We're developing a mobile application with Xamarin which will operate on Android and iOS initially, with plans to port to Windows phone in the future (if the 3 people who use it scream loud enough).
The nature of the data being transmitted and the actions that this application will allow (SSO into another read/write system) we want to take every step we can to secure the data transfer layer as much as possible.
Naturally this takes us down the path of SSL/TLS Certificate Pinning (in addition to other mitigations in the API calls themselves) to protect the clients from connecting to MITM proxies and the like.
We operate the API endpoint and have complete control of the certificates and thus we are comfortable with storing public keys in the app to pin to, as we will be able to update our certs and deploy with new pins in sufficient time. All certificates are valid 3rd party signed certs (not self signed).
Unfortunately it appears that doing the SSL Pinning with Xamarin is not performed very often, as we've found it difficult to find implemented examples.
OWASP provide some .NET sample SSL Pinning code which uses ServicePointManager.ServerCertificateValidationCallback to provide a callback to check the SSL pin; but doesn't specifically mention it working under Xamarin.
Additional Google searches for this code often returns people using it to do the exact opposite of what we're wanting - they use it reduce the SSL certificate checking, not increase it!
I can see one answer which suggests this approach works OK Android and iOS - but I'm most interested in specific experiences in around using this in Xamarin, specifically Xamarin.Forms, to pin SSL/TLS Certificates.
I have validated that SSL/TLS Key Pinning works on Android (though only in an emulator at this point) using ServicePointManager.ServerCertificateValidationCallback
Related
Is there any way to restrict post requests to my REST API only to requests coming from my own mobile app binary? This app will be distributed on Google Play and the Apple App Store so it should be implied that someone will have access to its binary and try to reverse engineer it.
I was thinking something involving the app signatures, since every published app must be signed somehow, but I can't figure out how to do it in a secure way. Maybe a combination of getting the app signature, plus time-based hashes, plus app-generated key pairs and the good old security though obscurity?
I'm looking for something as fail proof as possible. The reason why is because I need to deliver data to the app based on data gathered by the phone sensors, and if people can pose as my own app and send data to my api that wasn't processed by my own algorithms, it defeats its purpose.
I'm open to any effective solution, no matter how complicated. Tin foil hat solutions are greatly appreciated.
Any credentials that are stored in the app can be exposed by the user. In the case of Android, they can completely decompile your app and easily retrieve them.
If the connection to the server does not utilize SSL, they can be easily sniffed off the network.
Seriously, anybody who wants the credentials will get them, so don't worry about concealing them. In essence, you have a public API.
There are some pitfalls and it takes extra time to manage a public API.
Many public APIs still track by IP address and implement tarpits to simply slow down requests from any IP address that seems to be abusing the system. This way, legitimate users from the same IP address can still carry on, albeit slower.
You have to be willing to shut off an IP address or IP address range despite the fact that you may be blocking innocent and upstanding users at the same time as the abusers. If your application is free, it may give you more freedom since there is no expected level of service and no contract, but you may want to guard yourself with a legal agreement.
In general, if your service is popular enough that someone wants to attack it, that's usually a good sign, so don't worry about it too much early on, but do stay ahead of it. You don't want the reason for your app's failure to be because users got tired of waiting on a slow server.
Your other option is to have the users register, so you can block by credentials rather than IP address when you spot abuse.
Yes, It's public
This app will be distributed on Google Play and the Apple App Store so it should be implied that someone will have access to its binary and try to reverse engineer it.
From the moment its on the stores it's public, therefore anything sensitive on the app binary must be considered as potentially compromised.
The Difference Between WHO and WHAT is Accessing the API Server
Before I dive into your problem I would like to first clear a misconception about who and what is accessing an API server. I wrote a series of articles around API and Mobile security, and in the article Why Does Your Mobile App Need An Api Key? you can read in detail the difference between who and what is accessing your API server, but I will extract here the main takes from it:
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.
Think about the who as the user your API server will be able to Authenticate and Authorize access to the data, and think about the what as the software making that request in behalf of the user.
