How do I secure a REST-API?

Question

I've set up an API with authentication but I want to only allow certain applications and websites to access it. What do I do?

I've got authentication set up for users that are Logged in only being able to access the API, however, how do I prevent them from just logging in from anywhere?

Answer 1

Before I address your question, I think is important that first we clear a common misconception among developers, regarding WHO and WHAT is accessing an API.

THE DIFFERENCE BETWEEN WHO AND WHAT IS COMMUNICATING WITH YOUR API SERVER

To better understand the differences between the WHO and the WHAT are accessing your mobile app, let’s use this picture:

The Intended Communication Channel represents your mobile being used as you expected, by a legit user without any malicious intentions, using an untampered version of your mobile app, and communicating directly with your API server without being man in the middle attacked.

The actual channel may represent several different scenarios, like a legit user with malicious intentions that may be using a repackaged version of your mobile app, a hacker using the genuine version of you mobile app while man in the middle attacking it to understand how the communication between the mobile app and the API server is being done in order to be able to automate attacks against your API. Many other scenarios are possible, but we will not enumerate each one here.

I hope that by now you may already have a clue why the WHO and the WHAT are not the same, but if not it will become clear in a moment.

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.

OAUTH

Generally, OAuth provides to clients a "secure delegated access" to server resources on behalf of a resource owner. It specifies a process for resource owners to authorize third-party access to their server resources without sharing their credentials. Designed specifically to work with Hypertext Transfer Protocol (HTTP), OAuth essentially allows access tokens to be issued to third-party clients by an authorization server, with the approval of the resource owner. The third party then uses the access token to access the protected resources hosted by the resource server.

OpenID Connect

OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It allows Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server, as well as to obtain basic profile information about the End-User in an interoperable and REST-like manner.

While user authentication may let your API server know WHO is using the API, it cannot guarantee that the requests have originated from WHAT you expect, your mobile app.

Now we need a way to identify WHAT is calling your API server, and here things become more tricky than most developers may think. The WHAT is the thing making the request to the API server. Is it really a genuine instance of your mobile app, or is a bot, an automated script or an attacker manually poking around your API server with a tool like Postman?

For your surprise you may end up discovering that It can be one of your legit users using a repackaged version of your mobile app or an automated script trying to gamify and take advantage of your service.

Well, to identify the WHAT, developers tend to resort to an API key that usually they hard-code in the code of their mobile app. Some developers go the extra mile and compute the key at run-time in the mobile app, thus it becomes a runtime secret as opposed to the former approach when a static secret is embedded in the code.

The above write-up was extracted from an article I wrote, entitled WHY DOES YOUR MOBILE APP NEED AN API KEY?, and that you can read in full here, that is the first article in a series of articles about API keys.

YOUR QUESTIONS

I've got authentication set up for users that are Logged in only being able to access the API, however, how do I prevent them from just logging in from anywhere?

If by logging in from anywhere you mean any physical location, then you can use blocking by IP address as already suggested by @hanshenrik, but if you mean blocking from logging from other applications, that are not the ones you have issued the API keys for, then you have a very hard problem in your hands to solve, that leads to your first question:

I've set up an API with authentication but I want to only allow certain applications and websites to access it. What do I do?

This will depend if WHAT is accessing the API is a web or a mobile application.

Web application

In a web app we only need to inspect the source code with the browser dev tools or by right click on view page source and search for the API key, and then use it in any tool, like Postman or in any kind of automation we want, just by replicating the calls as we saw them being made in the network tab of the browser.

For an API serving a web app you can employ several layers of dense, starting with reCaptcha V3, followed by Web Application Firewall(WAF) and finally if you can afford it a User Behavior Analytics(UBA) solution.

Google reCAPTCHA V3:

reCAPTCHA is a free service that protects your website from spam and abuse. reCAPTCHA uses an advanced risk analysis engine and adaptive challenges to keep automated software from engaging in abusive activities on your site. It does this while letting your valid users pass through with ease.

