MS05-012 is not categorized as virus, worm, Trojan or backdoor. It is a set of two vulnerabilities in COM (Component Object Model) and OLE (Object Linking and Embedding).
The vulnerability in COM affects Windows 2003/XP/2000 computers. It allows users who could log on to the computer and run programs, to elevate their privileges and take complete control of the affected computer. In order to do this, the attacking user must be able to log on to the vulnerable computer and run a specially crafted application. It is not possible to remotely exploit this vulnerability.
On the other hand, the vulnerability in OLE affects both operating systems and certain applications: Windows 2003/XP/2000, Exchange Server 5.0, 5.5, 2000 and 2003, and Office XP and 2003. If exploited successfully, it 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.
There are several methods to exploit the vulnerability in OLE, depending on which the vulnerable program under attack is, but the most frequent vector is an e-mail message or a file that contains a malicious OLE object.
If you have any of the vulnerable programs installed on your computer, it is recommendable to download and apply the security patch for this vulnerability. Click here to access the web page for downloading the patch.