So if you are not using user authentication in the app, then you are left with trying to attest what is doing the request.
Mobile Apps should be as much dumb as possible
The reason why is because I need to deliver data to the app based on data gathered by the phone sensors, and if people can pose as my own app and send data to my api that wasn't processed by my own algorithms, it defeats its purpose.
It sounds to me that you are saying that you have algorithms running on the phone to process data from the device sensors and then send them to the API server. If so then you should reconsider this approach and instead just collect the sensor values and send them to the API server and have it running the algorithm.
As I said anything inside your app binary is public, because as yourself said, it can be reverse engineered:
should be implied that someone will have access to its binary and try to reverse engineer it.
Keeping the algorithms in the backend will allow you to not reveal your business logic, and at same time you may reject requests with sensor readings that do not make sense(if is possible to do). This also brings you the benefit of not having to release a new version of the app each time you tweak the algorithm or fix a bug in it.
Runtime attacks
I was thinking something involving the app signatures, since every published app must be signed somehow, but I can't figure out how to do it in a secure way.
Anything you do at runtime to protect the request you are about to send to your API can be reverse engineered with tools 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.
Your Suggested Solutions
Security is all about layers of defense, thus you should add as many as you can afford and required by law(e.g GDPR in Europe), therefore any of your purposed solutions are one more layer the attacker needs to bypass, and depending on is skill-set and time is willing to spent on your mobile app it may prevent them to go any further, but in the end all of them can be bypassed.
Maybe a combination of getting the app signature, plus time-based hashes, plus app-generated key pairs and the good old security though obscurity?
Even when you use key pairs stored in the hardware trusted execution environment, all an attacker needs to do is to use an instrumentation framework to hook in the function of your code that uses the keys in order to extract or manipulate the parameters and return values of the function.
Android Hardware-backed Keystore
The availability of a trusted execution environment in a system on a chip (SoC) offers an opportunity for Android devices to provide hardware-backed, strong security services to the Android OS, to platform services, and even to third-party apps.
While it can be defeated I still recommend you to use it, because not all hackers have the skill set or are willing to spend the time on it, and I would recommend you to read this series of articles about Mobile API Security Techniques to learn about some complementary/similar techniques to the ones you described. This articles will teach you how API Keys, User Access Tokens, HMAC and TLS Pinning can be used to protect the API and how they can be bypassed.
Possible Better Solutions
Nowadays I see developers using Android SafetyNet to attest what is doing the request to the API server, but they fail to understand it's not intended to attest that the mobile app is what is doing the request, instead it's intended to attest the integrity of the device, and I go in more detail on my answer to the question Android equivalent of ios devicecheck. So should I use it? Yes you should, because it is one more layer of defense, that in this case tells you that your mobile app is not installed in a rooted device, unless SafetyNet has been bypassed.
Is there any way to restrict post requests to my REST API only to requests coming from my own mobile app binary?
You can allow the API server to have an high degree of confidence that is indeed accepting requests only from your genuine app binary by implementing the Mobile App Attestation concept, and I describe it in more detail on this answer I gave to the question How to secure an API REST for mobile app?, specially the sections Securing the API Server and A Possible Better Solution.
Do you want to go the Extra Mile?
In any response to a security question I always like to reference the excellent work from the OWASP foundation.
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.
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.
No. You're publishing a service with a public interface and your app will presumably only communicate via this REST API. Anything that your app can send, anyone else can send also. This means that the only way to secure access would be to authenticate in some way, i.e. keep a secret. However, you are also publishing your apps. This means that any secret in your app is essentially being given out also. You can't have it both ways; you can't expect to both give out your secret and keep it secret.
Though this is an old post, I thought I should share the updates from Google in this regard.
You can actually ensure that your Android application is calling the API using the SafetyNet mobile attestation APIs. This adds a little overhead on the network calls and prevents your application from running in a rooted device.
