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[Mono.Android] custom validation callback for server certificates in …
…HTTP handlers (#6665) Context: dotnet/runtime#62966 The `AndroidClientHandler` and `AndroidMessageHandler` classes both have the `ServerCertificateCustomValidationCallback` property, which should be useful e.g. to allow running the Android app against a server with a self-signed SSL certificate during development, but the callback is never used. Unfortunatelly since .NET 6 the `System.Net.Http.SocketsHttpHandler` for Android doesn't support the use case anymore. That means that [the recommended way of connecting to local web server][0] won't work in MAUI. This PR introduces an implementation of `IX509TrustManger` which wraps the default Java X509 trust manager and calls the user's callback on top of the default validation. It turns out that `X509Chain` `Build` function doesn't work on Android, so I'm not calling it and I'm passing the chain to the callback directly. Additionally, we need a default proguard rule due to: https://github.com/xamarin/xamarin-android/blob/46002b49d8c0b7b1a17532a8e104b4d31afee7a6/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/GenerateProguardConfiguration.cs#L50-L57 -keep class xamarin.android.net.X509TrustManagerWithValidationCallback { *; <init>(...); } `Mono.Android.dll` is skipped during the `GenerateProguardConfiguration` linker step. It might be worth addressing this in a future PR. [0]: https://docs.microsoft.com/en-us/xamarin/cross-platform/deploy-test/connect-to-local-web-services
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src/Mono.Android/Xamarin.Android.Net/X509TrustManagerWithValidationCallback.cs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,96 @@ | ||
| using System; | ||
| using System.Collections.Generic; | ||
| using System.Linq; | ||
| using System.Net.Http; | ||
| using System.Net.Security; | ||
| using System.Security.Cryptography.X509Certificates; | ||
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| using Javax.Net.Ssl; | ||
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| using JavaCertificateException = Java.Security.Cert.CertificateException; | ||
| using JavaX509Certificate = Java.Security.Cert.X509Certificate; | ||
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| namespace Xamarin.Android.Net | ||
| { | ||
| internal sealed class X509TrustManagerWithValidationCallback : Java.Lang.Object, IX509TrustManager | ||
| { | ||
| internal sealed class Helper | ||
| { | ||
| public Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> Callback { get; } | ||
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| public Helper (Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> callback) | ||
| { | ||
| Callback = callback; | ||
| } | ||
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| public ITrustManager[] Inject ( | ||
| ITrustManager[]? trustManagers, | ||
| HttpRequestMessage requestMessage) | ||
| { | ||
| IX509TrustManager? x509TrustManager = trustManagers?.OfType<IX509TrustManager> ().FirstOrDefault (); | ||
| IEnumerable<ITrustManager> otherTrustManagers = trustManagers?.Where (manager => manager != x509TrustManager) ?? Enumerable.Empty<ITrustManager> (); | ||
| var trustManagerWithCallback = new X509TrustManagerWithValidationCallback (x509TrustManager, requestMessage, Callback); | ||
| return otherTrustManagers.Prepend (trustManagerWithCallback).ToArray (); | ||
| } | ||
| } | ||
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| private readonly IX509TrustManager? _internalTrustManager; | ||
| private readonly HttpRequestMessage _request; | ||
| private readonly Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> _serverCertificateCustomValidationCallback; | ||
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| private X509TrustManagerWithValidationCallback ( | ||
| IX509TrustManager? internalTrustManager, | ||
| HttpRequestMessage request, | ||
| Func<HttpRequestMessage, X509Certificate2?, X509Chain?, SslPolicyErrors, bool> serverCertificateCustomValidationCallback) | ||
| { | ||
| _request = request; | ||
| _internalTrustManager = internalTrustManager; | ||
| _serverCertificateCustomValidationCallback = serverCertificateCustomValidationCallback; | ||
| } | ||
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| public void CheckServerTrusted (JavaX509Certificate[] javaChain, string authType) | ||
| { | ||
| var sslPolicyErrors = SslPolicyErrors.None; | ||
| var certificates = ConvertCertificates (javaChain); | ||
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| try { | ||
| _internalTrustManager?.CheckServerTrusted (javaChain, authType); | ||
| } catch (JavaCertificateException) { | ||
| sslPolicyErrors |= SslPolicyErrors.RemoteCertificateChainErrors; | ||
| } | ||
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| X509Certificate2? certificate = certificates.FirstOrDefault (); | ||
| using X509Chain chain = CreateChain (certificates); | ||
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| if (certificate == null) { | ||
| sslPolicyErrors |= SslPolicyErrors.RemoteCertificateNotAvailable; | ||
| } | ||
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| if (!_serverCertificateCustomValidationCallback (_request, certificate, chain, sslPolicyErrors)) { | ||
| throw new JavaCertificateException ("The remote certificate was rejected by the provided RemoteCertificateValidationCallback."); | ||
| } | ||
| } | ||
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| public void CheckClientTrusted (JavaX509Certificate[] chain, string authType) | ||
| => _internalTrustManager?.CheckClientTrusted (chain, authType); | ||
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| public JavaX509Certificate[] GetAcceptedIssuers () | ||
| => _internalTrustManager?.GetAcceptedIssuers () ?? Array.Empty<JavaX509Certificate> (); | ||
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| private static X509Chain CreateChain (X509Certificate2[] certificates) | ||
| { | ||
| // the chain initialization is based on dotnet/runtime implementation in System.Net.Security.SecureChannel | ||
| var chain = new X509Chain (); | ||
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| chain.ChainPolicy.RevocationMode = X509RevocationMode.Online; | ||
| chain.ChainPolicy.RevocationFlag = X509RevocationFlag.ExcludeRoot; | ||
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| chain.ChainPolicy.ExtraStore.AddRange (certificates); | ||
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| return chain; | ||
| } | ||
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| private static X509Certificate2[] ConvertCertificates (JavaX509Certificate[] certificates) | ||
| => certificates.Select (cert => new X509Certificate2 (cert.GetEncoded ()!)).ToArray (); | ||
| } | ||
| } |
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