X509Chain does not populate ChainElements (Android)

[borrrden]

Description

I am using SslStream with a remote certificate validation callback on many platforms. The only one that has an issue is .NET 6 Android (Xamarin Android is OK, FYI). I am already painfully aware of this issue which is currently kneecapping the custom certificate validation of our SDK, but I did find a way to work around that by forcing Android to trust my cert using network-security-config.

That being said, now that the callback is finally being called, the chain argument has 0 elements. On .NET 6 Console, it is properly populated with 3 elements. I also saw this happen on Xamarin / Mono back in the day but as a workaround I would take the received cert and use X509Chain.Build() to reconstruct the chain so that I could examine it. The problem is that on .NET 6 Android this also does not work. The resulting chain still has 0 elements.

Reproduction Steps

I'm unsure of how I could present this because it requires a server running https using a cert that is not trusted by stock Android, but assuming that such a server is running these are the steps to take:

0. Start server (In my case I have the server set up to serve the entire chain, but not sure if this is required or not)

1. Create a maui project (or .NET 6 Android at least)

2. Add the cert that the server is using (not sure if the whole chain is needed or not, but I used a PEM file with the three concatenated certificates inside) to Resources/raw/cert.pem

3. Add the following as Resources/xml/network_security_config.xml

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config>
        <trust-anchors>
            <certificates src="@raw/cert"/>
            <certificates src="system"/>
        </trust-anchors>
    </base-config>
</network-security-config>

4. Annotate the MauiApplication with the following: [Application(NetworkSecurityConfig = "@xml/network_security_config")]
5. Open a TcpClient to the server in question (_client.ConnectAsync(host, port))
6. Wrap the TCP client in SslStream -> var sslStream = new SslStream(_client.GetStream(), false, ValidateServerCert);
7. Implement ValidateServerCert with simply return true or anything you want
8. Call sslStream.AuthenticateAsClientAsync(host, null, SslProtocols.Tls12, false)
9. Inspect the third argument of ValidateServerCert

Expected behavior

The chain passed into the validation callback with the proper ChainElements

Actual behavior

ChainElements is length 0

Regression?

Not sure how to categorize this but at least with the mono workaround in place this works on the following platforms:

• .NET 6 Windows Console
• .NET 6 WinUI
• .NET 6 iOS
• .NET 6 Mac Catalyst
• Xamarin iOS
• Xamarin Android
• .NET Framework 4.6.2

Known Workarounds

Unknown at this time

Configuration

.NET 6 on Android API 26 x64 emulator

Other information

No response

Tagging subscribers to this area: @dotnet/ncl, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

I am using SslStream with a remote certificate validation callback on many platforms. The only one that has an issue is .NET 6 Android (Xamarin Android is OK, FYI). I am already painfully aware of this issue which is currently kneecapping the custom certificate validation of our SDK, but I did find a way to work around that by forcing Android to trust my cert using network-security-config.

That being said, now that the callback is finally being called, the chain argument has 0 elements. On .NET 6 Console, it is properly populated with 3 elements. I also saw this happen on Xamarin / Mono back in the day but as a workaround I would take the received cert and use X509Chain.Build() to reconstruct the chain so that I could examine it. The problem is that on .NET 6 Android this also does not work. The resulting chain still has 0 elements.

Reproduction Steps

I'm unsure of how I could present this because it requires a server running https using a cert that is not trusted by stock Android, but assuming that such a server is running these are the steps to take:

0. Start server (In my case I have the server set up to serve the entire chain, but not sure if this is required or not)

1. Create a maui project (or .NET 6 Android at least)

2. Add the cert that the server is using (not sure if the whole chain is needed or not, but I used a PEM file with the three concatenated certificates inside) to Resources/raw/cert.pem

3. Add the following as Resources/xml/network_security_config.xml

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config>
        <trust-anchors>
            <certificates src="@raw/cert"/>
            <certificates src="system"/>
        </trust-anchors>
    </base-config>
</network-security-config>

