MS06-021 is not categorized as virus, worm, Trojan or backdoor. It is a group of critical vulnerabilities in Internet Explorer on Windows 2003/XP/2000/Me/98 computers, which allows arbitrary code to be remotely executed in the vulnerable computer. Affected versions of Internet Explorer are 5.01 and 6.
If exploited successfully, MS06-021 allows hackers to gain remote control of the affected computer with the same privileges as the logged on user. If this user had administrator rights, the hacker could take complete control of the system: create, modify or delete files, install programs, create new user accounts, etc. On the other hand, other vulnerabilities allow information disclosure or Internet Explorer address bar spoofing.
These vulnerabilities are usually exploited by creating a specially crafted web page and enticing users into accessing it.
If you have a Windows 2003/XP/2000/Me/98 computer, it is recommended to download and apply the security patch for this vulnerability. Click here to access the web page for downloading the patch. However, provided that this is a cumulative patch, make sure that you download the latest security patch available.