Computer Viruses The Nasty Truth

Computer Viruses The Nasty Truth

Computer Viruses: the​ Nasty Truth
The term,​ virus,​ in​ computer technology,​ refers to​ a​ self replicating application that spreads by making copies of​ itself by inserting into other programs,​ other executables or​ documents,​ and when executed begins to​ perform harmful actions on​ the​ system .​
All computer viruses are deliberately created,​ not always malicious and some of​ them may be benign and simply annoying.
Non-Memory Resident and Memory Resident Viruses:
Non-Memory resident viruses,​ when they are executed,​ immediately look for other hosts that can be infected .​
When they infect these targets,​ they transfer control to​ the​ application program they infected .​
a​ non-resident virus has a​ finder module and a​ replication module .​
The finder module,​ once it​ finds a​ new file to​ infect,​ calls upon the​ replication module to​ infect that file.
Memory-Resident virus stays in​ the​ memory and do not look for hosts to​ infect when they are executed .​
It stays active in​ the​ background after its host program is​ terminated,​ and infects files as​ soon as​ they are opened or​ accessed by other programs or​ the​ operating system .​
It does have the​ replication module like the​ non-memory resident virus,​ but without the​ finder module.
Types of​ Computer Viruses:
File Viruses: These types of​ viruses are the​ most common,​ and mostly infect open files and program libraries on​ an​ operating system .​
The virus functions by inserting itself into a​ host file,​ modifies it​ in​ such a​ way that the​ virus is​ executed when the​ file is​ opened .​
They are also known as​ left viruses .​
Today,​ there are known viruses infecting all kinds of​ executables of​ standard DOS: batch command files (BAT),​ loadable drivers (SYS,​ including special purpose files IO.SYS and MS- DOS.SYS) and binary executables (EXE,​ COM) .​
There are also viruses targeting executables of​ other operating systems - Windows 3.x,​ Windows95/NT,​ OS/2,​ Macintosh,​ Unix,​ including the​ VxD drivers of​ Windows 3.x and Windows95.
Macro viruses: Macros are used in​ most word processing programs such as​ Microsoft Office in​ order to​ automate or​ simplify recurring tasks in​ documents .​
Macro viruses are those viruses that use the​ application's own macro programming language to​ distribute themselves,​ in​ which an​ unwanted sequence of​ actions is​ performed automatically when the​ application is​ started or​ something else triggers it .​
These macro viruses may inflict damage to​ the​ document or​ to​ other computer software but are relatively harmless,​ and are often spread as​ an​ e-mail virus.
Boot Viruses: These were one of​ the​ most common viruses prevalent during the​ early and mid 1990s,​ when the​ use of​ diskettes was popular .​
These viruses infect or​ substitute their own code for either the​ DOS boot sector or​ the​ Master Boot Record (MBR),​ which controls the​ boot sequence of​ the​ PC .​
The MBR is​ executed every time a​ computer is​ booted so the​ virus will also be loaded into memory on​ every startup and spreads to​ every disk that the​ system reads .​
They are typically very difficult to​ remove,​ and most antivirus programs cannot clean the​ MBR while Windows is​ running .​
So,​ bootable antivirus disks are needed to​ fix boot sector viruses.
Script viruses: They are a​ division of​ file viruses,​ written in​ a​ variety of​ script languages such as​ VBS,​ JavaScript,​ BAT,​ PHP,​ HTML etc .​
They can form a​ part of​ multi-component viruses or​ infect other scripts such as​ Windows or​ Linux command and service files .​
If the​ file format,​ such as​ HTML,​ allows the​ execution of​ scripts,​ they can infect it.

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