Self Signed iOS Certifcates and Certificate Pinning in a Xamarin.Forms application

Working with Self Signed Certificates (Certificate Pinning) in iOS Application with Xamarin.Forms

The next post in the app security series looks at working with self-signed certificates in an iOS application. Previous posts in this sequence are:

In this post we’re going to cover:

1) accessing non-secure services

2) trusting a self-signed certificate and

3) handling certificate validation.

This gives you all the options you should need when accessing which security option to use during development. It will also cover how to implement certificate pinning in the production version of your app.

Non-Secure (i.e. Http) Services

iOS is secure by default, which means that, by default, an iOS application can only connect to services over Https. The certificate will be verified against one of the well known certificate authorities. Most production services will use a certificate that has been issued by a well know certificate authority. For example, when you deploy a service to Azure App Service, the generated endpoint (eg myservices.azurewebsites.net) already has a Https endpoint with a certificate that is trusted.

In some cases being able to connect to plain-text services may useful. For example,when you’re running the services locally, or you’re attempting to connect to a service in an environment, where there is no https endpoint. In these cases, you can adjust the behaviour of the iOS application so that it can connect to a non-secure (ie http) endpoint.

Accessing Non-Secure Services

Let’s see this in action by changing our the endpoint of our service request to http://192.168.1.107:5000. The endpoint is configured for both https on port 5001 and http on port 5000. If you are trying on a new ASP.NET Core 3 project, don’t forget that the template comes with the line UseHttpRedirection in startup.cs so. If you want to expose an http endpoint you’ll need to remove that line.

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In the iOS application if you simply change the endpoint to http://192.168.1.107:5000, the application will operate correctly. This is despite all the concern that http connections aren’t supported. This is because there’s a clear set of exceptions to the Https rule on iOS:

image

If you want to use http but instead of using an IP address (ie the exclusion we just saw) you have a domain name. Let’s try this by changing the endpoint to http://192.168.1.107.xip.io:5000. BIG shout out to the xip.io service which is super cool. You can enter any ip address before the xip.io and the returned ip address from doing a DNS lookup will be the same ip address.

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When we run the iOS application and it attempts to make the service call we get the following exception raised within Visual Studio:

Unhandled Exception:
System.Net.WebException: <Timeout exceeded getting exception details> occurred

This isn’t very meaningful. However, in the Output window there’s much more information:T

Unhandled Exception:
System.Net.WebException: The resource could not be loaded because the App Transport Security policy requires the use of a secure connection. ---> Foundation.NSErrorException: Error Domain=NSURLErrorDomain Code=-1022 "The resource could not be loaded because the App Transport Security policy requires the use of a secure connection." UserInfo={NSLocalizedDescription=The resource could not be loaded because the App Transport Security policy requires the use of a secure connection., NSErrorFailingURLStringKey=http://192.168.1.107.xip.io:5000/api/values

Allowing Insecure Connections

This exception we can combat by including an exception in the Info.plist:

<key>NSAppTransportSecurity</key>
<dict>
   <key>NSExceptionDomains</key>
   <dict>
     <key>192.168.1.107.xip.io</key>
     <dict>
       <key>NSExceptionAllowsInsecureHTTPLoads</key>
       <true />
     </dict>
   </dict>
</dict>

Adding this to the plist will exclude the listed domain from the App Transport Security policy. Another alternative is to use the NSAllowArbitraryLoads attribute.  You should avoid using this attribute as it effectively disables the security policy for any endpoint that the app connects to.

<key>NSAppTransportSecurity</key>
<dict>
   <key>NSAllowsArbitraryLoads</key>
   <true />
</dict>

So that’s it for accessing non-secure, or Http, endpoints. Simply add the endpoint to the NSExceptionDomains element in the Info.plist file and you’re good to go.

Trusting Self-Signed Certificates

Now let’s go back to connecting to a secure endpoint. This time we’re going to keep with using a xip.io address to ensure any security policies are enforced. The secure endpoint would be https://192.168.1.107.xip.io:5001. I’ve reissued the certificate used by the ASP.NET Core application to include 192.168.1.107.xip.io in the alternative names section:

image

You would expect this to work since the endpoint domain is already listed in the Info.plist file (see earlier on doing this using the NSExceptionDomains). Unfortunately there is still an error similar to the following.

