How to secure an API REST for mobile app? (if sniffing requests gives you the "key")

Question

It's probable a newbie question but I'll try to create an interesting debate.

I know there are some authentication methods for API Basic Authentication, API Keys, OAuth 2.0 ... all of those methods add a header or a formData param in the request.

Although you use SSL, it's "usually easy" to hack mobile apps (I'm thinking in Android right now: decompiling the app, changing manifest to allow custom SSL, compiling again and sniffing through a SSL proxy all the requests).

In those requests I've found a lot of auth keys that I can use in other calls from a console, simulating the app with no problems.

So, now I've hacked some APIs in mobile apps, my question is: is there any way to secure an API in a mobile app?

I wonder if one securitization layer would be to limit the number of requests per "key".

Am I wrong? Am I missing something? Is this a stupid question?

Answer 1

Am I wrong? Is this a stupid question?

No you are not wrong, and it's not a stupid question at all, because it's indeed easy to attack an API server of a mobile app, and you will be surprised to know how many senior developers are not aware how easy it can be done, and I have noticed that more often then not, this is due to their misconception about what vs who is accessing the API server.

The difference between what and who is accessing the API server.

This is discussed in more detail in this article I wrote, 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.

So if the quoted text is not enough to clarify you, then please go ahead and read the entire section of the article.

Impersonating the Mobile APP

In those request I've found a lot of auth keys that I can use in other calls from a console, simulating the app with no problems.

If by auth keys you mean the ones provide via user sign-in, with his username and password, then they just identify the who in the request.

For other keys, like api-keys, access-tokens, or any other convention used to name them, they have the purpose of providing to the API server a mechanism to only authorize requests from the genuine mobile app, they are indeed trying to allow the API server to identify what is doing the request, and has you already discovered it's easy to extract them with a proxy:

Although you use SSL, it's "usually easy" to hack mobile apps (I'm thinking in Android right now: decompiling the app, changing manifest to allow custom SSL, compiling again and sniffing through a SSL proxy all the requests).

So, in the end of the day all an attacker needs it's to use a proxy to learn how the API server works, and what is needed to simulate an API call as if it was done from the mobile app itself.

Hardening and Shielding the Mobile App

So, now I've hacked some API's in mobile apps, my question is: is there any way to secure an API in a mobile app?

You can use Mobile Hardening and Shielding solutions, that will try to prevent the mobile app to work in compromised/rooted devices, with modified/tampered apps and/or when some instrumentation framework is being used during runtime, but all of them have the draw-back of performing all that decisions in the mobile app, therefore subject to be manipulated or totally bypassed by the alreay dito instrumentation frameworks, and a good example of one it's 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.

While is better to use an in app solution, than not using anything, it's still not the ideal solution, because the control of deciding what to do is in the client side, not in the server side, thus the attacker can resort to use Frida to introspect the code at runtime and learn how to impersonate the mobile app.

Securing the API Server

The Basic API Security Defenses

Now that you understand the difference between who vs what is accessing your API server and you know that an attacker can learn how to impersonate your genuine mobile app you may want to go an read my article about the basic techniques to secure an API:

In this article we will explore the most common techniques used to protect an API, including how important it is to use HTTPS to protect the communication channel between mobile app and API, how API keys are used to identify the mobile app on each API request, how user agents, captchas and IP addresses are used for bot mitigation, and finally how user authentication is important for the mobile security and api security. We will discuss each of these techniques and discuss how they impact the business risk profile, i.e. how easy they are get around.

This is only a very basic technique that the majority of APIs may already employ, but they can be reinforced with some more advanced techniques.

More Advanced API Security Defenses

You can start to read this series of articles on Mobile API Security Techniques to understand how API keys, HMAC, OAUTH and certificate pinning can be used to enhance the security and at same time learn how they can be abused/defeated.

Afterwards, and depending on your budget and resources you may employ an array of different approaches and techniques to defend your API server, and I will start to enumerate some of the most usual ones.

You can start 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.

Regarding APIs serving mobile apps a positive identification model can be used by implementing a Mobile App Attestation solution that attests the integrity of your mobile app and device its running on before any request is made to the API server.

A Possible Better Solution

The current implementations of a mobile app and API server may look like this:

This approach is what leaves the API keys vulnerable to be extracted by attackers via proxy interception(red line), just like you already noticed by using a proxy to intercept them.

A better approach would be something like this:

Wait, but I don't see any API key in the mobile app anymore:

Am I missing something ?

Yes, a Mobile App Attestation solution.

To be in a position where you don't need to ship any secrets with your mobile app, then you need to resort to the Mobile App Attestation concept, and from this article section I will quote the relevant parts to explain it's role:

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.

A great benefit of using a Mobile App Attestation service is its proactive and positive authentication model, which does not create false positives, and thus does not block legitimate users while it keeps the bad guys at bay.

The Mobile App Attestation releases your mobile app from having an embedded secret in its code, instead now it only needs to pass to the reverse proxy or backend the JWT token it receives from the mobile app attestation service. Now the reverse proxy or backend can verify the JWT token, and on successful validation they can accept requests with a very high confidence that they are originated from what they expect, a true and genuine instance of the mobile app, with the added benefit of not exposing the API keys to access your API server or any Third Party services.

GOING THE EXTRA MILE

I cannot finish without recommending you the excellent work done by the OWASP foundation.

For Mobile Apps

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.