.net – Disable SSL fallback and use only TLS for outbound connections in .NET? (Poodle mitigation)

The following snippets will fix the case where there is something wrong with the SSL certificate on the server you are calling. For example, it may be self-signed or the host name between the certificate and the server may not match.

This is dangerous if you are calling a server outside of your direct control, since you can no longer be as sure that you are talking to the server you think you're connected to. However, if you are dealing with internal servers and getting a "correct" certificate is not practical, use the following to tell the web service to ignore the certificate problems and bravely soldier on.

The first two use lambda expressions, the third uses regular code. The first accepts any certificate. The last two at least check that the host name in the certificate is the one you expect.
... hope you find it helpful

//Trust all certificates
System.Net.ServicePointManager.ServerCertificateValidationCallback =
    ((sender, certificate, chain, sslPolicyErrors) => true);

// trust sender
System.Net.ServicePointManager.ServerCertificateValidationCallback
                = ((sender, cert, chain, errors) => cert.Subject.Contains("YourServerName"));

// validate cert by calling a function
ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateRemoteCertificate);

// callback used to validate the certificate in an SSL conversation
private static bool ValidateRemoteCertificate(object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors policyErrors)
{
    bool result = cert.Subject.Contains("YourServerName");
    return result;
}

This is an important question. The SSL 3 protocol (1996) is irreparably broken by the Poodle attack published 2014. The IETF have published "SSLv3 MUST NOT be used". Web browsers are ditching it. Mozilla Firefox and Google Chrome have already done so.

Two excellent tools for checking protocol support in browsers are SSL Lab's client test and https://www.howsmyssl.com/ . The latter does not require Javascript, so you can try it from .NET's HttpClient:

// set proxy if you need to
// WebRequest.DefaultWebProxy = new WebProxy("http://localhost:3128");

File.WriteAllText("howsmyssl-httpclient.html", new HttpClient().GetStringAsync("https://www.howsmyssl.com").Result);

// alternative using WebClient for older framework versions
// new WebClient().DownloadFile("https://www.howsmyssl.com/", "howsmyssl-webclient.html");

The result is damning:

Your client is using TLS 1.0, which is very old, possibly susceptible to the BEAST attack, and doesn't have the best cipher suites available on it. Additions like AES-GCM, and SHA256 to replace MD5-SHA-1 are unavailable to a TLS 1.0 client as well as many more modern cipher suites.

That's concerning. It's comparable to 2006's Internet Explorer 7.

To list exactly which protocols a HTTP client supports, you can try the version-specific test servers below:

var test_servers = new Dictionary<string, string>();
test_servers["SSL 2"] = "https://www.ssllabs.com:10200";
test_servers["SSL 3"] = "https://www.ssllabs.com:10300";
test_servers["TLS 1.0"] = "https://www.ssllabs.com:10301";
test_servers["TLS 1.1"] = "https://www.ssllabs.com:10302";
test_servers["TLS 1.2"] = "https://www.ssllabs.com:10303";

var supported = new Func<string, bool>(url =>
{
    try { return new HttpClient().GetAsync(url).Result.IsSuccessStatusCode; }
    catch { return false; }
});

var supported_protocols = test_servers.Where(server => supported(server.Value));
Console.WriteLine(string.Join(", ", supported_protocols.Select(x => x.Key)));

I'm using .NET Framework 4.6.2. I found HttpClient supports only SSL 3 and TLS 1.0. That's concerning. This is comparable to 2006's Internet Explorer 7.

Update: It turns HttpClient does support TLS 1.1 and 1.2, but you have to turn them on manually at System.Net.ServicePointManager.SecurityProtocol. See https://stackoverflow.com/a/26392698/284795

I don't know why it uses bad protocols out-the-box. That seems a poor setup choice, tantamount to a major security bug (I bet plenty of applications don't change the default). How can we report it?