- It must execute itself. It will often place its own code in the path of execution of another program.
- It must replicate itself. For example, it may replace other executable files with a copy of the virus infected file. Viruses can infect desktop computers and network servers alike.
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply to replicate themselves and make their presence known by presenting text, video, and audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
There a five recognized types of viruses:
- File infector viruses: File infector viruses infect program files. These viruses normally infect executable code, such as .com and .exe files. The can infect other files when an infected program is run from floppy, hard drive, or from the network. Many of these viruses are memory resident. After memory becomes infected, any noninfected executable that runs becomes infected. Examples of known file infector viruses include Jerusalem and Cascade.
- Boot sector viruses: Boot sector viruses infect the system area of a disk--that is, the boot record on floppy disks and hard disks. All floppy disks and hard disks (including disks containing only data) contain a small program in the boot record that is run when the computer starts up. Boot sector viruses attach themselves to this part of the disk and activate when the user attempts to start up from the infected disk. These viruses are always memory resident in nature. Most were written for DOS, but, all PCs, regardless of the operating system, are potential targets of this type of virus. All that is required to become infected is to attempt to start up your computer with an infected floppy disk Thereafter, while the virus remains in memory, all floppy disks that are not write protected will become infected when the floppy disk is accessed. Examples of boot sector viruses are Form, Disk Killer, Michelangelo, and Stoned.
- Master boot record viruses: Master boot record viruses are memory resident viruses that infect disks in the same manner as boot sector viruses. The difference between these two virus types is where the viral code is located. Master boot record infectors normally save a legitimate copy of the master boot record in an different location. Windows NT computers that become infected by either boot sector viruses or master boot sector viruses will not boot. This is due to the difference in how the operating system accesses its boot information, as compared to Windows 95/98. If your Windows NT systems is formatted with FAT partitions you can usually remove the virus by booting to DOS and using antivirus software. If the boot partition is NTFS, the system must be recovered by using the three Windows NT Setup disks. Examples of master boot record infectors are NYB, AntiExe, and Unashamed.
- Multi-partite viruses: Multi-partite (also known as polypartite) viruses infect both boot records and program files. These are particularly difficult to repair. If the boot area is cleaned, but the files are not, the boot area will be reinfected. The same holds true for cleaning infected files. If the virus is not removed from the boot area, any files that you have cleaned will be reinfected. Examples of multi-partite viruses include One_Half, Emperor, Anthrax and Tequilla.
- Macro viruses: These types of viruses infect data files. They are the most common and have cost corporations the most money and time trying to repair. With the advent of Visual Basic in Microsoft's Office 97, a macro virus can be written that not only infects data files, but also can infect other files as well. Macro viruses infect Microsoft Office Word, Excel, PowerPoint and Access files. Newer strains are now turning up in other programs as well. All of these viruses use another program's internal programming language, which was created to allow users to automate certain tasks within that program. Because of the ease with which these viruses can be created, there are now thousands of them in circulation. Examples of macro viruses include W97M.Melissa, WM.NiceDay and W97M.Groov.
Trojan Horses are impostors--files that claim to be something desirable but, in fact, are malicious. A very important distinction from true viruses is that they do not replicate themselves, as viruses do. Trojans contain malicious code, that, when triggered, cause loss, or even theft, of data. In order for a Trojan Horse to spread, you must, in effect, invite these programs onto your computers--for example, by opening an email attachment. The PWSteal.Trojan is a Trojan.
Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file. Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer to computer, so the entire document should be considered the worm. PrettyPark.Worm is a particularly prevalent example.
Virus hoaxes are messages, almost always sent by email, that amount to little more than chain letters. Some of the common phrases used in these hoaxes are:
- If you receive an email titled [email virus hoax name here], do not open it!
- Delete it immediately!
- It contains the [hoax name] virus.
- It will delete everything on your hard drive and [extreme and improbable danger specified here].
- This virus was announced today by [reputable organization name here].
- Forward this warning to everyone you know!