...helps you detect abusive traffic on your website without any user friction. It returns a score based on the interactions with your website and provides you more flexibility to take appropriate actions.

WAF - Web Application Firewall:

A web application firewall (or WAF) filters, monitors, and blocks HTTP traffic to and from a web application. A WAF is differentiated from a regular firewall in that a WAF is able to filter the content of specific web applications while regular firewalls serve as a safety gate between servers. By inspecting HTTP traffic, it can prevent attacks stemming from web application security flaws, such as SQL injection, cross-site scripting (XSS), file inclusion, and security misconfigurations.

UBA - User Behavior Analytics:

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. Instead of tracking devices or security events, UBA tracks a system's users. Big data platforms like Apache Hadoop are increasing UBA functionality by allowing them to analyze petabytes worth of data to detect insider threats and advanced persistent threats.

All this solutions work based on a negative identification model, by other words they try their best to differentiate the bad from the good by identifying WHAT is bad, not WHAT is good, thus they are prone to false positives, despite of the advanced technology used by some of them, like machine learning and artificial intelligence.

So you may find yourself more often than not in having to relax how you block the access to the API server in order to not affect the good users. This also means that this solutions require constant monitoring to validate that the false positives are not blocking your legit users and that at same time they are properly keeping at bay the unauthorized ones.

Mobile Application

From your reply to a comment:

What about for mobile applications?

Some may think that once a mobile app is released in a binary format that their API key will be safe, but turns out that is not true, and extracting it from a binary is sometimes almost as easy as extracting it from a web application.

Reverse engineering a mobile app is made easy by plethora of open source tools, like the Mobile Security Framework(MobSF), Frida, XPosed, MitmProxy, and many other more, but as you can see in this article, it can be done with MobSF or with the strings utility that is installed in a normal Linux distribution.

Mobile Security Framework

Mobile Security Framework is an automated, all-in-one mobile application (Android/iOS/Windows) pen-testing framework capable of performing static analysis, dynamic analysis, malware analysis and web API testing.

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.

xPosed

Xposed is a framework for modules that can change the behavior of the system and apps without touching any APKs. That's great because it means that modules can work for different versions and even ROMs without any changes (as long as the original code was not changed too much). It's also easy to undo.

MiTM Proxy

An interactive TLS-capable intercepting HTTP proxy for penetration testers and software developers.

Regarding APIs serving mobile apps a positive identification model can be used by using a Mobile App Attestation solution that guarantees to the API server that WHAT is making the requests can be trusted, without the possibility of false positives.

The Mobile App Attestation

The role of a Mobile App Attestation service is to guarantee at run-time that your mobile app was not tampered or is not running in a rooted device by running a SDK in the background that will communicate with a service running in the cloud to attest the integrity of the mobile app and device is running on.

On 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 are aware. In the case of failure on the mobile app attestation the JWT token is signed with a secret that the API server does not know.

Now the App must sent with every API call the JWT token in the headers of the request. This will allow the API server to only serve requests when it can verify the signature and expiration time in the JWT token and refuse them when it fails the verification.

Once the secret used by the Mobile App Attestation service is not known by the mobile app, is not possible to reverse engineer it at run-time even when the App is tampered, running in a rooted device or communicating over a connection that is being the target of a Man in the Middle Attack.

The Mobile App Attestation service already exists as a SAAS solution at Approov(I work here) that provides SDKs for several platforms, including iOS, Android, React Native and others. The integration will also need a small check in the API server code to verify the JWT token issued by the cloud service. This check is necessary for the API server to be able to decide what requests to serve and what ones to deny.

CONCLUSION

In the end the solution to use in order to protect your API server must be chosen in accordance with the value of what you are trying to protect and the legal requirements for that type of data, like the GDPR regulations in Europe.

So using API keys may sound like locking the door of your home and leave the key under the mat, but not using them is liking leaving your car parked with the door closed, but the key in the ignition.