PUBLIC KEY PINNING
I am implementing SSL pinning in our android app. I have pinned 2 certificates (current and backup) at the client by embedding them in the app.
If you pin against the public key you do not need to update your mobile app each time a certificate is rotated in the server, once you will sign it with the same public key, and you can read the article Hands On Mobile APi Security: Pinning Client Connections for more details in how this can be done:
For networking, the Android client uses the OKHttp library. If our digital certificate is signed by a CA recognized by Android, the default trust manager can be used to validate the certificate. To pin the connection it is enough to add the host name and a hash of the certificate’s public key to the client builder(). See this OKHttp recipe for an example. All certificates with the same host name and public key will match the hash, so techniques such as certificate rotation can be employed without requiring client updates. Multiple host name - public key tuples can also be added to the client builder().
For a situation where the private key used to sign the certificate gets compromised you will end-up in the same situation that you are trying to solve now, that is the need to release a new mobile app to update what you trust to pin against. By other words, the public key cannot be trusted anymore, thus the server must rotate the certificate with one signed with the backup public key you have released with your mobile app. This approach will give you time for a new release, that removes the public key used to sign the compromised certificate, without locking out all your users.
You should always store the backup private keys in separated places, so that if one is compromised you don't get all compromised at once, because then having a backup pin being release with the mobile app is useless.
DON'T DO THIS
Now, I want to have a mechanism in place to update these certificates without requiring to roll out an app upgrade in case certificates are expired or private key is compromised. How can I implement that?
Unfortunately the safer method to deal with a compromised private key is to release a new mobile app that doesn't trust on it anymore. Any remote solution you may devise to update the certificates will open the mobile app doors for attackers to replace the certificates you are pinning against.
So my advice is to not go down this road, because you will shoot yourself on the foot more easily than you can think off.
One possible solution I am seeing is through app notification. I can broadcast a notification with new certificates and store them in the client. Is there any problem in this approach or is there any better approach?
While the mobile app have the connection pinned it can be bypassed, thus a MitM attack can be performed and the new certificates retrieved from the attackers server, instead from your server. Please read the article The Problem with Pinning for more insights on bypassing it:
Unpinning works by hooking, or intercepting, function calls in the app as it runs. Once intercepted the hooking framework can alter the values passed to or from the function. When you use an HTTP library to implement pinning, the functions called by the library are well known so people have written modules which specifically hook these checking functions so they always pass regardless of the actual certificates used in the TLS handshake. Similar approaches exist for iOS too.
While certificate pinning can be bypassed is still strongly advised to use it, because security is all about layers of defense, the more you have the more hard it will be to overcome all them... This is nothing new, if you think of medieval castles, they where built with this approach.
A POSSIBLE BETTER APPROACH
But you also asked for a better approach:
Is there any problem in this approach or is there any better approach?
As already mentioned you should pin against the public key of the certificate to avoid lockouts of the client when you rotate the server certificates.
While I cannot point you a better approach to deal with compromised private keys, I can point out to protect the certificate pinning from being bypassed with introspection frameworks, like xPosed or Frida, we can employ the Mobile App Attestation technique, that will attest the authenticity of the mobile app.
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.
Before we dive into the Mobile App Attestation technique, I would like to clear first a usual misconception among developers, regarding the WHO and the WHAT is calling the API server.
The Difference Between WHO and WHAT is Accessing the API Server
To better understand the differences between the WHO and the WHAT are accessing an API server, let’s use this picture:
![Man in the Middle Attack]()
The Intended Communication Channel represents the mobile app being used as you expected, by a legit user without any malicious intentions, using an untampered version of the mobile app, and communicating directly with the 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 the mobile app, a hacker using the genuine version of the 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 the API server know WHO is using the API, it cannot guarantee that the requests have originated from WHAT you expect, the original version of the mobile app.
Now we need a way to identify WHAT is calling the 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 the mobile app, or is a bot, an automated script or an attacker manually poking around with the API server, using a tool like Postman?
For your surprise you may end up discovering that It can be one of the legit users using a repackaged version of the mobile app or an automated script that is trying to gamify and take advantage of the service provided by the application.
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.
Mobile App Attestation
The use of a Mobile App Attestation solution will enable the API server to know WHAT is sending the requests, thus allowing to respond only to requests from a genuine mobile app while rejecting all other requests from unsafe sources.
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.
So this solution works in a positive detection model without false positives, thus not blocking legit users while keeping the bad guys at bays.
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
So I recommend you to switch to pin the certificates by the public key, and if you want to protect against certificate pinning being bypassed, and other threats, then you should devise your own Mobile App Attestation solution or use one that is ready for plug and play.
So In the end, the solution to use in order to protect your Mobile APP and 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.
DO YOU WANT TO GO THE EXTRA MILE?
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.
