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Antivirus, Anti-Virus, or AV Software

Antivirus, Anti-Virus, or AV Software is computer software used to prevent, detect and remove malicious computer viruses. Most software described as antivirus also works against other types of malware, such as malicious Browser Helper Objects (BHOs), browser hijackers, ransomware, keyloggers, backdoors, rootkits, trojan horses, worms, malicious LSPs, dialers, fraudtools, adware and spyware.  

A specific component of the Anti virus and anti-malware software commonly referred as the on-access or real-time scanner, hooks deep into the operating system's core or kernel functions in a manner similar to how certain malware itself would attempt to operate, though with the user's informed permission for protecting the system.

Any time the operating system accesses a file, the on-access scanner checks if the file is a 'legitimate' file or not. If the file is considered a malware by the scanner, the access operation will be stopped, the file will be dealt by the scanner in pre-defined way (how the Anti-virus program was configured during/post installation) and the user will be notified. This may considerably slow down the operating system depending on how well the scanner was programmed. The goal is to stop any operations the malware may attempt on the system before they occur, including activities which might exploit bugs or trigger unexpected operating system behavior. 

Anti-malware programs can combat malware in two ways 

1. They can provide real time protection against the installation of malware software on a computer. This type of malware protection works the same way as that of antivirus protection in that the anti-malware software scans all incoming network data for malware and blocks any threats it comes across. 

2. Anti-malware software programs can be used solely for detection and removal of malware software that has already been installed onto a computer. This type of anti-malware software scans the contents of the Windows registry, operating system files, and installed programs on a computer and will provide a list of any threats found, allowing the user to choose which files to delete or keep, or to compare this list to a list of known malware components, removing files that match. 

Real-time protection from malware works identically to real-time antivirus protection: the software scans disk files at download time, and blocks the activity of components known to represent malware. In some cases, it may also intercept attempts to install start-up items or to modify browser settings. Because many malware components are installed as a result of browser exploits or user error, using security software (some of which are anti-malware, though many are not) to "sandbox" browsers (essentially isolate the browser from the computer and hence any malware induced change) can also be effective in helping to restrict any damage done.

Computer Security, including protection from social engineering techniques, is commonly offered in products and services of antivirus software companies. This article discusses the software used for the prevention, detection, and removal of malware threats, rather than computer security implemented by software methods.

A variety of strategies are typically employed. Signature-based detection involves searching for known patterns of data within executable code. However, it is possible for a computer to be infected with new malware for which no signature is yet known; and malware is often modified to change its signature without affecting functionality. To counter such so-called zero-day threats, heuristics can be used.

One type of heuristic approach, generic signatures, can identify variants by looking for slight variations of known malicious code in files. Some antivirus software can also predict what a file will do by running it in a sandbox and analyzing what it does to see if it performs any actions which could be malicious.

Antivirus software has some drawbacks. It can impair a computer's performance. Inexperienced users can be lulled into a false sense of security when using the computer, considering themselves to be totally protected, and may have problems understanding the prompts and decisions that antivirus software presents them with. An incorrect decision may lead to a security breach. If the antivirus software employs heuristic detection, it must be fine-tuned to minimize misidentifying harmless software as malicious (false positive). 

Many users install antivirus software that can detect and eliminate known viruses when the computer attempts to download or run the executable (which may be distributed as an email attachment, or on USB flash drives, for example). Some antivirus software blocks known malicious web sites that attempt to install malware. Antivirus software does not change the underlying capability of hosts to transmit viruses. Users must update their software regularly to patch security vulnerabilities ("holes"). Antivirus software also needs to be regularly updated in order to recognize the latest threats. The German AV-TEST Institute publishes evaluations of antivirus software for Windows and Android.

Examples of Microsoft Windows anti virus and anti-malware software include the optional Microsoft Security Essentials for Windows, for real-time protection, the Windows Malicious Software Removal Tool (now included with Windows (Security) Updates on "Patch Tuesday", the second Tuesday of each month), and Windows Defender. Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use).

Some such free programs are almost as good as commercial competitors. Common security vulnerabilities are assigned CVE IDs and listed in the US National Vulnerability Database. Secunia PSI is an example of software, free for personal use, that will check a PC for vulnerable out-of-date software, and attempt to update it. Ransomware and phishing scam alerts appear as press releases on the Internet Crime Complaint Center noticeboard.

Other commonly used preventative measures include timely operating system updates, software updates, careful Internet browsing, and installation of only trusted software.

There are two common methods that an antivirus software application uses to detect viruses.

