Back Orifice 2000 (often shortened to BO2k) is a computer program designed for remote system administration. It enables a user to control a computer running the Microsoft Windows operating system from a remote location. The name is a pun on Microsoft BackOffice Server software.

Back Orifice 2000 debuted on July 10, 1999 at DEF CON 7. The original code was written by Dildog, a member of US hacker group CULT OF THE DEAD COW (cDc). It was the successor to the cDc's Back Orifice remote administration tool, released the previous year.

Whereas the original Back Orifice was limited to the Windows 95 and Windows 98 operating systems, BO2k also supports Windows NT, Windows 2000 and Windows XP.

Some BO2k client functionality has also been implemented for Linux-systems. In addition, BO2k was released as free software. As of 2005, BO2k is being actively developed.

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"The members of The Cult of the Dead Cow demonstrate the ease with which their software program, "Back Orifice 2000" can seize control of a Windows computer connected to the internet."