Polymorphic viruses. A polymorphic virus is a type of malware that has the ability to change or mutate its underlying code without changing its basic functions or features. This process helps a virus evade detection from many antimalware and threat detection products that rely on identifying signatures of malware; once a polymorphic virus’ signature is identified by a security product, the virus can then alter itself so that it will no longer be detected using that signature.
Resident viruses. This type of virus embeds itself in the memory of a system. The original virus program isn’t needed to infect new files or applications; even if the original virus is deleted, the version stored in memory can be activated when the operating system loads a specific application or function. Resident viruses are problematic because they can evade antivirus and antimalware software by hiding in the system’s RAM.
Rootkit viruses. A rootkit virus is a type of malware that installs an unauthorized rootkit on an infected system, giving attackers full control of the system with the ability to fundamentally modify or disable functions and programs. Rootkit viruses were designed to bypass antivirus software, which typically scanned only applications and files. More recent versions of major antivirus and antimalware programs include rootkit scanning to identify and mitigate these types of viruses.
System or boot-record infectors. These viruses infect executable code found in certain system areas on a disk. They attach to the DOS bootsector on diskettes and USB thumb drives or the Master Boot Record on hard disks. In a typical attack scenario, the victim receives storage device that contains a boot disk virus. When the victim’s operating system is running, files on the external storage device can infect the system; rebooting the system will trigger the boot disk virus. An infected storage device connected to a computer can modify or even replace the existing boot code on the infected system so that when the system is booted next, the virus will be loaded and run immediately as part of the master boot record. Boot viruses are less common now as today’s devices rely less on physical storage media.
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