Most virus hoax warnings do not deviate far from this pattern. If you are unsure if a virus warning is legitimate or a hoax, visit the Security Response Hoaxes Web site.
Because of the publicity that viruses have received, it is easy to blame any computer problem on a virus. The following are not likely to be caused by a virus or other malicious code:
- Hardware problems. There are no viruses that can physically damage computer hardware, such as chips, boards, and monitors.
- The computer beeps at startup with no screen display. This is usually caused by a hardware problem during the boot process. Consult your computer documentation for the meaning of the beep codes.
- The computer does not register 640 K of conventional memory. This can be a sign of a virus, but it is not conclusive. Some hardware drivers such as those for the monitor or SCSI card can use some of this memory. Consult with your computer manufacturer or hardware vendor to determine if this is the case.
- You have two antivirus programs installed and one of them reports a virus. While this could be a virus, it can also be caused by one antivirus program detect the other program's signatures in memory. For additional information, see Should you run more than one antivirus program at the same time?
- You are using Microsoft Word and Word warns you that a document contains a macro. This does not mean that the macro is a virus.
- You are not able to open a particular document. This is not necessarily an indication of a virus. Try opening another document or a backup of the document in question. If other documents open correctly, the document may be damaged.
- The label on a hard drive has changed. Every disk is allowed to have a label. You can assign a label to a disk by using the DOS Label command of from within Windows.
For the most up-to-date information on viruses, visit the Symantec Security Response Web site.
To submit a file or disk that you suspect is infected with a virus, please see one of the following documents:
What can I do to prevent being infected by Virus?
- Do not open any files attached to an email from an unknown, suspicious or untrustworthy source.
- Do not open any files attached to an email unless you know what it is, even if it appears to come from a dear friend or someone you know. Some viruses can replicate themselves and spread through email. Better be safe than sorry and confirm that they really sent it.
- Do not open any files attached to an email if the subject line is questionable or unexpected. If the need to do so is there always save the file to your hard drive before doing so.
- Delete chain emails and junk email. Do not forward or reply to any to them. These types of email are considered spam, which is unsolicited, intrusive mail that clogs up the network.
- Do not download any files from strangers.
- Exercise caution when downloading files from the Internet. Ensure that the source is a legitimate and reputable one. Verify that an anti-virus program checks the files on the download site. If you're uncertain, don't download the file at all or download the file to a floppy and test it with your own anti-virus software.
- Update your anti-virus software regularly. Over 500 viruses are discovered each month, so you'll want to be protected. These updates should be at the least the products virus signature files. The McAfee Viruscan program installed on your PC is configured to automatically download the latest viruscan updates. To check if your Viruscan program is correctly configured to do this, review the configuration notes for Configuring Automatic Updates - VirusScan (For Laucala Campus).
- Back up your files on a regular basis. If a virus destroys your files, at least you can replace them with your back-up copy. You should store your backup copy in a separate location from your work files, one that is preferably not on your computer.
- When in doubt, always err on the side of caution and do not open, download, or execute any files or email attachments. Not executing is the more important of these caveats. Check with your product vendors for updates which include those for your operating system web browser, and email. One example is the security site section of Microsoft located at http://www.microsoft.com/security.
- If you are in doubt about any potential virus-related situation you find yourself in, you may report a virus to our virus team.
With all the hype, it is easy to believe that viruses lurk in every file, every email, every Web site. However, a few basic precautions can minimize your risk of infection. Practice safe computing and encourage everyone you know to do so as well.
- Do not leave a floppy disk in the floppy disk drive when you shut down or restart the computer.
- Write-protect your floppy disks after you have finished writing to them.
- Be suspicious of email attachments from unknown sources.
- Verify that attachments have been sent by the author of the email. Newer viruses can send email messages that appear to be from people you know.
- Do not set your email program to "auto-run" attachments.
- Obtain all Microsoft security updates.
- Back up your data frequently. Keep the (write protected) media in a safe place--preferably in a different location than your computer.