The flaw exposes Mac users to risks that are more familiar to Windows users: Visiting a malicious Web site using Apple's Safari Web browser could result in a rootkit, a backdoor or other malicious software being installed on the computer without the user noticing anything, experts said. Apple is developing a patch for the flaw, a company representative said. Word of the new vulnerability comes after the recent discovery of a Trojan horse and a worm that target Mac users. The operating system had not been in the security crosshairs previously.