The Mac OS X operating system — long touted as immune to viruses and hacking — has been compromised by a Trojan horse attack that installs a backdoor hacker tool, according to Mac security specialist Intego.

The Trojan program pops up an error message when Mac users visit certain booby-trapped Web sites, stating that a ‘missing plug-in’ must be installed before they can view the site. If the user clicks ‘OK’ to install the plug-in the backdoor remote control program is actually installed.

Once installed, the backdoor program lets cyber criminals remotely control the infected Mac and use it as a platform to broadcast spam or infect other computers.

The usual cautions and safety recommendations apply: Don’t install any software that is offered to you unsolicited, either via an e-mail or in a pop-up advisory at a Web site.