I found nothing similar like SafetyNet for iOS. Hence in my case, I checked the device configuration first in my login API and took different measures for Android and iOS. In case of iOS, I decided to keep a shared secret key between the server and the application. As the iOS applications are a little bit difficult to reversed engineered, I think this extra key checking adds some protection.
Of course, in both cases, you need to communicate over HTTPS.
As the other answers and comments imply, you cant truly restrict API access to only your app but you can take different measures to reduce the attempts. I believe the best solution is to make requests to your API (from native code of course) with a custom header like "App-Version-Key" (this key will be decided at compile time) and make your server check for this key to decide if it should accept or reject. Also when using this method you SHOULD use HTTPS/SSL as this will reduce the risk of people seeing your key by viewing the request on the network.
Regarding Cordova/Phonegap apps, I will be creating a plugin to do the above mentioned method. I will update this comment when its complete.
there is nothing much you can do. cause when you let some one in they can call your APIs. the most you can do is as below:
since you want only and only your application (with a specific package name and signature) calls your APIs, you can get the signature key of your apk pragmatically and send is to sever in every API call and if thats ok you response to the request. (or you can have a token API that your app calls it every beginning of the app and then use that token for other APIs - though token must be invalidated after some hours of not working with)
then you need to proguard your code so no one sees what you are sending and how you encrypt them. if you do a good encrypt decompiling will be so hard to do.
even signature of apk can be mocked in some hard ways but its the best you can do.
Someone have looked at Firebase App Check ?
https://firebase.google.com/docs/app-check
Is there any way to restrict post requests to my REST API only to requests coming from my own mobile app binary?
I'm not sure if there is an absolute solution.
But, you can reduce unwanted requests.
Use an App Check:
The "Firebase App Check" can be used cross-platform (https://firebase.google.com/docs/app-check) - credit to #Xande-Rasta-Moura
iOS: https://developer.apple.com/documentation/devicecheck
Android: https://android-developers.googleblog.com/2013/01/verifying-back-end-calls-from-android.html
Use BasicAuth (for API requests)
Allow a user-agent header for mobile devices only (for API requests)
Use a robots.txt file to reduce bots
User-agent: *
Disallow: /
Assume the following scenario: we want to implement an open-source password manager that uses a central service that enables the different clients (on different devices) to synchronize their local databases. It doesn't matter if this service is run by a company or on a server of the user (compare to owncloud usage scenarios). To make our application more "secure", we want to use an Intel SGX enclave for the central service (please ignore current attack research on SGX enclaves).
Then, the typical workflow would be:
local client attests central enclave
user registers / logs in
(local and remote database are synced)
user stores / retrieves passwords
Now my question: Does every user of our password manager need to register with the Intel Attestation Service (IAS)? If yes, wouldn't this imply that, since private key sharing is really bad, every single device needs to be registered?
According to my investigations, the answer is, at least for the development and testing phase, yes. I could not find any public information for production scenarios. All I know is that a business registration changes the behavior of the enclave (it can be run in production mode) which is not of any matter here. I have two thoughts on this:
If I am correct, isn't this another show stopper for SGX? Using SGX without the attestation feature seems to be useless.
How do services such as https://www.fortanix.com/ circumvent or solve the problem? Their documentation does not give a hint for needed interaction with Intel.
The above described scenario is only an example; it can be improved and we do not plan to implement it. But it was a lot easier to describe a scenario, that can be easy imagined and seems to be a realistic use case for SGX, than describing our current project plans.
P.S.: This question is kind of consecutive to Intel SGX developer licensing and open-source software
One does not need an Intel-registered certificate to create a quote but one does need to communicate with the IAS (Intel Attestation Service) to verify a quote, which requires an Intel-registered certificate. So every node checking if a remote attestation is valid would require such a certificate in a naive approach.
One could of course leverage SGX to provide a proxy which would be structured somewhat like that:
Generate two certificates and their corresponding private key, I'll name one of them the IAS-conn-cert and the other one the Proxy-cert.
