Public Key Certificate Pinning and MITM attack

Question

So I was looking at an example of how to implement public key pinning for when your app connects to your web service and you want to have your app ensure it's really talking to your server.

http://blogs.windows.com/buildingapps/2015/10/13/create-more-secure-apps-with-less-effort-10-by-10/

They do a test HTTPS request and then check the thumbprint of the certificate:

private async Task DemoSSLRoot()
{
    // Send a get request to Bing
    HttpClient client = new HttpClient();
    Uri bingUri = new Uri("https://www.bing.com");
    HttpResponseMessage response = await client.GetAsync(bingUri);

    // Get the list of certificates that were used to validate the server's identity
    IReadOnlyList<Certificate> serverCertificates = response.RequestMessage.TransportInformation.ServerIntermediateCertificates;

    // Perform validation
    if (!ValidCertificates(serverCertificates))
    {
        // Close connection as chain is not valid
        return;
    }

    PrintResults("Validation passed\n");
    // Validation passed, continue with connection to service
}

private bool ValidCertificates(IReadOnlyList<Certificate> certs)
{
    // In this example, we iterate through the certificates and check that the chain contains
    // one specific certificate we are expecting
    for(int i=0; i<certs.Count; i++)
    {
        PrintResults("Cert# " + i + ": " + certs[i].Subject + "\n");
        byte[] thumbprint = certs[i].GetHashValue();

        // Check if the thumbprint matches whatever you are expecting
        // ‎d4 de 20 d0 5e 66 fc 53 fe 1a 50 88 2c 78 db 28 52 ca e4 74
        byte[] expected = new byte[] { 212, 222, 32, 208, 94, 102, 252, 83, 254, 26, 80, 136, 44, 120, 219, 40, 82, 202, 228, 116 };

        if (ThumbprintMatches(thumbprint, expected))
        {
            return true;
        }
    }

    return false;
}

Now my question is: With this approach, couldn't an attacker selectively allow your app to authenticate the first time to your web server and establish that it's trusted, and then immediately switch over to their malicious HTTPS server? It seems so, since only the initial "Hello World" request is validated...

Answer 1

During the initial TLS handshake, a "master secret" is created

The client and server then use the random numbers and PreMasterSecret to compute a common secret, called the "master secret". All other key data for this connection is derived from this master secret (and the client- and server-generated random values), which is passed through a carefully designed pseudorandom function.

As this secret is used throughout the communication a MiTM attempting to impersonate either the client or server mid-conversation would need to have the master secret (and likely other secrets) to encrypt and decrypt communication. But being that this secret is only known by the original client and server, a MiTM is impossible (or at least very difficult).