Prevent untrusted clients to use login/register endpoints of REST API

Question

I have actually one SPA in ReactJs + one mobile application in Flutter + one REST API developed with SailsJs running on a separate server. I managed user authentication with a secured session cookie sent back by the API when we are login with valid information (id/password).

So all the endpoints that require users to be authenticated are protected (unless there are others security best practices that I'm not aware of?). The session cookie expiration and validity are checked with each call to one of the protected endpoints.

I really read a massive amount of topics and blog posts talking about securing REST API. And my problem is never or barely represented. So now my main problem is :

How can I restrict my public API endpoints (login & register currently) that does not require users to be authenticated (since there are the endpoints used to achieve this mission...) to be used only in my trusted client apps (web and mobile)?

How can I prevent another app developed by another person to use these endpoints?

I don't want anyone to login via my API unless it is done in the client apps I am developing... I don't want anyone to replicate my applications and successfully use my API that way with 0 protection, without knowing it...

I see a lot of popular services with login API routes (Heroku for example) that can't be accessed in Postman with the same parameters (403 error code). So it is possible. But how they do that? There is nothing in specialized forums that handle this or I missed something!

I tough of a secret token stored in the client to authenticate it but it is literally public with web developer tools for example.

Need some advice.

Thanks

USER AUTHENTICATION IS NOT APP AUTHENTICATION

So all the endpoints that require users to be authenticated are protected...

This endpoints are only protected regarding to identify, authenticate and authorize Who his in the request, but not for What is doing the request, and this is a topic not very well understood among developers, be them juniors or seniors.

The Difference Between WHO and WHAT is Accessing the API Server

In an article I wrote, entitled Why Does Your Mobile App Need An Api Key? you can read with more detail the difference between Who and What is accessing your API server, from where I quote the following:

The what is the thing making the request to the API server. Is it really a genuine instance of your mobile app, or is it a bot, an automated script or an attacker manually poking around your API server with a tool like Postman?

The who is the user of the mobile app that we can authenticate, authorize and identify in several ways, like using OpenID Connect or OAUTH2 flows.

So the Who is the user of your API server that you will be able to Authenticate and Authorize access to the data, and the What is the software making that request in behalf of the user, your genuine app, a tampered one, an automated script or someone manually poking around with your API via cURL, Postman or similar tools.

By now I hope that you have enough knowledge to understand why user(who) authentication is not the same as app(what) authentication.

LOCK THE API SERVER TO THE APPS

How can I restrict my public API endpoints (login & register currently) that does not require users to be authenticated (since there are the endpoints used to achieve this mission...) to be used only in my trusted client apps (web and mobile)?

I think that by now it may be clear to you that it's not only the login and registration endpoints that need to be protected from What is doing the request.

How can I prevent another app developed by another person to use these endpoints? I don't want anyone to login via my API unless it is done in the client apps I am developing... I don't want anyone to replicate my applications and successfully use my API that way with 0 protection, without knowing it...

This is extremely hard to achieve for web apps, but possible with an high degree of confidence for mobile apps when the Mobile App Attestation concept is implemented.

For web apps

Due to the nature of how the web was built, all it's necessary to inspect a web app is to hit F12 or inspect the page source, and then search for whatever you need to access the API server from another tool.

You can learn some useful techniques to help your API server to try to respond only to requests coming from What you expect, your genuine web app, and to do so I invite you to read my answer to the question Secure api data from calls out of the app, specially the section dedicated to Defending the API Server.

For mobile apps

To learn how you can lock your API server to your mobile app I recommend you to read my answer to the question How to secure an API REST for mobile app? for the sections on Securing the API Server and A Possible Better Solution.

Endpoints to Secure

So all the endpoints that require users to be authenticated are protected (unless there are others security best practices that I'm not aware of?).

It's up to you if you only want to enhance the security of your login and register endpoints, but my advice is that you enhance the security of all them regarding the detection for What is accessing them.

POSTMAN WITH HEROKU AND OTHERS

I see a lot of popular services with login API routes (Heroku for example) that can't be accessed in Postman with the same parameters (403 error code). So it is possible. But how they do that? There is nothing in specialized forums that handle this or I missed something!

I never used Heroku, but when I am using an API that doesn't work in Postman, but works in other clients, let's say from cURL, then I disable Postman from sending it's own user-agent and normally the API will start accepting the requests.

If doesn't then they may be doing device fingerprinting:

A device fingerprint or machine fingerprint is information collected about the software and hardware of a remote computing device for the purpose of identification. The information is usually assimilated into a brief identifier using a fingerprinting algorithm. A browser fingerprint is information collected specifically by interaction with the web browser of the device.

The fingerprinting can be done in active or passive mode. In active mode some Javascript runs on the client to collect some data to send back to the API server, while in passive mode it uses the information available from the request in the server, like the http headers and request parameters.

While this raises the bar to fake What is doing the request, it can be bypassed by observing how a trusted client sends the request and mimic it. For an attacker it's just a little more work to enumerate all variants and then automate them.

DO YOU WANT TO GO THE EXTRA MILE?

I really read a massive amount of topics and blog posts talking about securing REST API.

First and foremost my congratulations for putting such effort in educating yourself about securing your API.

I don't know if you already read some of the OWASP resources I am about to link, but in any response to a security question I always like to reference the excellent work from the OWASP foundation ;)

For Web Apps

OWASP Web Top 10 Risks

The OWASP Top 10 is a powerful awareness document for web application security. It represents a broad consensus about the most critical security risks to web applications. Project members include a variety of security experts from around the world who have shared their expertise to produce this list.

The Web Security Testing Guide:

The OWASP Web Security Testing Guide includes a "best practice" penetration testing framework which users can implement in their own organizations and a "low level" penetration testing guide that describes techniques for testing most common web application and web service security issues.

For Mobile Apps

OWASP Mobile Security Project - Top 10 risks

The OWASP Mobile Security Project is a centralized resource intended to give developers and security teams the resources they need to build and maintain secure mobile applications. Through the project, our goal is to classify mobile security risks and provide developmental controls to reduce their impact or likelihood of exploitation.

OWASP - Mobile Security Testing Guide:

The Mobile Security Testing Guide (MSTG) is a comprehensive manual for mobile app security development, testing and reverse engineering.

For APIS

OWASP API Security Top 10

The OWASP API Security Project seeks to provide value to software developers and security assessors by underscoring the potential risks in insecure APIs, and illustrating how these risks may be mitigated. In order to facilitate this goal, the OWASP API Security Project will create and maintain a Top 10 API Security Risks document, as well as a documentation portal for best practices when creating or assessing APIs.

Comments

[codebrane]

have a look at JWT

[Mister_Ideas]

please develop? one JWT per request + the session cookie JWT already here? how can i achieve that?

[Rajneesh Chaurasia]

You can use CORS in your server to enable only your trusted clients can request to your api.

[Mister_Ideas]

Thanks a lot for your time, probably best answer I saw on SO... Will be back if I have other questions after reading all your links.