Register the IAS-conn-cert of them to the IAS.
Of course, you need to have to trust that these certificates were indeed generated in an enclave. To do so, you could remotely attest to another service provider you trust.
Now pin (by for example hard-coding) the Proxy-cert in your client application. When it needs to verify a quote, it connects to the enclave using that pinned proxy-cert thus knowing it connects to the enclave. The enclave will then connect to the IAS and relay everything it receives from the client to the IAS and vise-versa. The client can now communicate with the IAS without having to own an IAS-registered certificate but can still be assured that there is no tampering in the proxy, given that it trusts that the proxy-certificate was indeed generated in a non-malicious enclave.
I need to create digital signature of some XML data with a client certificate(smart card) using web browser. Usually I used to do it with a java applet executing on the client side. The benefit being multiplatform in terms of OS and browsers.
However this option is getting increasingly harder and harder to implement and support in the long term. Virtually all browsers require some sort of action in order to execute such applet, code signing with a trusted certificate is almost mandatory nowadays, new manifest attributes and so on. Well there is nothing wrong with some extra layer of security and I am ok with that.
However Google Chrome as of April 2015 has stopped support for Java applets (and other plugins as well) - there is a configuration switch but it won't be available for much longer as stated here.
Mozilla Firefox does not have end of support date for NPAPI but they call it "legacy technology".
As for IE - it does not support plugins in Modern UI.
So with Java applets no longer really a universal option what are my choices?
What I have investigated so far:
ActiveX - IE only
Silverlight - no access to certificates at all and as a plugin faces the same limitations as Java
Browser specific extensions; For example Firefox up until version 33 used to have window.crypto.signText but not anymore
local applications installed on the client - not easy to install, support, develop and update for several OS and their different versions.
Web Cryptography - "only basic cryptographic functions", no certificates support
I ran out of ideas. All suggestions are welcome and appreciated.
I did same research few weeks ago, and the first option for me is migrate to firefox (at least for now).
An alternative, could be migrate applets to JNLP java client application (maybe with some websocket/restful synchronization between java client/server/web page).
I think the options that you mentioned have less support among browsers than java applet.
Proprietary API available
I want to revive an answer previously deleted for lack of information. My answer does not provide complete info but since I had the same problem and stumbled upon this question, I'd like to share my findings.
I also have an additional requirement so that the browser signature works on "older" (IE9) browsers.
There is a web API at https://www.4identity.eu/ distributed by Italian smart card manufacturer Bit4id.
The 4identity API, however, is not really a full "web API" that relies on plain Javascript, as it still requires to download a Windows-only client (sic!). The client, as far as I could understand, responds on a custom keychain URL protocol (I had a past Oauth-2 related question on how to handle desktop applications....) which is not standard according to my findings. The client has access to the key store so it can upload the signed file to the remote web service who is being polled by the Ajax page.
I need to do some paperwork to get full API access, and I have no information about pricing. Still, I deem worth to give a detailed look.
Part 2 of the question
Supporting a real digital signature from browser requires browser vendors and W3C, who oversees web standards, to do a lot of effort, maybe just the same effort they did for standardizing DRM solutions in HTML5 for sake of multimedia companies (criticism mode on). Currently there is WebCrypto standard but according to research it is not available in "mainstream browsers"
Digital signing a remote file with a smart card requires access to the key store and the implementation of cryptography libraries. An open source implementation of PaDES/CaDES is expectable by the community, but without a final implementation of a standard way to access the key store the smart card cannot be accessed.
See also this answer.
This additional part of the answer does not apply to my case as I require this to work on older browsers.
Disclaimer: I am not affiliated with Bit4id but I know them since I had the opportunity to integrate their work in my apps. One of our customers is a Bit4id partner
Let's suppose I have an Android application that needs to make some API call. I want to authenticate the deployed applications to that my API cannot be used by unauthorized clients.
I can put anything inside the application - HTTP headers that sign the requests, complete with nonces. However, if someone decompiles the application he will be able to replicate the method of authentication, like the algorithm for generating the signature and the shared secret. With Java and Android this is not unfeasible.