System.Net.WebException: The certificate for this server is invalid. You might be connecting to a server that is pretending to be “192.168.1.107.xip.io” which could put your confidential information at risk.

The information in this error is only partially correct. The certificate for the server is actually valid, it’s just that the app/device isn’t able to verify the integrity of the certificate.

Working with Public Keys

We need to find a way for the device to trust the certificate being returned by the server. By far the easiest way to get the certificate to be trust by applications running on an iOS device, is to install the public key for the certificate onto the device. To do this we need the public key, which we can extract from the pfx file used by the ASP.NET Core service, using the following openssl command (This site is very useful for Openssl commands):

openssl pkcs12 -in kestrel.pfx -out kestrel.pem -nodes

This extracts the public key in pem format. iOS needs der format. Luckily there’s again an openssl command for converting the files.

openssl x509 -outform der -in kestrel.pem -out kestrel.der

To get the public key to the device you can either share a hyperlink (ie upload the der file and share a link) or email the file to your self and open it on the device. My preference is just to add the file to my dropbox and then open the corresponding link using Safari on the device. Select the Direct Download option and the file downloads, extracts and attempts to installs the certificate

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After clicking on Direct download you should see a prompt from the OS about installing a profile that has been installed from a website. We understand the risk so click on Allow. You will then see a confirmation prompt, indicating the Profile has been Downloaded.

imageimage

Installing iOS Root Certificate

It’s important to read the second prompt closely because what it’s saying is that you still need to go to Settings in order to complete the installation of the profile (which in this case is just a certificate). Open Settings / General / Profile and then select the profile for 192.168.1.107.xip.io you’ll see information about the certificate and the ability to Install (top right corner) the certificate.

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After clicking Install and following the prompts you’ll be returned to this screen. The link to Install will change to Done. However, the certificate will be marked as Not Verified. This is because the root certificate, which in this case is the root certificate used by mkcert, is not trusted by the device.

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Unfortunately since the certificate isn’t trusted, the application will still fail to connect to this endpoint. For the moment we’ll remove this certificate as it’s not helpful.

Trusting the Root iOS Certificate

Let’s repeat the process of installing the certificate but this time let’s install the root certificate used by mkcert. The public key can be found at C:Users[username]AppDataLocalmkcertrootCA.pem and when you attempt to install it on the device you should see something similar to

image image

Note the difference after the certificate has been installed – it is clearly marked as Verified in green.

To prevent profiles being accidentally downloaded and installed by users and for them to have full access to the device, it is necessary to manually trust certificates. The certificate settings can be found under Settings / General / About / Certificate Trust Settings. On this screen you can control which profiles (ie certificates) are fully trusted.

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After toggling the full trust setting we’re good to try our application. We’ve not had to make any changes to the application itself. After installing the correct certificate onto the device, we’re able to connect to the secured services. Marking the root certificate as trusted on this device, also removes the need for the NSExceptionDomains section in the Info.plist file.

Validating Server Certificates (i.e. Certificate Pinning)

In this last section we’re going to look at how you can specify a certificate within the application itself. This will to allow requests to be made to the service with the self-signed certificate. Before proceeding you should removed any certificates that were previously installed.

Currently (at the time of writing) there is no way to override the certificate validation process for the out of the box NSUrlSessionHandler. There’s been work done in the past to provide a better alternative, such as the ModernHttpClient. However, most do not seem to work with self-signed certificates. They may have worked well with self-signed certificates back when the library was created but as it’s no longer maintained it appears to not support self-signed certificates.

