You might need to think twice about what the guys at the Apple store told you about Macs being impenetrable. More than 600,000 Apple computers have installed malware known as the Flashback Trojan, a Russian antivirus company said Thursday. The trojan first appeared last September, presenting itself as an Adobe Flash update. It disables certain Mac security features, enabling criminal hackers to control the machine. Apple has released a security update to guard against the malware, but users who haven’t installed it remain exposed.