Problem

After the installation of the October 8, 2019 — KB4517389 or KB4520003 or KB4519998 or KB4519990 update (depending on OS version), all TLS encrypted data connections to the affected FTP servers fail.

Error you may see is: “TLS session of data connection has not resumed or the session does not match the control connection”

Detailed explanation

In FTP protocol, data connection does not directly authenticate the client.

Client uses control connection to authenticate, then it established data connection using PASV command followed by the STOR (upload) or RETR (download) command.

The server opens a port and waits for the client to connect to it and upload/download files.

An attacker could figure out the port the server listens to, connect to it before the client, and upload a piece of malware.

TLS session resumption prevents this. It acts as a form of authentication. If the TLS session of the data connection matches the session of the control connection, both the client and the server have the guarantee, that the data connection is genuine. Any mismatch in sessions indicates a potential attack.

Ftp.dll library uses .NET’s SslStream that relies on Schannel (Microsoft Secure Channel – a security package that facilitates the use of Secure Sockets Layer (SSL) and/or Transport Layer Security (TLS))

Cause

The KB4517389 addresses the following issue:

“Addresses an issue in security bulletin CVE-2019-1318 that may cause client or server computers that don’t support Extended Master Secret (EMS) RFC 7627 to have increased connection latency and CPU utilization. This issue occurs while performing full Transport Layer Security (TLS) handshakes from devices that don’t support EMS, especially on servers. EMS support has been available for all the supported versions of Windows since calendar year 2015 and is being incrementally enforced by the installation of the October 8, 2019 and later monthly updates.”

It looks like Schannel stared enforcing EMS. If the server runs a TLS stack which is not compatible with this change, the FTP data connection fails.

OpenSSL, which is used by most servers, supports EMS since version 1.1.0 (released 25th August 2016).

Affected Servers

• All FTP servers using OpenSSL older than version 1.1.0
• FileZilla Server (all versions). The latest version uses an insecure/outdated OpenSSL version 1.0.2.11 from 2017.

Solution

Contact the server administrator, explain the situation and and request an upgrade of the FTP server software and of the installed OpenSSL version.

As a temporary workaround, the KB4517389 (or equivalent for non-Windows 10 machines) can be uninstalled.

As a temporary workaround on FileZilla server you can go to “FileZilla Server Interface/Edit/Settings/Ftp over TLS setting” and uncheck “Require TLS resumption on data connection when using FTP over TLS”:

There are two main content types used in all HTML emails: text/plain and text/html MIME types.

You should always include a plain text version of your email that closely matches to the HTML version of your email. Mail.dll will generate (extract) plain text automatically, if you provide HTML text only.

Usually Apple watch displays only the plain text part of your email. In most cases it considers the HTML too complicated (e.g. external images are referenced), so it shows the plain text version instead.

External image is an image that isn’t embedded in the email, using cid: protocol, but loaded from a remote HTTP server using a standard <img src=’…’ /> tag.

You can use ‘text/watch-html’ type to send a limited HTML version of your message to Apple Watch users, resulting in rich text-like messages on Apple Watch devices.

Here’s how to create and add such MIME entity using Mail.dll:

MailBuilder builder = new MailBuilder();
builder.Subject = "Apple Watch Example";
builder.From.Add(new MailBox("alice@example.com"));
builder.To.Add(new MailBox("bob@example.com"));

builder.Html = "This is <strong>HTML<strong> text.";
builder.Text = "Plain text.";

MimeText appleWatchText = new MimeFactory().CreateMimeText();
appleWatchText.ContentType = ContentType.Parse("text/watch-html");
appleWatchText.Text = "This is <strong>Watch HTML<strong> text.";

builder.Alternatives.Add(appleWatchText);

IMail mail = builder.Create();

Please have in mind that apple watch supports limited version of HTML only.

Using the above code will create an email with following content:

Content-Type: multipart/alternative;
 boundary="----=_NextPart_19511516.440335455040"
MIME-Version: 1.0
Date: Tue, 02 Jul 2019 15:38:53 +0200
Message-ID: <d65fcc07-c988-48e1-a466-166e18998d02@mail.dll>
Subject: Apple Watch Example
From: <alice@example.com>
To: <bob@example.com>

------=_NextPart_19511516.440335455040
Content-Type: text/plain;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

Plain text.
------=_NextPart_19511516.440335455040
Content-Type: text/html;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

This is <strong>HTML<strong> text.
------=_NextPart_19511516.440335455040
Content-Type: text/watch-html;
 charset="utf-8"
Content-Transfer-Encoding: 7bit

This is <strong>Watch HTML<strong> text.
------=_NextPart_19511516.440335455040--

Entire sample, including sending process:

MailBuilder builder = new MailBuilder();
builder.Subject = "Apple Watch Example";
builder.From.Add(new MailBox("alice@example.com"));
builder.To.Add(new MailBox("bob@example.com"));

builder.Text = "Plain text";
builder.Html = "This is <strong>HTML<strong> text.";