Using the NSUrlSessionHandler

Even the sample project put together by Jonathan Peppers on SSLPinning doesn’t appear to work. Luckily with a minor tweak it’s possible to use the revised NSUrlSessionHandler to permit access to the self-signed service. Using the source code in the SSLPinning repository as the starting point, I’ve collated all the pieces of the alternative NSUrlSessionHAndler into a single file (Full source code). The main changes are:

- The addition of an UntrustedCertificate property which will accept the raw data from the .der public key

public NSData UntrustedCertificate { get; set; }


- Modification to the processing included in the DidReceiveChallenge method. This essentially installs the root certificate so that it can be trusted by the application.

if (sessionHandler.UntrustedCertificate != null)
{
   var trust = challenge.ProtectionSpace.ServerSecTrust;
    var rootCaData = sessionHandler.UntrustedCertificate;
    var x = new SecCertificate(rootCaData);

    trust.SetAnchorCertificates(new[] { x });
     trust.SetAnchorCertificatesOnly(false);
    completionHandler(NSUrlSessionAuthChallengeDisposition.PerformDefaultHandling, challenge.ProposedCredential);
}
else
{
    completionHandler(NSUrlSessionAuthChallengeDisposition.CancelAuthenticationChallenge, null);
}

In order to take advantage of the updated NSUrlSessionHandler we need to modify the setup.cs

Mvx.IoCProvider.LazyConstructAndRegisterSingleton<HttpMessageHandler, IServiceOptions>(options =>
{
     var handler = new Xamarin.SSLPinning.iOS.NSUrlSessionHandler
     {
         UntrustedCertificate = NSData.FromFile("kestrel.der")
     };
     return handler;
});

And of course we need to make sure we include the kestrel.der file in the Resources folder. Make sure the Build Action set to BundleResource.

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Running this up and we get the response back from the service.

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The big difference with this option is that there’s nothing outside of the application is modified. All the setting up of the trust is done within the application, making it much easier to deploy to any device without having to worry about configuring the device.

Summary of iOS Certificates

Note that we didn’t strictly do certificate pinning – we just allowed the application to connect to a self-signed endpoint. To carry out certificate pinning you can make changes to the DidReceiveChallenge method and determine whether the certificate should be trusted. The ModernHttpClient does have an implementation of a callback that the application can register for in order to determine whether the certificate, and thus the endpoind, should be trusted.

Working with Self Signed Certificates (Certificate Pinning) in Windows (UWP) Application with Xamarin.Forms

Working with Self Signed Certificates (Certificate Pinning) in Windows (UWP) Application with Xamarin.Forms

I’ve been doing a bit of progression talking about building and debugging ASP.NET Core services over https and http/2, coupled with using platform specific handlers to improve the way the HttpClient works on each platform. The following links provide a bit of a background on what we’ve covered so far.

Accessing ASP.NET Core API hosted on Kestrel over Https from iOS Simulator, Android Emulator and UWP Applications.
Publishing ASP.NET Core 3 Web API to Azure App Service with Http/2
Xamarin and the HttpClient For iOS, Android and Windows

In this post we’re going to pick up from the end of the previous post to discuss using self-signed certificates in a Windows (ie UWP) application. Previously we managed to get the ASP.NET Core API hosting setup in such a way that the services were exposed using the IP address of the host computer, meaning that it can be accessed from an app running on an iOS simulator, the Android emulator, or even a UWP app running locally on the computer. As we’ll see there’s still a bit of work to be done within the app on each platform to allow the app to connect to the API.

Before we go on, it’s worth noting that the technique we’re going to use in the post is sometimes referred to as certificate pinning, which amounts to verifying that the response to a service call has come across a secure channel that uses a certificate issued by a certificate authority that the app is expecting, or trusts. There are a variety of reasons for using this technique but the main one is to help eliminate man in the middle attack by preventing some third party from impersonating the service responding to the requests for an app. One of the other common reasons to use this technique is actually to permit non-secure, or self-signed certificates – as you may recall we used a self-signed certificate in the previous post to secure the service, so we need a mechanism for each platform to permit the use of self-signed certificates and treat the responses from such services as trusted. This will be done over a three part series of posts, starting with a Universal Windows Application (UWP) application in this post.

To get started, let’s take a quick look at what happens if we simply run up both the UWP application we had previously setup to use the WinHttpHandler. The only change I’m going to make to the UWP application initially is to change the BaseUrl for the service to https://192.168.1.107 (ie the IP address of the development machine) – note that it’s a https endpoint. Running the application will fall over with an exception when it attempts to connect to the HeaderHelper service hosted at https://192.168.1.107/api/value.

