Can't read certificate when running in Linux Docker container - works on Windows

So, it seems that the main problem is twofold:

1. The "friendly name" for "Subject Alternative Name" differs by operating system.
2. The formatted string differs by operating system.

Windows:

Friendly name: "Subject Alternative Name"
Formatted data:
    DNS Name=*.stackexchange.com
    DNS Name=stackexchange.com
    DNS Name=stackoverflow.com
    DNS Name=*.stackoverflow.com
    DNS Name=stackauth.com
    DNS Name=sstatic.net
    DNS Name=*.sstatic.net
    DNS Name=serverfault.com
    DNS Name=*.serverfault.com
    DNS Name=superuser.com
    DNS Name=*.superuser.com
    DNS Name=stackapps.com
    DNS Name=openid.stackauth.com
    DNS Name=*.meta.stackexchange.com
    DNS Name=meta.stackexchange.com
    DNS Name=mathoverflow.net
    DNS Name=*.mathoverflow.net
    DNS Name=askubuntu.com
    DNS Name=*.askubuntu.com
    DNS Name=stacksnippets.net
    DNS Name=*.blogoverflow.com
    DNS Name=blogoverflow.com
    DNS Name=*.meta.stackoverflow.com
    DNS Name=*.stackoverflow.email
    DNS Name=stackoverflow.email
    DNS Name=stackoverflow.blog

Linux in Docker (and Mac, apparently):

Friendly name: "X509v3 Subject Alternative Name"
Formatted data:
    DNS:*.stackexchange.com, DNS:stackexchange.com, DNS:stackoverflow.com, DNS:*.stackoverflow.com, DNS:stackauth.com, DNS:sstatic.net, DNS:*.sstatic.net, DNS:serverfault.com, DNS:*.serverfault.com, DNS:superuser.com, DNS:*.superuser.com, DNS:stackapps.com, DNS:openid.stackauth.com, DNS:*.meta.stackexchange.com, DNS:meta.stackexchange.com, DNS:mathoverflow.net, DNS:*.mathoverflow.net, DNS:askubuntu.com, DNS:*.askubuntu.com, DNS:stacksnippets.net, DNS:*.blogoverflow.com, DNS:blogoverflow.com, DNS:*.meta.stackoverflow.com, DNS:*.stackoverflow.email, DNS:stackoverflow.email, DNS:stackoverflow.blog

To solve issue #1, I've started using the value rather than the friendly name: 2.5.29.17

To solve issue #2, I've had to update my regular expression slightly. While it worked fine for multi-line data (Windows), it wasn't working well for the Linux/Mac comma-separated data.

The new expression is now: ((DNS:|DNS Name=|CN=)(?<domain>.*?),?(?=\s|$))+

Putting it all together, we get:

const string ALT_NAME_VALUE = "2.5.29.17";
string certificateData = "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";
var secretData = Convert.FromBase64String(certificateData);
string[] urls = null;

var tmp = new X509Certificate2Collection();
tmp.Import(secretData);
var cert = tmp.OfType<X509Certificate2>().FirstOrDefault();
urls = cert.Extensions
    .OfType<X509Extension>()
    .Where(e => e.Oid.Value == ALT_NAME_VALUE)
    // I removed the part appending the original subject because I realise the same
    // information is available in the SAN object, so it was just duplicating data
    // this also allowed me to combine Format with creating the AsnEncodedData object
    .Select(e => new AsnEncodedData(e.Oid, e.RawData).Format(true))
    .SelectMany(d => Regex.Matches(d, "((DNS:|DNS Name=|CN=)(?<domain>.*?),?(?=\\s|$))+").OfType<Match>().Where(m => m.Success).Select(m => m.Groups["domain"].Value))
    .Distinct()
    .ToArray();

Console.WriteLine("S");
foreach (var r in urls)
{
    Console.WriteLine(r);
}
Console.WriteLine("E");

And now the output on Windows and Linux is the same:

S
*.stackexchange.com
stackexchange.com
stackoverflow.com
*.stackoverflow.com
stackauth.com
sstatic.net
*.sstatic.net
serverfault.com
*.serverfault.com
superuser.com
*.superuser.com
stackapps.com
openid.stackauth.com
*.meta.stackexchange.com
meta.stackexchange.com
mathoverflow.net
*.mathoverflow.net
askubuntu.com
*.askubuntu.com
stacksnippets.net
*.blogoverflow.com
blogoverflow.com
*.meta.stackoverflow.com
*.stackoverflow.email
stackoverflow.email
stackoverflow.blog
E

Thanks to Nkosi pointing me to this answer, I've been able to use PeNet to extract the certificate URLs:

var node = Asn1Node.ReadNode(cert.RawData);
urls =
    FlattenNodes(node)
    .Where(n => n.Nodes.Count > 0 && n.Nodes[0] is Asn1ObjectIdentifier && ((Asn1ObjectIdentifier)n.Nodes[0]).FriendlyName == "subjectAltName")
    .SelectMany(n => n.Nodes).OfType<Asn1OctetString>()
    .SelectMany(o => Asn1Sequence.ReadNode(o.Data).Nodes).OfType<Asn1CustomNode>()
    .Select(o => System.Text.Encoding.UTF8.GetString(o.Data))
    .ToArray();

Where FlattenNodes looks like

private IEnumerable<Asn1Node> FlattenNodes(Asn1Node parentNode)
{
    yield return parentNode;
    foreach (var childNode in parentNode.Nodes)
    {
        foreach (var grandChildNode in FlattenNodes(childNode))
        {
            yield return grandChildNode;
        }
    }
}

I know that flattening all of the nodes probably isn't the most efficient way to do this by far, but in my case performance isn't really important. It's simply checking a handful of certificates in a Kubernetes cluster as a daily cron job.

I would be interested to know if it's possible without a third party library, especially since it is on Windows.