MimeText appleWatchText = new MimeFactory().CreateMimeText();
appleWatchText.ContentType = ContentType.Parse("text/watch-html");
appleWatchText.Text = "This is <strong>Watch HTML<strong> text.";

builder.Alternatives.Add(appleWatchText);

IMail mail = builder.Create();

// Send the message
using (Smtp smtp = new Smtp())
{
    smtp.Connect("server.example.com");   // or ConnectSSL for SSL
    smtp.UseBestLogin("user", "password"); // remove if not needed

    smtp.SendMessage(mail);

    smtp.Close();
}

By default most systems allow SSL 3.0, TLS 1.0, 1.2 and 1.2 to be used, when connecting using SMTP client.

TLS 1.2 is the most secure version of SSL/TLS protocols. It is easy to force the connection to use it. All you need to do is to set Smtp.SSLConfiguration.EnabledSslProtocols property to SslProtocols.Tls12:

// C#

using (Smtp smtp = new Smtp())
{
    smtp.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12;

    smtp.ConnectSSL("smtp.example.com");

    smtp.UseBestLogin("user","password");

    // ... 

    smtp.Close();
}

' VB.NET

Using smtp As New Smtp()
	smtp.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12

	smtp.ConnectSSL("smtp.example.com")

	smtp.UseBestLogin("user@example.com", "password")

	'...

	smtp.Close()
End Using

For explicit SSL/TLS, code is almost the same. You first connect to non-secure port and secure the connection using Smtp.StartTLS command:

// C#

using (Smtp smtp= new Smtp())
{
    smtp.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12;

    smtp.Connect("smtp.example.com");
    smtp.StartTLS();

    smtp.UseBestLogin("user@example.com","password");

    // ... 
    
    smtp.Close();
}

' VB.NET

Using smtp As New Smtp()
	smtp.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12

	smtp.Connect("smtp.example.com")
	smtp.StartTLS()

	smtp.UseBestLogin("user@example.com", "password")

	'...

	smtp.Close()
End Using

To use TLS 1.2 at least .NET Framework 4.5+ must be installed on your machine and you application should target .NET 4.5+.

It is possible to use TLS 1.2 in applications targeting earlier .NET framework versions, but 4.5 must be installed on the machine. After you have .NET 4.5 installed, your 2.0-4.0 apps will use the 4.5 System.dll and you can enable TLS 1.2 using this code:

// C#

imap.SSLConfiguration.EnabledSslProtocols = (SecurityProtocolType)3072;

By default most systems allow SSL 3.0, TLS 1.0, 1.2 and 1.2 to be used, when connecting using POP3 client.

TLS 1.2 is the most secure version of SSL/TLS protocols. It is easy to force the connection to use it. All you need to do is to set Pop3.SSLConfiguration.EnabledSslProtocols property to SslProtocols.Tls12:

// C#

using (Pop3 pop3 = new Pop3())
{
    pop3.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12;

    pop3.ConnectSSL("pop.example.com");

    pop3.UseBestLogin("user","password");

    // ... 

    pop3.Close();
}

' VB.NET

Using pop3As New Pop3()
	pop3.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12

	pop3.ConnectSSL("pop.example.com")

	pop3.UseBestLogin("user@example.com", "password")

	'...

	pop3.Close()
End Using

For explicit SSL/TLS, code is almost the same. You first connect to non-secure port and secure the connection using Pop3.StartTLS command:

// C#

using (Pop3 pop3 = new Pop3())
{
    pop3.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12;

    pop3.Connect("pop.example.com");
    pop3.StartTLS();

    pop3.UseBestLogin("user@example.com","password");

    // ... 
    
    pop3.Close();
}

' VB.NET

Using pop3 As New Pop3()
	pop3.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12

	pop3.Connect("pop.example.com")
	pop3.StartTLS()

	pop3.UseBestLogin("user@example.com", "password")

	'...

	pop3.Close()
End Using

To use TLS 1.2 at least .NET Framework 4.5+ must be installed on your machine and you application should target .NET 4.5+.

It is possible to use TLS 1.2 in applications targeting earlier .NET framework versions, but 4.5 must be installed on the machine. After you have .NET 4.5 installed, your 2.0-4.0 apps will use the 4.5 System.dll and you can enable TLS 1.2 using this code:

// C#

pop3.SSLConfiguration.EnabledSslProtocols = (SecurityProtocolType)3072;

We will be conducting planned maintenance to ordering system next week:

Tuesday, May 7, 2019 from From 5:00 AM CDT/12:00 PM CEST, lasting approximately 6 hours.

During the planned maintenance, the system will continue to take orders. However customers may see temporary delays in fulfillment and order confirmation emails.

Once the maintenance is finished, we expect all functionality to resume; orders will be processed, and order confirmation emails will be sent to customers.