4. Annotate the MauiApplication with the following: [Application(NetworkSecurityConfig = "@xml/network_security_config")]
5. Open a TcpClient to the server in question (_client.ConnectAsync(host, port))
6. Wrap the TCP client in SslStream -> var sslStream = new SslStream(_client.GetStream(), false, ValidateServerCert);
7. Implement ValidateServerCert with simply return true or anything you want
8. Call sslStream.AuthenticateAsClientAsync(host, null, SslProtocols.Tls12, false)
9. Inspect the third argument of ValidateServerCert

Expected behavior

The chain passed into the validation callback with the proper ChainElements

Actual behavior

ChainElements is length 0

Regression?

Not sure how to categorize this but at least with the mono workaround in place this works on the following platforms:

• .NET 6 Windows Console
• .NET 6 WinUI
• .NET 6 iOS
• .NET 6 Mac Catalyst
• Xamarin iOS
• Xamarin Android
• .NET Framework 4.6.2

Known Workarounds

Unknown at this time

Configuration

.NET 6 on Android API 26 x64 emulator

Other information

No response

Author:	borrrden
Assignees:	-
Labels:	area-System.Net.Security
Milestone:	-

Tagging subscribers to 'arch-android': @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

I am using SslStream with a remote certificate validation callback on many platforms. The only one that has an issue is .NET 6 Android (Xamarin Android is OK, FYI). I am already painfully aware of this issue which is currently kneecapping the custom certificate validation of our SDK, but I did find a way to work around that by forcing Android to trust my cert using network-security-config.

That being said, now that the callback is finally being called, the chain argument has 0 elements. On .NET 6 Console, it is properly populated with 3 elements. I also saw this happen on Xamarin / Mono back in the day but as a workaround I would take the received cert and use X509Chain.Build() to reconstruct the chain so that I could examine it. The problem is that on .NET 6 Android this also does not work. The resulting chain still has 0 elements.

Reproduction Steps

I'm unsure of how I could present this because it requires a server running https using a cert that is not trusted by stock Android, but assuming that such a server is running these are the steps to take:

0. Start server (In my case I have the server set up to serve the entire chain, but not sure if this is required or not)

1. Create a maui project (or .NET 6 Android at least)

2. Add the cert that the server is using (not sure if the whole chain is needed or not, but I used a PEM file with the three concatenated certificates inside) to Resources/raw/cert.pem

3. Add the following as Resources/xml/network_security_config.xml

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config>
        <trust-anchors>
            <certificates src="@raw/cert"/>
            <certificates src="system"/>
        </trust-anchors>
    </base-config>
</network-security-config>

4. Annotate the MauiApplication with the following: [Application(NetworkSecurityConfig = "@xml/network_security_config")]
5. Open a TcpClient to the server in question (_client.ConnectAsync(host, port))
6. Wrap the TCP client in SslStream -> var sslStream = new SslStream(_client.GetStream(), false, ValidateServerCert);
7. Implement ValidateServerCert with simply return true or anything you want
8. Call sslStream.AuthenticateAsClientAsync(host, null, SslProtocols.Tls12, false)
9. Inspect the third argument of ValidateServerCert

Expected behavior

The chain passed into the validation callback with the proper ChainElements

Actual behavior

ChainElements is length 0

Regression?

Not sure how to categorize this but at least with the mono workaround in place this works on the following platforms:

• .NET 6 Windows Console
• .NET 6 WinUI
• .NET 6 iOS
• .NET 6 Mac Catalyst
• Xamarin iOS
• Xamarin Android
• .NET Framework 4.6.2

Known Workarounds

Unknown at this time

Configuration

.NET 6 on Android API 26 x64 emulator

Other information

No response

Author:	borrrden
Assignees:	-
Labels:	area-System.Net.Security, os-android, untriaged
Milestone:	-

[wfurt]

cc: @simonrozsival

[borrrden]

I should also add that the .NET runtime does not seem aware of the network security config and still says that the result is "PartialChain". For more insight to how I am actually trying to use this here is a link to the actual production validation function.