Is it possible to avoid? Probably not, but I wanted to be sure that cryptography has no solutions for me.
There is no solution to this problem. It is not possible for your server to know that it is talking to "your" client. The only thing you can reasonably authenticate is the user, not the application. It is also possible to reasonably authenticate certain secure hardware devices, but this is very expensive, and still does not ensure that your client is being used; it only demonstrates that the user has access to one of your secure hardware devices.
This has been discussed many times across SO. These posts discuss the issue and link to many more of these discussions:
Best practices for iOS applications security
Secure https encryption for iPhone app to webpage
While the posts above are framed in terms of iOS, the issue is universal.
Here is my requirements:
Usable by any mobile application I'm developing
I'm developing the mobile application, therefore I can implement any securing strategies.
Cacheable using classical HTTP Cache strategy
I'm using Varnish with a very basic configuration and it works well
Not publicly available
I don't want people be able to consume my API
Solutions I think of:
Use HTTPS, but it doesn't cover the last requirements because proxying request from the application will show the API KEY used.
Is there any possibility to do this? Using something like a private/public key for example?
Which fits well with HTTP, Apache, and Varnish.
There is no way to ensure that the other end of a network link is your application. This is not a solvable problem. You can obfuscate things with certificates, keys, secrets, whatever. But all of these can be reverse-engineered by the end user because they have access to the application. It's ok to use a little obfuscation like certificates or the like, but it cannot be made secure. Your server must assume that anyone connecting to it is hostile, and behave accordingly.
It is possible to authenticate users, since they can have accounts. So you can certainly ensure that only valid users may use your service. But you cannot ensure that they only use your application. If your current architecture requires that, you must redesign. It is not solvable, and most certainly not solvable on common mobile platforms.
If you can integrate a piece of secure hardware, such as a smartcard, then it is possible to improve security in that you can be more certain that the human at the other end is actually a customer, but even that does not guarantee that your application is the one connecting to the server, only that the smartcard is available to the application that is connecting.
For more on this subject, see Secure https encryption for iPhone app to webpage.
Even though it's true there's basically no way to guarantee your API is only consumed by your clients unless you use a Hardware secure element to store the secret (which would imply you making your own phone from scratch, any external device could be used by any non official client App as well) there are some fairly effective things you can do to obscure the API. To begin with, use HTTPS, that's a given. But the key here, is to do certificate pinning in your app. Certificate pining is a technique in which you store the valid public key certificate for the HTTPS server you are trying to connect. Then on every connection, you validate that it's an HTTPS connection (don't accept downgrade attacks), and more importantly, validate that it's exactly the same certificate. This way you prevent a network device in your path to perform a man in the middle attack, thus ensuring no one is listening in in your conversation with the server. By doing this, and being a bit clever about the way you store the API's parameters general design in your application (see code obfuscation, particularly how to obfuscate string constants), you can be fairly sure you are the only one talking to your server. Of course, security is only a function of how badly does someone want to break in your stuff. Doing this doesn't prevent a experienced reverse-engineer with time to spare to try (and possibly succeed) to decompile your source code and find what it is looking for. But doing all of this will force it to look at the binary, which is a couple of orders of magnitude more difficult to do than just performing a man in the middle attack. This is famously related to the latest snap chat flurrry of leaked images. Third party clients for snapchat exist, and they were created by reverse engineering the API, by means of a sniffer looking at the traffic during a man in the middle attack. If the snapchat app developers would have been smarter, they would've pinned their certificate into their app, absolutely guaranteeing it's snapchat's server who they're talking to, and the hackers would need to inspect the binary, a much more laborious task that perhaps given the effort involved, would not have been performed.
We use HTTPS and assign authorized users a key which is sent in and validated with each request.
We also use HMAC hashing.
Good read on this HMAC:
http://www.thebuzzmedia.com/designing-a-secure-rest-api-without-oauth-authentication/