For the last few months i've been working on a Rest API for a web app for the company I work for. The endpoints supply data such as transaction history, user data, and data for support tickets. However, I keep running into one issue that always seems to set me back to some extent.
The issue I seem to keep running into is how do I handle user authentication for the Rest API securely? All data is going to be sent over a SSL connection, but there's a part of me that's paranoid about potential security problems that could arise. As it currently stands when a client attempts to login the client must provide a username or email address, and a password to a login endpoint (E.G "/api/login"). Along with with this information, a browser fingerprint must be supplied through header of the request that's sending the login credentials. The API then validates whether or not the specified user exists, checks whether or not the password supplied is correct, and stores the fingerprint in a database model. To access any other endpoints in the API a valid token from logging in, and a valid browser fingerprint are required.
I've been using browser fingerprints as a means to prevent token-hijacking, and as a way make sure that the same device used to login is being used to make the requests. However, I have noticed a scenario where this practice backfires on me. The client-side library i'm using to generate browser fingerprints isn't always accurate. Sometimes the library spits out a different fingerprint entirely. Which causes some client requests to fail as the different fingerprint isn't recognized by the API as being valid. I would like to keep track of what devices are used to make requests to the API. Is there a more consistent way of doing so, while still protecting tokens from being hijacked?
When thinking of the previous question, there is another one that also comes to mind. How do I store auth tokens on client-side securely, or in a way that makes it difficult for someone to obtain the tokens through malicious means such as a xss-attack? I understand setting a strict Content-Security Policy on browser based clients can be effective in defending against xss-attacks. However, I still get paranoid about storing tokens as cookies or in local storage.
I understand oauth2 is usually a good solution to user authentication, and I have considered using it before to deal with this problem. Although, i'm writing the API using Flask, and i'm also using JSON Web tokens. As it currently stands, Flask's implementation of oauth2 has no way to use JWTs as access tokens when using oauth for authentication.
This is my first large-scale project where I have had to deal with this issue and i am not sure what to do. Any help, advice, or critiques are appreciated. I'm in need of the help right now.
Put  an API Gateway in front of your API , your API Gateway is publicly ( i.e in the DMZ ) exposed while the actual API are internal.
You can look into Kong..
How to handle Rest API user authentication securely?