A computer virus is a self executable computer program that injects itself into other computer programs and thus reproduced. Classification as virus ‘refers to the dissemination and infection function.

Once started, it cannot be controlled by the user changes the status of the hardware, the operating system or the software do (harmful function). Computer viruses can be carried by the Creator desired or not desired functions the computer security compromise and to include malware.

The term computer virus is used colloquially for computer worms and Trojan horses used, as it is often mixed forms for users and the difference is barely noticeable.

Functioning

As its biological model uses a computer virus, the resources of its host computer and hurts him frequently. Also it increases mostly uncontrolled. By the virus writer installed malicious software or from errors in the virus, the virus can affect the host system or its programs in various ways, from more benign disorders or data loss to hardware damage.

Viruses need, as opposed to computer worms, a host program to their machine code to execute. Will host this program is called, is – according to the type of virus sooner or later – the virus is executed, which then redistribute themselves in uninfected programs or to perform its existing possibly harmful.

Nowadays, computer viruses have been almost entirely supplanted by worms, since nearly every computer on the Internet or local network is connected and active dissemination strategy of the worms in less time allows for greater distribution. Viruses are only in new niches (see below) are of importance.

Difference between virus and worm

Computer viruses and – worms spread both on computer systems, but they are partly based on completely different concepts and techniques.

A virus spreads by copying itself in uninfected files and adjusts so that the virus is to run when the host program is started. The infected files are normal executable files, libraries , scripts , documents with macros , or other executable content and boot sectors (even if the latter are usually not a representative of a file).

The proliferation of new systems by copying an infected host files to the new system by a user. It does not matter in which way these host file is copied: Formerly the main distribution channels removable media such as floppy disks , now there are computer networks (for example, via e-mail sent by FTP servers, web servers, or from sharing downloaded). There are also viruses that files in shared folders in local networks become infected when they have appropriate rights.

Unlike viruses, worms do not wait passively for disseminated by a user on a new system to be, but try to actively enter new systems. They need is for security problems on the target system, such as:

• Network services, the default passwords or no password use,
• Design and programming errors in network services,
• Design and programming errors in user programs that use network services (e.g. e-mail clients).

A worm can be like a virus to insert into another program file, and usually he tries but only in an inconspicuous place in the system to hide with a nondescript name and changed the target system so that is invoked at boot time, the worm (such as the Auto Start function in Microsoft Windows systems).

In colloquial speech, computer worms such as “will I Love You “often called viruses because the difference for users is often not apparent.

Degree of risk of different operating systems

The operating system has a major influence on how likely a viral infection is or how high the probability of a system-wide infection. Basically, all operating systems are vulnerable to allow a program to manipulate a different file. Whether security systems such as user rights systems are in place and used to be affected, the extent to which a virus can spread on a system.

Operating systems without any rights systems such as MS-DOS, MS-DOS-based Microsoft Windows – or Amiga systems are the most susceptible systems. If the user works only as administrator and thus engage the rights system of the operating system cannot, however, are more recent Microsoft Windows NT – Unix – and Unix-like systems such as Linux and Mac OS X is just as vulnerable.

Especially with Windows NT and underlying systems such as Windows 2000 or XP, the problem is that while a good user rights system is in place, this is not used by default to limit the rights of the user. One reason for this is that after the installation of some versions of Windows will automatically set up user accounts have administrator rights. Unlike Windows Vista, however, allow for the creation of a Standard account has not full administrator rights and using the User Account Control (UAC), the system is also protected. Most Linux distributions set at installation, a user account without administrative rights, so that the normal use of the computer initially limited rights are available, and only the special root account has administrator rights.

When a user works with a user account with limited rights, a virus can spread only to the files for which the user has the appropriate rights to modify. This usually means that system files can not be infected by the virus, as long as the administrator or system administrator privileges provided with services not call files the infected user. Possibly working on the same system user can also usually not be infected, as long as they. Not run an infected file the infected user or allow the rights of the infected user is to change the files from other users

Since Windows systems today have the widest distribution on PCs, they are currently the main target of virus writers. The fact that many Windows users work with accounts that have administrator rights, and lack of knowledge of safety practices in the relatively high number of inexperienced home makes Windows systems even more rewarding as a target of virus writers.

While for Windows systems over hundreds of thousands of viruses are known, the number of known viruses for Linux, and is the classic Mac OS significantly lower. In “the wild” but far less numerous viruses are observed, as theoretically known. The first virus for Apple’s Mac OS X operating system on 13 Published in February 2006 Forum a U.S. gossip site. Until then, the operating system of the Macintosh computer was completely unloaded as viruses and worms. The maker of Windows antivirus Sophos notes in his Security Report 2006 publicly stated that Mac OS X is more secure than Windows.

On Unix and Linux systems also ensure the high safety standards and the distribution of these systems still rare among end users that they currently for virus writers constitute a worthwhile goal and viruses “in the wild” do not occur in practice. It is different from computer worms. Unix or Linux systems because of the high market shares of Internet servers has become a frequent target of worm authors.

Universal prevention for all operating systems

Users should never unknown files or programs run from unreliable sources and generally exercise caution when opening files. This is especially true for files that were received by e-mail. Such files – even seemingly innocuous documents as images or PDF documents – can vulnerabilities in the associated applications enable various ways malicious programs. Therefore, their review is to recommend a current antivirus program.

Operating system and applications should be updated regularly and vendor-supplied service packs and patches / hotfixes are imported. It should be noted that it may take some time, be provided to patches.  Some operating systems simplify this procedure by automatically downloading and installing updates support. Some even support the specific download and install only those updates that resolve security-critical problems. In addition there is also the possibility of the service packs and hotfixes for Windows 2000 and Windows XP via “offline update” einzuspielen. This offline updates are especially recommended for new PCs, otherwise the PC might be infected already when you first connect to the Internet.

Built-in protective functions of the operating system should be utilized. This includes in particular, not as an administrator with full rights, but as a user with restricted rights to work, because he may not install software system-wide.

The automatic opening of files from the Internet, and automatically hide known file attachments should be disabled to avoid accidentally execute files that you would otherwise recognize as a disguised malware. Even through the autostart function for CD-ROMs and DVD -ROMs, programs upon insertion of such media and executed so that a system can be infected.

It is recommended that on most private computers preinstalled software from Microsoft to avoid or to configure more secure because they are usually configured so that it for the user to offer maximum comfort and safety is not the highest. Nor do they offer through their extremely high penetration a large attack surface. Especially Internet Explorer and Outlook Express are set out below. They are the most frequently attacked by pests applications because it is prevalent and in the default settings are vulnerable to attack. The currently most important alternatives to Internet Explorer is Firefox and Opera, as both promise much more security. The alternatives are Outlook Express, for example, Mozilla Thunderbird, Opera, or The Bat.

There are also computer viruses for non-Microsoft operating systems such as Symbian OS, Linux , Mac OS operating systems and the BSD series.

Because these viruses are not very common, they do not much danger to the user represent a reason for this is on the one hand, the extent of occurrence of these platforms (whose distribution was early 2009, about five percent),  so that virus writers these systems in the Historically, in much and the other for the malware has a significant difficulty in finding more victims of infection. Another technical reason is the explicit separation of rights of many other operating systems. With open-source operating systems is also the fact that there are many different distributions, which in turn places a limit for viruses.