Adventures in Machine Learning

Secure Your Flask App: Implementing User Authentication with Flask-Login and SQLAlchemy

Flask User Authentication: How to secure your Flask app with Flask-Login and SQLAlchemy

As a web developer, user authentication is one of the most significant parts of building a web application. It secures the users’ data and prevents unauthorized access to the application.

Flask is an open-source web framework written in Python, which is widely used for building web applications. In this article, we will discuss how to implement user authentication in a Flask app with the Flask-Login library and the SQLAlchemy library.

Cookie-based Authentication

Flask-Login is the most popular user authentication library in Flask. It provides cookie-based authentication, which is a preferred way of authentication over token-based authentication in most cases.

Token-based authentication can be vulnerable to several types of attacks, including XSS and CSRF attacks. To implement cookie-based authentication in a Flask-Login app, we will need to install Flask-Login and SQLAlchemy libraries.

Flask-Login is responsible for authenticating and authorizing users, while SQLAlchemy is responsible for communicating with our database.

Setup a Password Hash

Before creating a user authentication system in our Flask app, we need to create a password hash. A password hash is a secure way of storing passwords in our database.

The reason for hashing passwords is to prevent an attacker from gaining access to the users’ passwords in plain text. We will use the library for hashing the passwords.

We can create a password hash using the generate_password_hash method, and we can verify the passwords using the check_password_hash method.

Adding Hashed Passwords to Your Database

Next, we need to store the hashed passwords in our database. Flask-SQLAlchemy is a library that supports SQLAlchemy to work with Flask.

We will use Flask-SQLAlchemy to create a user model and add it to our database. We should use SQLite for the database in this example as it is lightweight and easy to deploy.

However, we can use any database system we prefer by changing the SQLALCHEMY_DATABASE_URI.

Setting the Flask-login Extension

The Flask-login extension requires that we define a user_loader function in our Flask app. This function should accept a user id and return the corresponding user object.

The user object should be an instance of the user class that we defined earlier.

Complete Code

After adding the hashed passwords to our database with our user model created, we can complete the code. Flask_login and UserMixin should be imported for Flask-SQLAlchemy and our User model.

Coding our main Flask application file

Now that we’ve implemented the authentication system on another file, we can integrate it into our main Flask app file. Here is how:

Linking Database to Our Flask File

We need to link our database to our Flask app file. We should create a SQLAlchemy instance that we will use to connect to our database.

Adding User Authentication to our App

Once our database is connected to our app, we can add user authentication to our app. We need to use the login_required decorator provided by Flask-Login to validate that a user is logged in before they can access certain views.

Coding a Simple View

We can create a simple view that only displays a message to verify that the user is indeed logged in. Here, we will use the current_user variable provided by Flask-Login to determine whether a user is logged in.

Coding the Log-in View

Now we will be building a login view that allows the user to log in to the app. Our login view should display the login form and handle the POST request that comes with the form submission.

Coding the Register View

The register view will allow the user to create a new account and persist their information to our database. We will use Flask’s request object to get the values of the submitted form.

Coding the Logout View

Finally, we will create the logout view, which logs out the user and redirects them to the login page.


In this article, we have learned how to implement user authentication in a Flask app with Flask-Login and SQLAlchemy. We have covered adding hashed passwords to our database, setting up Flask-Login, and integrating our authentication system into our main Flask app file.

By following the steps outlined in this article, we can secure our Flask app and ensure that it is only accessible to authorized users. Implementation of the Flask User Authentication Application: Testing the App

After implementing the Flask user authentication application, it is crucial to test it to ensure functionality.

Testing the app includes making sure that endpoints work as expected and that we are following best security practices.


We can use a simple blog application as an example to test our application. Our blog application would allow users to create new posts, view posts, and delete their posts if they wish to.

Users should only be able to view and delete posts created by them.


The main endpoint for our blog application will be ‘/posts.’ This endpoint should serve the user’s posts in a list view. To limit the user’s access to only their posts, we will use Flask-Login’s current_user to filter the posts to only include the ones created by the logged-in user.

Security Warning

While testing the app, we may come across security warnings, such as SSL verification warnings. These warnings may arise because we are still developing and testing the app on a development server.

Once we are ready to deploy the app, we should use a production server and secure it with SSL certificates. Before deploying our app, we can use tools like Qualys SSL Labs to check the SSL configuration and verify that it meets security best practices.

The tool checks our server for the TLS protocol version, cipher suites, security headers, and more. This testing tool helps us ensure that our app’s SSL configuration is up-to-date and secure.

In conclusion, testing our Flask user authentication application is crucial to ensure that it is functioning correctly. By developing and testing a simple blog application, we can ensure that users can create, view, and delete their posts without accessing posts created by other users.

We can also use SSL certificate verification tools to ensure that our server meets security best practices before deployment. With proper testing and security measures, we can create a secure and reliable Flask application.

In this article, we discussed the importance of implementing user authentication in a Flask app and how to do it using Flask-Login and SQLAlchemy. We reviewed the process of creating hashed passwords, adding them to a database, and linking the database to our Flask app file.

We also explored how to create views for login, registration, and logout, as well as testing the app and taking security measures to ensure it meets best practices. The key takeaway is that user authentication is critical for securing web applications and preventing unauthorized access to sensitive information.

With proper techniques and testing, we can create a safe and reliable Flask application for users.

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