Wednesday, October 30, 2019

How to test SMTP using Opportunistic TLS with Powershell and grab the public certificate a SMTP server is using

Most email services these day employ Opportunistic TLS when trying to send Messages which means that wherever possible the Messages will be encrypted rather then the plain text legacy of SMTP.  This method was defined in RFC 3207 "SMTP Service Extension for Secure SMTP over Transport Layer Security" and  there's a quite a good explanation of Opportunistic TLS on Wikipedia https://en.wikipedia.org/wiki/Opportunistic_TLS .  This is used for both Server to Server (eg MTA to MTA) and Client to server (Eg a Message client like Outlook which acts as a MSA) the later being generally Authenticated.

Basically it allows you to have a normal plain text SMTP conversation that is then upgraded to TLS using the STARTTLS verb. Not all servers will support this verb so if its not supported then a message is just sent as Plain text. TLS relies on PKI certificates and the administrative issues that come around certificate management like expired certificates which is why I wrote this script. Essentially I wanted to see the Public Certificate that was in used by Recipient SMTP server and couldn't find any easy to use method to get it. Eg in a web browser you can always view a certificate to check its authenticity, but with SMTP there aren't a lot of good tools around for this, you can use Telnet to test in Plan text a SMTP server, but its not easy to retrieve the TLS public certificate from the server for inspection over Telnet (or using something like putty etc).

In PowerShell this is pretty easy back in 2006 I wrote a plain text SMTP test script https://gsexdev.blogspot.com/2006/09/doing-smtp-telnet-test-with-powershell.html and a variant to do alerting on verbs https://gsexdev.blogspot.com/2007/06/testing-smtp-verbs-and-sending-alert.html  so this is more just a modern version of this with the addition of using the System.Net.Security.SslStream class that supports creating the TLS connection and also allows you to easily export the recipient servers Public cert eg

        Write-Host("STARTTLS") -ForegroundColor Green
        $streamWriter.WriteLine("STARTTLS");
        $startTLSResponse = $streamReader.ReadLine();
        Write-Host($startTLSResponse)
        $ccCol = New-Object System.Security.Cryptography.X509Certificates.X509CertificateCollection
        $sslStream.AuthenticateAsClient($ServerName,$ccCol,[System.Security.Authentication.SslProtocols]::Tls12,$false);        
        $Cert = $sslStream.RemoteCertificate.Export([System.Security.Cryptography.X509Certificates.X509ContentType]::Cert);
        [System.IO.File]::WriteAllBytes($CertificateFilePath, $Cert);

I've created a small Powershell Script module that has two cmdlets the first is called Get-SMTPTLSCert which can be used to get the Public cert being used by the SMTP endpoint eg for Gmail you could use Get-SMTPTLSCert -ServerName smtp.gmail.com -Sendingdomain youdomain.com -CertificateFilePath c:\temp\gmailpubCer.cer By default this uses the client submission port 587 SMTP-MSA (Port 25 is often blocked from most locations) so its testing client(Message Submission Agent) to server (rather then server to server between to SMTP Mesage Transfer Agents). The Sending domain is required becuase most SMTP servers don't allows a empty helo/ehlo statement. I've also included a cmldet "Invoke-TestSMTPTLS" that does a test of the SMTP server and does Authentication if necessary. eg usually to use Port 587 you need to authenticate on the SMTP server. So in this instance once you have used the STARTTLS verb to upgrade the Plain text conversation to TLS you can then use the AUTH LOGIN verb to submit the username and password as base64 strings to authenticate eg this is what the Auth looks like in script
            $command = "AUTH LOGIN" 
            write-host -foregroundcolor DarkGreen $command
            $SSLstreamWriter.WriteLine($command) 
            $AuthLoginResponse = $SSLstreamReader.ReadLine()
            write-host ($AuthLoginResponse)
            $Bytes = [System.Text.Encoding]::ASCII.GetBytes($Credentials.UserName)
            $Base64UserName =  [Convert]::ToBase64String($Bytes)              
            $SSLstreamWriter.WriteLine($Base64UserName)
            $UserNameResponse = $SSLstreamReader.ReadLine()
            write-host ($UserNameResponse)
            $Bytes = [System.Text.Encoding]::ASCII.GetBytes($Credentials.GetNetworkCredential().password.ToString())
            $Base64Password = [Convert]::ToBase64String($Bytes)     
            $SSLstreamWriter.WriteLine($Base64Password)
            $PassWordResponse = $SSLstreamReader.ReadLine()
            write-host $PassWordResponse    

So the above code takes a PSCredential object and changes that into necessary SMTP verbs to authenticate.  So run against office365 this looks like


(The base64 values above decode to Useraname: and Password: )

This cmdlet doesn't actually send a Message it just invokes the envelope verbs and no Data verb is sent (where the MIME message would go). I've put a copy of the script on GitHub here https://github.com/gscales/Powershell-Scripts/blob/master/TLS-SMTPMod.ps1