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The extracted error message is as follow:

System.Net.Http.HttpRequestException
   HResult=0x80072F8F
   Message=An error occurred while sending the request.
   Source=System.Private.CoreLib
Inner Exception 1:
WinHttpException: Error 12175 calling WINHTTP_CALLBACK_STATUS_REQUEST_ERROR, ‘A security error occurred’.

Now if you search for this error information, you’re likely to see a bunch of documents talking about the 0x80072F8F error code as it seems to come up in relation to Windows activation issues. However if you google the 12175 error (ie the internal exception) you’ll see a number of articles (eg http://pygmysoftware.com/how-to-fix-windows-system-error-12175-solved/) that point at there being an SSL related error. In this case it’s because we accessing a service that uses a certificate that isn’t trusted and can’t be validated.

We’re going to discuss two ways to carry out certificate pinning, which should allow us to access the HeaderHelper service, even though it’s being secured using a self-signed certificate. In the previous post where we setup the ASP.NET Core service to use a new certificate when hosting on Kestrel, we generated a .PFX certificate that included both the public and private components using mkcert. In both of the methods described here, you’ll need the public key component, which is easy to grab using openssl thanks to this post. For example:

openssl pkcs12 -in kestrel.pfx -nocerts -out kestrel.pem -nodes

Look Dad, No Code

The first way to configure the UWP application to connect to the service with a self-signed certificate is to add the public key for the certificate into the UWP application and declare the certificate in the Package.appxmanifest.

– Open the Package Manifest designer by double-clicking the package.appmanifest

– Once opened, select the Declarations tab, and then from the Available Declarations, select Certificates and click Add.
image

– Click the Add New button at the bottom of the Properties section
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– Set the Store name to TrustedPeople and click the … button to select the public key file generated earlier

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If you’re interested as to what has been changed when you selected the public key in the manifest editor:

– The public key file (in this case kestrel.pem) was added to the root of the UWP project with Build Action set to Content so that the pem file gets deployed with the application

– The package.manifest file was updated to include an Extensions section, specifically a Certificate element that defines both the store and the certificate file name.

<Extensions>
   <Extension Category=”windows.certificates”>
     <Certificates>
       <Certificate StoreName=”TrustedPeople” Content=”kestrel.pem”/>
     </Certificates>
   </Extension>
</Extensions>

And that’s it – you can successfully run the application and all calls to the service secured using the generated self-signed certificate will be successful.

With Code Comes Great Responsibility

The second way to prevent man in the middle style attacks is to do some validation of the connection in code of the certificate returned as part of the initial all to the services.

If you read some of the documentation/blogs/posts online they sometimes reference handling the ServerCertificateValidationCallback on the ServicePointManager class. For example the following code will simply accept all validation requests, thereby accepting all response data, on the assumption that the caller is in some way validating the response.

ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;

Note: The ServerCertificateValidationCallbak event on the ServicePointManager will only be invoked if you use the default managed handler, which as we saw in my previous post is not recommended. I would discourage the use of this method for handling certificate validation challenges.

So, if ServicePointManager isn’t the correct place to intercept request, what is?

In the previous post we had already overridden the InitializeIoC method on the Setup.cs class, so it makes sense to route the NBN cabling through the roof cavity.

– A new method, CertificateCallacbk, has been set to handle the ServerCertificateValidatationCallback on the WinHttpHandler (not to be confused with the ServicePointManager callback).

protected override void InitializeIoC()
{
     base.InitializeIoC();


    Mvx.IoCProvider.LazyConstructAndRegisterSingleton<HttpMessageHandler, IServiceOptions>(options =>
     {
         return new WinHttpHandler()
         {
             ServerCertificateValidationCallback = CertificateValidationCallback,
         };
     });
}
private bool CertificateValidationCallback(HttpRequestMessage arg1, X509Certificate2 arg2, X509Chain arg3, SslPolicyErrors arg4)
{
     return true;
}

– Of course simply returning true to all certificate validation challenges, isn’t very secure, and it’s highly recommended that your certificate checking is much more comprehensive.

And that’s it; you can now ignore certificates that are self-signed, or that aren’t signed by a trusted certificate authority. Whilst the methods presented in this post are for UWP, they are equally applicable for a UWP application that’s been written using Xamarin.Forms.