SMTP Security

By default, an SMTP server attempts to make a TCP port 25 connection to your Exchange server via an anonymous connection. Anonymous does not mean that a user account set up in your Active Directory proxies the connection request, as is the case with the IIS Anonymous user account, IUSR_<machinename>. In the SMTP world, anonymous means that no user name or password is required for the remote SMTP service to make a port 25 connection. Hence, any SMTP server on the Internet can make, by default, a port 25 connection to your Exchange server.

To make SMTP more secure, you could require either Basic or Integrated Windows Authentication (IWA) before the SMTP Virtual Server (VS) could accept an inbound connection. But this configuration isn’t practical on the Internet because you can’t predict who will be connecting to your Exchange server in the future and thus can’t assume that the user has an appropriate user name and password to make a connection. Moreover, not many messaging administrators are interested in implementing such a security measure at their end. So even though an anonymous connection to port 25 on your Exchange server represents a vulnerability, it is one that must be managed using a different approach than removing anonymous connections.

How do you protect against these kinds of attacks? With Exchange Server 2007, you can use an Edge Transport server that offloads the security burden from your primary Exchange servers. You learn about implementing the Edge Transport server in Chapter 20, “Antivirus and Anti-Spam.” This chapter also discusses how the Edge Transport server can help improve the overall security of your Exchange infrastructure. However, more traditional ways of protecting Exchange also apply even when Edge Transport servers are used.

Perhaps the most common way to protect an Exchange infrastructure is through the use two firewalls. A dual firewall topology allows you to protect your internal Exchange servers while also filtering incoming e-mail against potential attacks. The area between the two firewalls is called the perimeter network (also known as DMZ or demilitarized zone). The philosophy is to put up a line of defense against potential attacks. Hence, you’re willing to sacrifice your Exchange servers in the perimeter network, but not willing to sacrifice your Exchange servers on the internal network. Because the Exchange servers in the perimeter network do not host any important information—no mailboxes or public folders—they can be both sacrificed during an attack and easily rebuilt. And because they act only as relay servers, they can be used to sanitize incoming e-mail over port 25.

Take a look at Figure 19-9. Note that there are three network levels. Starting from the top, each network becomes more trusted, with the External, or Internet, zone being completely untrusted. The perimeter network is more trusted as it resides behind at least one organizational firewall and generally houses servers that can be considered “expendable.” In this diagram, the external firewall has port 25 open in order to facilitate incoming SMTP traffic. Mail is routed to the Exchange Server 2007 Edge Transport server where it is processed for viruses, checked using various spam filters, and run through various incoming transport rules. Your external MX records must point to this Edge Transport server. There is another important note in this diagram. Note that the external firewall also provides the ability to scan incoming content for viruses and spyware. When possible, always run your e-mail through a similarly configured firewall even before that mail hits the Edge Transport server’s content-scanning engines. Many of today’s security appliances, such as the Cisco ASA and Sonicwall’s family of firewalls, provide this additional protection.

From a software perspective, also consider running Microsoft Forefront Security for Exchange Server. Forefront has the ability to scan every incoming message with up to five completely separate virus scanners. By instituting this multilayer security infrastructure, all incoming mail is scanned by many different virus scanning engines, some hardware-based and some software-based, which results in a much higher likelihood you will be protected against even the newest viruses.

Figure 19-9 One way to secure your Exchange infrastructure

However, even the best virus-scanning infrastructure on the planet does not always protect you. Think back to some of the major viruses in the last few years, which were able to spread worldwide very quickly, usually in a matter of hours. It is almost impossible for any antivirus company to get the virus, study it, write a definition for it, and then push out the new definition for that virus before it spreads worldwide. You can tell an Edge Transport server, however, to quarantine or delete any message that contains certain types of attachments and, in effect, block most viruses based on their type of content rather than on a comparison to a virus definition file.essent.

