How do I trust an application to use my access token? - authentication

Question

So I grant access of my resource to application A, and that application gets that access token. It is just a string. Sometimes the application will pass that access token to its backend server, and then use it to retrieve my resource. Usually the access token is valid for say one month.
Now my question is, what if application A leaks my access token, so any application B can use that access token to get my resource, which is not what I want. I only want application A can access my resource, not application B.
How do we trust application and give it my access token?

Answer 1

YOUR PROBLEM
Usually the access token is valid for say one month.
Using 1 month to the access token expiration time is way to long, not matter if is for admin or normal users.
Depending on your use case I recommend you to use the access tokens with an expiration time in the range of minute, lesser time you can have better will be for security.
So I would recommend you to switch to use refresh tokens, that will keep the access tokens short lived while refresh tokens can be long lived, but in the hours range, not days, weeks or years.
Refresh Token flow example:
Sourced from: Mobile API Security Techniques - part 2
NOTE: While the above graphic belongs to a series of articles written in the context of mobile APIs, they have a lot of information that is also valid for APIs serving web apps and third party clients.
By using the refresh tokens approach, when a client request fails to validate the short lived access token will mean that the client needs to request a new one by sending a refresh token in order to get a new short lived access token.
The important bit here is that the refresh token should not be sent to the browser, only the access token can be sent, therefore your third party clients must kept refresh tokens private, aka in their backends, therefore they MUST NOT send refresh tokens from javascript, instead any renewal of the short lived acess tokens MUST BE delegated to their backends.
Now my question is, what if application A leaks my access token, so any application B can use that access token to get my resource, which is not what I want.
That's why you should use the Refresh Tokens approach I mentioned earlier, because you limit their access to the amount of time remaining in the access token, and this is why I said that the short lived access tokens should be in the range of minutes.
I only want application A can access my resource, not application B.
I am gonna tell you a cruel truth... this is not doable at 100%, specially for web apps, where you can just hit F12 to access the developer tools console and search for the access token, or if you prefer to right click on the page and select view source.
Mobile apps seem to be more secure at a first glance, because they are shipped as a binary, thus you would expect to be hard to reverse engineer it? Wrong, it's indeed easy with the plethora of open source tools that we can use to reverse engineer them, and my preference goes to the MobSF:
Mobile Security Framework (MobSF) is an automated, all-in-one mobile application (Android/iOS/Windows) pen-testing, malware analysis and security assessment framework capable of performing static and dynamic analysis.
So if you cannot find the access tokens via static analysis, then you can resort to dynamic analysis with open source tools to, like Frida:
Inject your own scripts into black box processes. Hook any function, spy on crypto APIs or trace private application code, no source code needed. Edit, hit save, and instantly see the results. All without compilation steps or program restarts.
And if that's is not enough you also perform a Man in the Middle(MitM) Attack wit another open source tools, like mitmproxy:
An interactive TLS-capable intercepting HTTP proxy for penetration testers and software developers.
So, stealing your access token for a mobile app is not as easy as in the Web App, but it's not that hard to.
POSSIBLE SOLUTIONS
How do we trust application and give it my access token?
I don't know if your application is a web or mobile app, therefore I will show you possible solutions for both.
Before I mention the possible solutions, I need to first clear out a usual misconception among developers, regarding who vs what is accessing the backend. This is discussed in detail in this article, where we can read:
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.
If you still have doubts please go and read the section of the linked article, that also includes a graphic to help with understanding this. The article is in the context of a mobile app, but for understanding the difference between what and who is accessing the backend, the references to mobile app can be replaced with web app.
For Web Apps
If your use case is a web app the most affordable solution to try to mitigate the who vs what is accessing your backend it's by using:
Google Recaptcha V3:
reCAPTCHA is a free service that protects your site from spam and abuse. It uses advanced risk analysis techniques to tell humans and bots apart.
This is uses User Behaviour Analytics(UBA) in a best effort basis to tell appart who and what is accessing your backend.
User behavior analytics (UBA) as defined by Gartner is a cybersecurity process about detection of insider threats, targeted attacks, and financial fraud. UBA solutions look at patterns of human behavior, and then apply algorithms and statistical analysis to detect meaningful anomalies from those patterns—anomalies that indicate potential threats.[1] Instead of tracking devices or security events, UBA tracks a system's users.
This is prone to false positives, therefore you need to be careful when deciding to accept or not the request based on the score returned by reCPATCHA V3 for each request:
reCAPTCHA v3 returns a score for each request without user friction. The score is based on interactions with your site and enables you to take an appropriate action for your site.
For Mobile Apps
As you saw by the plethora of tools available to reverse engineer the mobile apps, statically or dynamically, the access token to identify your user is not that safe, plus this only identifies the who in the request, not what is doing it.
The solution that can let your backend to be sure that the request is indeed from the same exact mobile app that was uploaded to the Google Play or Apple store is a Mobile App Attestation solution, that is a concept that introduces a new way of dealing with security for your mobile app and backend in an unified manner.
The usual approaches focus to much on the mobile app side, but in first place the data you want to protect is in your backend server, and it's here that you want to have a way to know that what is making the request is really the thinh you expect to be, your genuine mobile app.
The Mobile App Attestation concept is described in this section of another article I wrote, from where I will quote the following text:
The role of a Mobile App Attestation service is to authenticate what is sending the requests, thus only responding to requests coming from genuine mobile app instances and rejecting all other requests from unauthorized sources.
In order to know what is sending the requests to the API server, a Mobile App Attestation service, at run-time, will identify with high confidence that your mobile app is present, has not been tampered/repackaged, is not running in a rooted device, has not been hooked into by an instrumentation framework (Frida, xPosed, Cydia, etc.) and is not the object of a Man in the Middle Attack (MitM). This is achieved by running an SDK in the background that will communicate with a service running in the cloud to attest the integrity of the mobile app and device it is running on.
On a successful attestation of the mobile app integrity, a short time lived JWT token is issued and signed with a secret that only the API server and the Mobile App Attestation service in the cloud know. In the case that attestation fails the JWT token is signed with an incorrect secret. Since the secret used by the Mobile App Attestation service is not known by the mobile app, it is not possible to reverse engineer it at run-time even when the app has been tampered with, is running in a rooted device or communicating over a connection that is the target of a MitM attack.
The mobile app must send the JWT token in the header of every API request. This allows the API server to only serve requests when it can verify that the JWT token was signed with the shared secret and that it has not expired. All other requests will be refused. In other words a valid JWT token tells the API server that what is making the request is the genuine mobile app uploaded to the Google or Apple store, while an invalid or missing JWT token means that what is making the request is not authorized to do so, because it may be a bot, a repackaged app or an attacker making a MitM attack.
So this approach will let your backend server to trust with a very high degree of confidence that the request is coming indeed from the same exact mobile app you uploaded to the Google Play and Apple store, provided the JWT token has a valid signature and expire time, and discard all other requests as untrustworthy ones.
GOING THE EXTRA MILE
To finish my answer I cannot resist to recommend you the excellent work of the OWASP foundation, because off their excellent work and for me no security solution for web and mobile is complete without going through their guides:
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.
The Mobile Security Testing Guide:
The Mobile Security Testing Guide (MSTG) is a comprehensive manual for mobile app security development, testing and reverse engineering.