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9.5.3 How Viruses Spread

There are several scenarios for distribution. Let us start with the classical one. Virgil writes his virus, inserts it into some program he has written (or stolen), and starts distributing the program, for example, by putting it on a shareware Web site. Eventually, somebody downloads the program and runs it. At this point there are several options. To start with, the virus probably infects more files on the hard disk, just in case the victim decides to share some of these with a friend later. It can also try to infect the boot sector of the hard disk. Once the boot sector is infected, it is easy to start a kernel-mode memory-resident virus on subsequent boots.

In addition, the virus can check to see if there are any floppy disks in the drives, and if so, infect their files and boot sectors. Floppy disks are a good target because they get moved from machine to machine much more often than hard disks. If a floppy disk boot sector is infected and that disk is later used to boot a different machine, it can start infecting files and the hard disk boot sector on that machine. In the past, when floppy disks were the main transmission medium for programs, this mechanism was the main way viruses spread.

Nowadays, other options are available to Virgil. The virus can be written to check if the infected machine is on a LAN, something that is very likely on a machine belonging to a company or university. The virus can then start infecting unprotected files on the servers connected to this LAN. This infection will not extend to protected files, but that can be dealt with by making infected programs act strangely. A user who runs such a program will likely ask the system administrator for help. The administrator will then try out the strange program himself to see what is going on. If the administrator does this while logged in as superuser, the virus can now infect the system binaries, device drivers, operating system, and boot sectors. All it takes is one mistake like this and all the machines on the LAN are compromised.

Often machines on a LAN have authorization to log onto remote machines over the Internet or a private corporate, or even authorization to execute commands remotely without logging in. This ability provides more opportunity for viruses to spread. Thus one innocent mistake can infect the entire company. To prevent this scenario, all companies should have a general policy telling administrators never to make mistakes.

Another way to spread a virus is to post an infected program to a USENET newsgroup or bulletin board system to which programs are regularly posted. Also possible is to create a Web page that requires a special browser plug-in to view, and then make sure the plug-ins are infected.

A different attack is to infect a document and then email it to many people or broadcast it to a mailing list or USENET newsgroup, usually as an attachment. Even people who would never dream of running a program some stranger sent them might not realize that clicking on the attachment to open it can release a virus on their machine. To make matters worse, the virus can then look for the user's address book and then mail itself to everyone in the address book, usually with a Subject line that looks legitimate or interesting, like

     Subject: Change of plans 
     Subject: Re: that last email 
     Subject: The dog died last night 
     Subject: I am seriously ill 
     Subject: I love you 

When the email arrives, the receiver sees that the sender is a friend or colleague, and thus does not suspect trouble. Once the email has been opened, it is too late. The ''I LOVE YOU'' virus that spread around the world in June 2000 worked this way and did a billion dollars worth of damage.

Somewhat related to the actual spreading of active viruses is the spreading of virus technology. There are groups of virus writers who actively communicate over the Internet and help each other develop new technology, tools, and viruses. Most of these are probably hobbyists rather than career criminals, but the effects can be just as devastating. One other category of virus writers is the military, which sees viruses as a weapon of war potentially able to disable an enemy's computers.

Another issue related to spreading viruses is avoiding detection. Jails have notoriously bad computing facilities, so Virgil would prefer avoiding them. If he posts the initial virus from his home machine he is running a certain risk. If the attack is successful, the police might track him down by looking for the virus message with the youngest timestamp, since that is probably closest to the source of the attack.

To minimize his exposure, Virgil might go to an Internet cafe in a distant city and log in there. He can either bring the virus on a floppy disk and read it in himself, or if the machines do not all have floppy disk drives, ask the nice young lady at the desk to please read in the file book.doc so he can print it. Once it is on his hard disk, he renames the file virus.exe and executes it, infecting the entire LAN with a virus that triggers two weeks later, just in case the police decide to ask the airlines for a list of all people who flew in that week. An alternative is to forget the floppy disk and get the virus from a remote FTP site. Or bring a laptop and plug it in to an Ethernet or USB port that the Internet cafe has thoughtfully provided for laptop-toting tourists who want to read their email every day.

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