Note

Be aware of two issues regarding traditional antivirus servers. First, many products offered by the major antivirus vendors perform content scanning at the same time as the virus scanning. While there may be no problem with this method of scanning e-mail, be aware of a distinction between content scanning and antivirus scanning, which highlights the need to perform both types of scanning in the perimeter network, a capability enabled through the use of Exchange Server 2007’s Edge Transport server. Second, everyone may not be able afford to purchase everything required in order to achieve the configuration outlined in this chapter—namely, a separate Exchange server running Edge Transport as well as firewalls/security appliances that perform virus scanning functions. These ideas are presented to highlight the concepts being discussed. Other, less expensive (and potentially less secure) options include:

• Using a single firewall with multiple interfaces and creating a perimeter network using firewall rules
• Using a single firewall and running the Edge Transport server on the internal interface alongside your other Exchange servers
• Skipping the installation of the Edge Transport server and delivering mail directly to an internal Hub Transport server

Once scanned and approved, the e-mail is sent to an internal Hub Transport server. The internal Exchange Server 2007 Hub Transport server should be configured to accept inbound e-mail only from the perimeter network’s Edge Transport server. Inbound mail that has been approved by the Edge Transport server also rides on the standard SMTP TCP port 25, so you need to open this port on your internal firewall as well. To do this in the most secure way possible, create a firewall rule that only allows port 25 traffic specifically between the Edge Transport server and one of your internal Hub Transport servers. Then, secure the communication tunnel using IPsec, which is discussed further in Chapter 21. The internal Exchange server should also be running its own antivirus software, preferably from a vendor that is different from the one the servers are using in the perimeter network. The whole point of implementing this model is to ensure that port 25 traffic is as well protected as possible.

In order to use an Edge Transport server, subscribe the Edge Transport server to the Active Directory domain. The subscription process establishes one-way replication of recipient and configuration information from your Active Directory into an Active Directory Application Mode (ADAM) instance running on your Edge Transport server. Further, the Edge Subscription process creates the SMTP Send connectors required to enable mail flow from your Exchange servers to the Internet through an Edge Transport server. If you are using the recipient lookup or safe list aggregation features of the Edge Transport server, subscribe the Edge Transport server to the organization.

More Info

The complete process for installing, configuring, and subscribing the Edge Transport services is covered in Chapter 20, “Antivirus and Anti-Spam.”

No system is foolproof, but this dual firewall topology has multiple advantages:

• By passing incoming e-mail through the Edge Transport servers content filtering services, you filter for code types that virus scanners don’t.
• By passing your e-mail through a virus scanner, you do your best to ensure that all known viruses are cleaned out. Not passing your e-mail through an updated antivi-rus scanner after running it through a content scanner is unwise because older viruses might not be caught by the content scanner.
• By passing all of your outgoing e-mail through the Exchange Server 2007 Edge Transport server, the IP address (private or public) of the internal Exchange Server 2007 server does not need to be published in the public DNS records. This means that an attacker attempting to Telnet into your server is never able to reach it directly. Also, if you configure the internal Exchange Server 2007 server to accept e-mail only from DMZ-based Exchange servers, any attempts to make port 25 connections to the internal Exchange server from any other IP address will fail.

If a hacker decides to bring down your perimeter Exchange servers, you’ve really lost nothing of value other than your time in getting the servers functioning again. Your company might lose some money due to the inability to communicate via e-mail, but it hasn’t lost any current data. This is an important point. The server that hosts your data is the one most protected. And the ones most exposed do not host important data. If those servers are lost, at least all the business-critical data is saved on the internal Exchange Server 2007 server. For many companies, this is an acceptable level of risk to assume. This is the beginning stage of a defense that provides multiple layer of protection, starting with expendable services with the really important data protected in a variety of different ways.

As explained throughout this chapter, no answer is perfect, and this security scenario does have a few major holes, such as doing nothing to protect against messages sent to the Exchange server via Outlook Web Access. Port 25 is well protected, but port 80 access to your Exchange server is wide open. If you want to learn more about OWA, refer to Chapter 24, “Supporting Outlook Web Access.”

The second major hole in this model is one that cannot be plugged: messages are continuing to flow to your internal Exchange server. As long as a packet can reach your internal Exchange server, there is always the potential for harm. So remember the 80-percent rule: you can make your data only about 80-percent secure. But don’t let that discourage you from implementing appropriate security strategies.

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