C# Ignore certificate errors?

I am getting the following error during a web service request to a remote web service:

Could not establish trust relationship for the SSL/TLS secure channel. ---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.

Is there anyway to ignore this error, and continue?

It seems the remote certificate is not signed.

The site I connect to is www.czebox.cz - so feel free to visit the site, and notice even browsers throw security exceptions.

Add a certificate validation handler. Returning true will allow ignoring the validation error:

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

Allowing all certificates is very powerful but it could also be dangerous. If you would like to only allow valid certificates plus some certain certificates it could be done like this.

.Net core:

using (var httpClientHandler = new HttpClientHandler())
{
    httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, sslPolicyErrors) => {
        if (sslPolicyErrors == SslPolicyErrors.None)
        {
            return true;   //Is valid
        }

        if (cert.GetCertHashString() == "99E92D8447AEF30483B1D7527812C9B7B3A915A7")
        {
            return true;
        }
        return false;
    };
    using (var httpClient = new HttpClient(httpClientHandler))
    {
        var httpResponse = httpClient.GetAsync("https://example.com").Result;
    }
}

.Net framework:

System.Net.ServicePointManager.ServerCertificateValidationCallback += delegate (
    object sender,
    X509Certificate cert,
    X509Chain chain,
    SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
    {
        return true;   //Is valid
    }

    if (cert.GetCertHashString() == "99E92D8447AEF30483B1D7527812C9B7B3A915A7")
    {
        return true;
    }

    return false;
};

Update:

How to get cert.GetCertHashString() value in Chrome:

Click on Secure or Not Secure in the address bar.

Then click on Certificate -> Details -> Thumbprint and copy the value. Remember to do cert.GetCertHashString().ToLower().

IgnoreBadCertificates Method:

//I use a method to ignore bad certs caused by misc errors
IgnoreBadCertificates();

// after the Ignore call i can do what ever i want...
HttpWebRequest request_data = System.Net.WebRequest.Create(urlquerystring) as HttpWebRequest;

/*
and below the Methods we are using...
*/

/// <summary>
/// Together with the AcceptAllCertifications method right
/// below this causes to bypass errors caused by SLL-Errors.
/// </summary>
public static void IgnoreBadCertificates()
{
    System.Net.ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(AcceptAllCertifications);
}  

/// <summary>
/// In Short: the Method solves the Problem of broken Certificates.
/// Sometime when requesting Data and the sending Webserverconnection
/// is based on a SSL Connection, an Error is caused by Servers whoes
/// Certificate(s) have Errors. Like when the Cert is out of date
/// and much more... So at this point when calling the method,
/// this behaviour is prevented
/// </summary>
/// <param name="sender"></param>
/// <param name="certification"></param>
/// <param name="chain"></param>
/// <param name="sslPolicyErrors"></param>
/// <returns>true</returns>
private static bool AcceptAllCertifications(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certification, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors sslPolicyErrors)
{
    return true;
} 

The reason it's failing is not because it isn't signed but because the root certificate isn't trusted by your client. Rather than switch off SSL validation, an alternative approach would be to add the root CA cert to the list of CAs your app trusts.

This is the root CA cert that your app currently doesn't trust:

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

You can decode and view this certificate using

this certificate decoder or another certificate decoder

Bypass SSL Certificate....

        HttpClientHandler clientHandler = new HttpClientHandler();
        clientHandler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => { return true; };

        // Pass the handler to httpclient(from you are calling api)
        var client = new HttpClient(clientHandler)