These are perhaps the most damaging of all types of software attack, and certainly they are the most difficult to detect and remove. A trojan (or trojan horse) is a type of program that runs invisibly on your computer, but allows a remote attacker to log in or extract information from your disks

Rootkits can also be used legitimately to provide privileged access to a computer by a trusted third party, or to recover lost passwords or other data, though this is something of a grey area, and is not generally considered good practice, due to the risk of abuse.

Recently, Sony used a rootkit to install mediaplayer software when a copy-protected music CD is used on a PC. The intention was that though the CD could be played in a normal CD audio player, when used on a computer (i.e. a device able to duplicate the disc) the CD would appear to have errors, which could only be corrected using the installed mediaplayer. Unfortunately, the player software was quickly broken into and used by attackers; Sony has since withdrawn CDs using this mechanism.