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If you get “The remote certificate is invalid according to the validation procedure” exception while trying to establish SSL connection, most likely your’s server certificate is self-signed or you used incorrect host name to connect (Host name must match the name on certificate, for example ftp.example.com and example.com may point to the same server, but certificate is issued only to ftp.example.com and this is the address you should use).

Good news is that you can accept self-signed certificates using Ftp.dll FTP and FTPS .NET component.

First you need to subscribe to ServerCertificateValidate event.

Then you need to create ValidateCertificate method that validates the certificate (ignores certificate chain and name mismatch errors).

// C# version

using (Ftp client = new Ftp())
{
    // we will use custom validation
    client.ServerCertificateValidate +=
        new ServerCertificateValidateEventHandler(Validate);

    // Minimalistic version to accept any certificate:
    //client.ServerCertificateValidate += 
    //    (sender, e) => { e.IsValid = true; };

    client.ConnectSSL("ftp.example.org");
    client.Login("username", "password");

    foreach (FtpItem item in client.GetList())
    {
        if (item.IsFolder == true)
            Console.WriteLine("[{0}]", item.Name);
        else
            Console.WriteLine"{0}", item.Name);
    }
    client.Close();
}

private static void ValidateCertificate(
    object sender,
    ServerCertificateValidateEventArgs e)
{
    const SslPolicyErrors ignoredErrors =
        SslPolicyErrors.RemoteCertificateChainErrors |  // self-signed
        SslPolicyErrors.RemoteCertificateNameMismatch;  // name mismatch

    if ((e.SslPolicyErrors & ~ignoredErrors) == SslPolicyErrors.None)
    {
        e.IsValid = true;
        return;
    }
    e.IsValid = false;
}

You can download Ftp.dll FTP/FTPS component for .NET here.