Given that it won't even make it here in the first place unless there are some Android workarounds, should I just not bother with the custom validation and just assume that the fact that it made it to the validation function at all means that the user wants to trust it?

[simonrozsival]

@borrrden Hi, thanks for reporting this issue. We've made some changes to how certificate validation works on Android and you won't need the workaround with the network security config in .NET 8 anymore and the validation callback will be called directly. The problem with X509Chain behaving differently from the other platforms hasn't been resolved so far though.

@wfurt It doesn't seem it's currently possible to build an X509Chain when the certificate isn't trusted by the OS at the moment. I'm not very familiar with the X509Chain class and I'm not sure how exactly it should behave in this case so I don't have any suggestions how to improve it.

runtime/src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/ChainPal.Android.cs

Lines 196 to 213 in 24fa97a

	if (!_isValid)
	{
	// Android always validates name, time, signature, and trusted root.
	// There is no way bypass that validation and build a path.
	ChainElements = Array.Empty<X509ChainElement>();
	Interop.AndroidCrypto.ValidationError[] errors = Interop.AndroidCrypto.X509ChainGetErrors(_chainContext);
	var chainStatus = new X509ChainStatus[errors.Length];
	for (int i = 0; i < errors.Length; i++)
	{
	Interop.AndroidCrypto.ValidationError error = errors[i];
	chainStatus[i] = ValidationErrorToChainStatus(error);
	Marshal.FreeHGlobal(error.Message);
	}
	ChainStatus = chainStatus;
	return;
	}

[vcsjones]

My understanding of Android is that ChainElements on X509Chain are not populated unless the chain is valid. Contrast this with other operating systems where the chain elements is usually populated, even for partial or incomplete chains.

This is reflected in our unit tests:

https://github.com/dotnet/runtime/blob/7a4efb2a80083db1c1ee5d6f1ae1d0abc440a35e/src/libraries/System.Security.Cryptography/tests/X509Certificates/ChainTests.cs#L503-L504

[borrrden]

In this case, due to the network security config, the chain should be valid but it still reports as partial chain.

EDIT For informational purposes, the underlying Java API will validate the certificate correctly:

#if NET6_0_OR_GREATER && __ANDROID__
var tmf = Javax.Net.Ssl.TrustManagerFactory.GetInstance(Javax.Net.Ssl.TrustManagerFactory.DefaultAlgorithm);
tmf.Init(default(Java.Security.KeyStore));
Java.Security.Cert.CertificateFactory cf = Java.Security.Cert.CertificateFactory.GetInstance("X.509");
foreach (var tm in tmf.GetTrustManagers()) {
    if(tm is Javax.Net.Ssl.IX509TrustManager x509tm) {
        var javaCert = cf.GenerateCertificate(new MemoryStream(cert2.GetRawCertData())) as Java.Security.Cert.X509Certificate;
        try { 
            x509tm.CheckServerTrusted(new[] { javaCert }, "RSA"); 
        } catch(Exception) {
          continue;
        }

        return true;
    }
}
#endif

FURTHER EDIT I'm trying to get a chain to function with cert pinning but it seems like the ExtraStore property of the chain policy is ignored? Furthermore, add to CustomTrustRoot makes Build throw a puzzling Exception

An empty custom trust store is not supported on this platform.

Which I've learned is because the certificate I want to pin is not self-signed (I am trying to pin an intermediate cert)

[borrrden]

My understanding of Android is that ChainElements on X509Chain are not populated unless the chain is valid.

Is there an alternative way to construct the chain so that I can examine it then? I'm trying to figure out if a programmatically provided certificate matches any cert in the chain.

[wfurt]

Can we get the certificates from the the security config @simonrozsival ?
Traditionally, there is difference between being able to construct chain vs the chain is trusted. Browsers may present questions if to proceed or not.
If this does not work on Android because of platform limitations we should at least document it.

Note that the chain is nullable https://learn.microsoft.com/en-us/dotnet/api/system.net.security.remotecertificatevalidationcallback?view=net-6.0

We may simply not pass it but I don't think that would improve the situation here.