Computer virus - Wikipedia, the free encyclopedia. A computer virus is a type of malicious software program (. When this replication succeeds, the affected areas are then said to be . The majority of active malware threats are actually trojan horse programs or computer worms rather than computer viruses. The term computer virus was a misnomer until it was coined by Fred Cohen in 1. However, not all viruses carry a destructive . The vast majority of viruses target systems running Microsoft Windows. In response, free, open- source antivirus tools have been developed, and an industry of antivirus software has cropped up, selling or freely distributing virus protection to users of various operating systems. The work of von Neumann was later published as the . In his essay von Neumann described how a computer program could be designed to reproduce itself. The Reaper program was created to delete Creeper. On its 5. 0th use the Elk Cloner virus would be activated, infecting the personal computer and displaying a short poem beginning . In 1. 98. 7, Fred Cohen published a demonstration that there is no algorithm that can perfectly detect all possible viruses. Computer programs can often coordinate patches to update a target program. A security patch is a change applied to an asset to correct the weakness described by a vulnerability. However, antivirus professionals do not accept the concept of . Any virus will by definition make unauthorised changes to a computer, which is undesirable even if no damage is done or intended. 8 Tips to Protect Your Computer From Viruses and Malware. You don’t have to pay for software to protect your computer or for an annual. You should suspect a computer virus if your. VirusTotal is a free virus, malware and URL online scanning service. Files and URLs can be sent via web interface upload, email API or making use of VirusTotal's browser extensions and desktop applications. Conficker Worm: Help Protect Windows. Antivirus software may also be obtained from trusted third parties such as the members of the Virus. On page one of Dr Solomon's Virus Encyclopaedia, the undesirability of viruses, even those that do nothing but reproduce, is thoroughly explained. Gunn under the title . A few years later, in February 1. Australian hackers from the virus- writing crew Boza created the VLAD virus, which was the first known virus to target Windows 9. In late 1. 99. 7 the encrypted, memory- resident stealth virus Win. Cabanas was released. The first one to appear on the Commodore Amiga was a boot sector virus called SCA virus, which was detected in November 1. Users would be required to click on a link to activate the virus, which would then send an email containing user data to an anonymous email address, which was later found to be owned by Larose. Data sent would contain items such as user IP address and email addresses, contacts, website browsing history, and commonly used phrases. In 2. 00. 8, larger websites used part of the Win. Operations and functions. Secondly, every computer virus must contain a routine to copy itself into the program which the search routine locates. A virus typically has a search routine, which locates new files or new disks for infection. Payload activity might be noticeable (e. This life cycle can be divided into four phases: Dormant phase. The virus program has managed to access the target user's computer or software, but during this stage, the virus does not take any action. The virus will eventually be activated by the . Not all viruses have this stage. The virus places a copy of itself into other programs or into certain system areas on the disk. The copy may not be identical to the propagating version; viruses often . Each infected program will now contain a clone of the virus, which will itself enter a propagation phase. The triggering phase can be caused by a variety of system events, including a count of the number of times that this copy of the virus has made copies of itself.
It can be destructive such as deleting files on disk, crashing the system, or corrupting files or relatively harmless such as popping up humorous or political messages on screen. Resident viruses overwrite interrupt handling code or other functions, and when the operating system attempts to access the target file or disk sector, the virus code intercepts the request and redirects the control flow to the replication module, infecting the target. In contrast, a non- memory- resident virus (or . This is one of the reasons that it is dangerous to open unexpected or suspicious attachments in e- mails. Some old viruses, especially on the MS- DOS platform, make sure that the . This approach does not fool antivirus software, however, especially those which maintain and date cyclic redundancy checks on file changes. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses. For example, the CIH virus, or Chernobyl Virus, infects Portable Executable files. Because those files have many empty gaps, the virus, which was 1 KB in length, did not add to the size of the file. In the 2. 01. 0s, as computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access. In Microsoft Windows operating systems, the NTFS file system is proprietary. This leaves antivirus software little alternative but to send a . Some viruses trick antivirus software by intercepting its requests to the Operating system (OS). A virus can hide by intercepting the request to read the infected file, handling the request itself, and returning an uninfected version of the file to the antivirus software. The interception can occur by code injection of the actual operating system files that would handle the read request. Thus, an antivirus software attempting to detect the virus will either not be given permission to read the infected file, or, the . Security software can then be used to check the dormant operating system files. Most security software relies on virus signatures, or they employ heuristics. In older versions of Windows, file cryptographic hash functions of Windows OS files stored in Windows. Unfortunately, the term is misleading, in that viruses do not possess unique signatures in the way that human beings do. A better term would be . Different antivirus programs will employ different search strings, and indeed different search methods, when identifying viruses. If a virus scanner finds such a pattern in a file, it will perform other checks to make sure that it has found the virus, and not merely a coincidental sequence in an innocent file, before it notifies the user that the file is infected. The user can then delete, or (in some cases) . Some viruses employ techniques that make detection by means of signatures difficult but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus. If the virus is encrypted with a different key for each infected file, the only part of the virus that remains constant is the decrypting module, which would (for example) be appended to the end. In this case, a virus scanner cannot directly detect the virus using signatures, but it can still detect the decrypting module, which still makes indirect detection of the virus possible. Since these would be symmetric keys, stored on the infected host, it is entirely possible to decrypt the final virus, but this is probably not required, since self- modifying code is such a rarity that it may be reason for virus scanners to at least . It is suspicious for a code to modify itself, so the code to do the encryption/decryption may be part of the signature in many virus definitions. This is called cryptovirology. At said times, the executable will decrypt the virus and execute its hidden runtimes, infecting the computer and sometimes disabling the antivirus software. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption module. In the case of polymorphic viruses, however, this decryption module is also modified on each infection. A well- written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using . To enable polymorphic code, the virus has to have a polymorphic engine (also called . See polymorphic code for technical detail on how such engines operate. For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus. The advantage of using such slow polymorphic code is that it makes it more difficult for antivirus professionals and investigators to obtain representative samples of the virus, because . This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection. Metamorphic code. Viruses that utilize this technique are said to be in metamorphic code. To enable metamorphism, a . A metamorphic virus is usually very large and complex. For example, W3. 2/Simile consisted of over 1. Software development strategies that produce large numbers of . For this reason, many viruses attach themselves to executable files that may be part of legitimate programs (see code injection). If a user attempts to launch an infected program, the virus' code may be executed simultaneously. This makes it possible to create a file that is of a different type than it appears to the user. For example, an executable may be created and named . This is due to Microsoft's large market share of desktop computer users. Many Windows users are running the same set of applications, enabling viruses to rapidly spread among Microsoft Windows systems by targeting the same exploits on large numbers of hosts. This difference has continued partly due to the widespread use of administrator accounts in contemporary versions like Windows XP. In 1. 99. 7, researchers created and released a virus for Linux. Unlike Windows users, most Unix users do not log in as an administrator, or . The Bliss virus never became widespread, and remains chiefly a research curiosity. Its creator later posted the source code to Usenet, allowing researchers to see how it worked. Some antivirus software blocks known malicious websites that attempt to install malware. Antivirus software does not change the underlying capability of hosts to transmit viruses.
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