MS08-021 is not categorized as virus, worm, Trojan or backdoor. It is a group of critical vulnerabilities in the Microsoft Windows graphics device interface (GDI) on Windows Server 2008/Vista/2003/XP/2000 computers, which allows arbitrary code to be remotely executed in the vulnerable computer.
Windows graphics device interface (GDI) enables applications to use graphics and formatted text on both the video display and the printer.
If exploited successfully, MS08-021 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.
This vulnerability is usually exploited by creating a specially crafted image file and hosting it in a website or sending it in an attached file via email. An attacker would have to entice users into visiting the website or running the attachment.
If you have a Windows Server 2008/Vista/2003/XP/2000 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.
Additionally, bear in mind that this security bulletin replaces a previous bulletin, called MS07-046.