support it. In PostgreSQL we use the citext extension.

Note citext is part of PostgreSQL itself and is bundled with it in most operating systems and package managers. mix phx.gen.auth takes care of creating the extension and no extra work is necessary in the majority of cases. If by any chance your package manager splits citext into a separate package, you will get an error while migrating and you can most likely solve it by installing the postgres-contrib package.

Concurrent tests

The generated tests run concurrently if you are using a database that supports concurrent tests, which is the case of PostgreSQL.

Additional resources

The following links have more information regarding the motivation and design of the code this generates.