ZyXEL ZyWALL 5 UTM User Guide - Page 310

Chapter 15 Anti-Virus Screens 15.6 Technical Reference Types of Computer Viruses The following table describes some of the common computer viruses. Table 87 Common Computer Virus Types TYPE DESCRIPTION File Infector This is a small program that embeds itself in a legitimate program. A file infector is able to copy and attach itself to other programs that are executed on an infected computer. Boot Sector Virus This type of virus infects the area of a hard drive that a computer reads and executes during startup. The virus causes computer crashes and to some extend renders the infected computer inoperable. Macro Virus Macro viruses or Macros are small programs that are created to perform repetitive actions. Macros run automatically when a file to which they are attached is opened. Macros spread more rapidly than other types of viruses as data files are often shared on a network. E-mail Virus E-mail viruses are malicious programs that spread through e-mail. Polyrmorphic Virus A polymorphic virus (also known as a mutation virus) tries to evade detection by changing a portion of its code structure after each execution or self replication. This makes it harder for an anti-virus scanner to detect or intercept it. A polymorphic virus can also belong to any of the virus types discussed above. Computer Virus Infection and Prevention The following describes a simple life cycle of a computer virus. 1 A computer gets a copy of a virus from a source such as the Internet, e-mail, file sharing or any removable storage media. The virus is harmless until the execution of an infected program. 2 The virus spreads to other files and programs on the computer. 3 The infected files are unintentionally sent to another computer thus starting the spread of the virus. 4 Once the virus is spread through the network, the number of infected networked computers can grow exponentially. Types of Anti-Virus Scanner The section describes two types of anti-virus scanner: host-based and network-based. A host-based anti-virus (HAV) scanner is often software installed on computers and/or servers in the network. It inspects files for virus patterns as they are moved in and out of the hard drive. However, host-based anti-virus scanners cannot eliminate all viruses for a number of reasons: • HAV scanners are slow in stopping virus threats through real-time traffic (such as from the Internet). • HAV scanners may reduce computing performance as they also share the resources (such as CPU time) on the computer for file inspection. • You have to update the virus signatures and/or perform virus scans on all computers in the network regularly. 310 ZyWALL 5/35/70 Series User's Guide