Home > Sample chapters

Microsoft Exchange Server 2007 Security Basics

Computer Viruses

This section expands on computer viruses in general and discusses some implications for viruses on Exchange Server 2007.

What Is a Virus?

A virus is a piece of code that attaches itself to other programs or files. When these files run, the code is invoked and begins replicating itself. The replication occurs over the network. Viruses can now exploit the vulnerabilities of nearly every platform.

Some viruses reside in memory after the original program is shut down. When other programs are executed, the virus attaches itself to these new programs until the computer is shut down or turned off. Some viruses have a “dormant” phase and appear only at certain times or when certain actions are performed.

There are many types of viruses. Some overwrite existing code or data. Others include the ability to recognize whether an executable file is already infected. Self-recognition is required if the virus is to avoid multiple infections of a single executable, which can cause excessive growth in size of infected executables and corresponding excessive storage space, contributing to the detection of the virus.

Resident viruses install themselves as part of the operating system upon execution of an infected host program. The virus remains resident until the system is shut down. Once installed in memory, a resident virus is available to infect all suitable hosts that are accessed.

A stealth virus is a resident virus that attempts to evade detection by concealing its presence in infected files. For example, a stealth virus might remove the virus code from an executable when it is read (rather than executed) so that an antivirus software package sees only the noncompromised form of the executable.

Computer viruses can spread by the use of e-mail and usually appear in e-mail attachments. If the virus can find its way into the messaging stream, it uses the client capability to send and receive e-mail to replicate itself quickly and do its damage as fast as possible.

An essential aspect of protecting your messaging system against viruses is user education. Users should learn to be guarded about which attachments they are allowed to open. Your information security policies should also outline the types of e-mails and attachments that users are allowed to open. For example, users should be forbidden to open attachments in two instances: when they were not expecting the attachments, and when the attachments arrive from unrecognizable aliases.

Finally, whenever possible, consider a centralized antivirus service that updates the distributed clients from a centrally managed server. Most such solutions provide you with ways to more granularly manage each client and proactively fix problems that may take place.


A Trojan (also known as a Trojan horse) is a malicious program embedded inside a normal, safe-looking program. The difference between a virus and a Trojan is that the Trojan is embedded and the virus is attached to the file or executable.

When the normal program runs, the malicious code runs as well and can cause damage or steal critical information. An example of a Trojan is a word-processing program that, when executed, allows the user to compose a document while, in the background, malicious code is running that deletes files or destroys other programs.

Trojans generally are spread through e-mail or worms, which are programs that run by themselves. The damage that Trojans can cause is similar to that of a virus: from nominal to critical. Trojans are particularly frightening because in most cases, users are unaware of the damage the Trojan is causing. The malicious work is being masked by the Trojan effect of the program.


As just mentioned, worms are programs that run by themselves. They do not embed or attach themselves to other programs nor do they need to do this to replicate. They can travel from computer to computer across network connections and are self-replicating. Worms might have portions of themselves running on many different computers, or the entire program might run on a single computer. Typically, worms do not change other programs, although they might carry other code that does.

The first network worms were intended to perform useful network management functions by taking advantage of operating system properties. Malicious worms exploit system vulnerabilities for their own purposes. Release of a worm usually results in brief outbreaks, shutting down entire networks.

The damage that worms can cause, like Trojans and viruses, ranges from the nominal to the critical. The type and extent of damage must be assessed individually for each worm. However, worms can install viruses and Trojans that then run their own code.

An attack that combines a worm, Trojan, and/or virus can be a very difficult attack to survive without significant damage. The impact of viruses, Trojans, and worms on your messaging system and network should not be underestimated. Because they use e-mail to exploit system vulnerabilities, installing antivirus software is simply not enough. You must also ensure that known vulnerabilities in all your operating systems are updated. Don’t focus only on your servers. Every device should be updated with the most recent updates from each vendor as soon as possible. Most environments will want to test these updates before installing them. But after they have been tested, install them.