The first, and by far the most common method of virus detection is using a list of virus signature definitions. This works by examining the content of the computer's memory (its RAM, and boot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives, or USB flash drives), and comparing those files against a database of known virus "signatures".

Virus signatures are just strings of code that are used to identify individual viruses; for each virus, the anti-virus designer tries to choose a unique signature string that will not be found in a legitimate program. Different anti-virus programs use different "signatures" to identify viruses. The disadvantage of this detection method is that users are only protected from viruses that are detected by signatures in their most recent virus definition update, and not protected from new viruses (see "zero-day attack").

A second method to find viruses is to use a heuristic algorithm based on common virus behaviors. This method has the ability to detect new viruses for which anti-virus security firms have yet to define a "signature", but it also gives rise to more false positives than using signatures. False positives can be disruptive, especially in a commercial environment.

Recovery Strategies and Methods

One can also reduce the damage done by viruses by making regular backups of data (and the operating systems) on different media, that are either kept unconnected to the system (most of the time), read-only or not accessible for other reasons, such as using different file systems. This way, if data is lost through a virus, one can start again using the backup (which will hopefully be recent).

If a backup session on optical media like CD and DVD is closed, it becomes read-only and can no longer be affected by a virus (so long as a virus or infected file was not copied onto the CD/DVD). Likewise, an operating system on a bootable CD can be used to start the computer if the installed operating systems become unusable. Backups on removable media must be carefully inspected before restoration. The Gammima virus, for example, propagates via removable flash drives.

Computer Virus Removal

Many websites run by antivirus software companies provide free online virus scanning, with limited cleaning facilities (the purpose of the sites is to sell anti-virus products). Some websites like Google subsidiary VirusTotal.com, allow users to upload one or more suspicious files to be scanned and checked by one or more antivirus programs in one operation. Additionally, several capable antivirus software programs are available for free download from the Internet (usually restricted to non-commercial use). Microsoft offers an optional free antivirus utility called Microsoft Security Essentials, a Windows Malicious Software Removal Tool that is updated as part of the regular Windows update regime, and an older optional anti-malware (malware removal) tool Windows Defender that has been upgraded to an antivirus product in Windows 8. 

Some viruses disable System Restore and other important Windows tools such as Task Manager and Command Prompt. An example of a virus that does this is CiaDoor. Many such viruses can be removed by rebooting the computer, entering Windows safe mode with networking, and then using system tools or Microsoft Safety Scanner. System Restore on Windows can restore the registry and critical system files to a previous checkpoint. Often a virus will cause a system to hang, and a subsequent hard reboot will render a system restore point from the same day corrupt. Restore points from previous days should work provided the virus is not designed to corrupt the restore files and does not exist in previous restore points.

Operating System Reinstallation

Microsoft's System File Checker (improved in Windows 7 and later) can be used to check for, and repair, corrupted system files.

Restoring an earlier "clean" (virus-free) copy of the entire partition from a cloned disk, a disk image, or a backup copy is one solution, restoring an earlier backup disk image is relatively simple to do, usually removes any malware, and may be faster than disinfecting the computer, or reinstalling and reconfiguring the operating system and programs from scratch, as described below, then restoring user preferences.

Reinstalling the operating system is another approach to virus removal. It may be possible to recover copies of essential user data by booting from a live CD, or connecting the hard drive to another computer and booting from the second computer's operating system, taking great care not to infect that computer by executing any infected programs on the original drive. The original hard drive can then be reformatted and the OS and all programs installed from original media. Once the system has been restored, precautions must be taken to avoid reinfection from any restored executable files.

Quarantine is a function of antivirus software that isolates infected files on a computer's hard disk. Files put in quarantine are no longer capable of infecting their hosting system. 

  • allowed suspect files to beStandard executable were scanned, or, one could use up to twenty file matching patterns
    • Deleted
    • Moved to a quarantine area
    • Flagged in a report
  • Twenty exclusion patters were also available
  • Twenty directory paths could be included, or twenty excluded.

The 1990 version also allowed 

  • Background processing
  • Checking of executables and libraries as a file is opened
    • Timing of checks, e.g. if one opened a word file, WORD and all its libraries could be checked:
    • immediately
    • Every half an hour
    • once a day or every ten day, etc.

Quarantine allowed system managers to track all modifications of a selected files or file structures, hence Quarantine users also got early warnings of failing disks or disk interface cards.

Antivirus software itself usually runs at the highly trusted kernel level of the operating system to allow it access to all potential malicious process and files, creating a potential avenue of attack.

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