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For a malicious program to accomplish its goals, it must be able to run without being detected, shut down, or deleted. When a malicious program is disguised as something normal or desirable, users may willfully install it without realizing it. This is the technique of the Trojan horse or trojan. In broad terms, a Trojan horse is any program that invites the user to run it, concealing harmful or malicious code. The code may take effect immediately and can lead to many undesirable effects, such as deleting the user's files or installing additional harmful software.
One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads from the Internet. When the user installs the software, the spyware is installed along with it. Spyware authors who attempt to act in a legal fashion may include an end-user license agreement that states the behavior of the spyware in loose terms, which users may not read or understand.
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A backdoor is a method of bypassing normal authentication procedures. Once a system has been compromised, one or more backdoors may be installed in order to allow easier access in the future. Backdoors may also be installed prior to malicious software, to allow attackers entry.
The idea has often been suggested that computer manufacturers preinstall backdoors on their systems to provide technical support for customers, but this has never been reliably verified. Recently it came to light that government agencies have been preinstalling backdoors on private computers purchased online. Backdoors secure remote access to a computer, while attempting to remain hidden from casual inspection. To install backdoors crackers may use Trojan horses, worms, Implants or other methods.
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Companion Viruses is another kind of viruses. When user computer infect by this sort of viruses, it will create another type file from an existing file in same directory (such as creating file.com from file.exe in the same folder), some companion viruses create file.exe from any folder.
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A worm is a program very similar to a virus; it has the ability to self-replicate and can lead to negative effects n your system. But they can be detected and eliminated by an antivirus software.
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When talking about Macro Viruses, we refer to viruses which infect macro of other applications such as Microsoft Word, Microsoft Excel. The viruses are written in a macro language and use it to distribute themselves. Macro viruses will run automatically when user open document. Usually this type of virus cause harmless to your computer, but instead they are annoying by automatically inserting undesired texts or symbols.
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Multipartite Viruses is type of viruses which infect user computer on both part boot sector and executable files and programs at the same time, with this condition, the viruses spread faster than boot sector or file infector alone.
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This type of viruses is similar to Resident Viruses by using replication of module. Besides that, Nonresident Viruses role as finder module which can infect to files when it found one (it will select one or more files to infect each time the module is executed).
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Polymorphic Virus is similar to encrypted viruses; it can infect files with an encrypted copy of itself. The viruses use difference technique to replicate themselves. Some polymorphic viruses are hardly to detect by antivirus software using virus signature based, because it do not remain any identical after replication.
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Resident Viruses or known as Memory Resident Viruses is malicious module. The viruses can replicate module and installing malicious code into computer memory (RAM). The viruses are commonly classified into two main categories: Fast Infectors and Slow Infectors.
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Stealth Viruses is some sort of viruses which try to trick anti-virussoftware by intercepting its requests to the operating system. It has ability to hide itself from some antivirus software programs. Therefore, some antivirus program cannot detect them.