So you thought Macs couldn't fall prey to hackers? Think again. A Trojan horse is entering through the back door of Apple computers with a nasty payload -- and it hit more than a half-million users before Apple slammed it shut.

The Flashback Trojan infects computers running Mac OS X. The massive botnet is now using upward of 550,000 infected Macs, most of which are reportedly in the United States and Canada, according to Russian anti-virus vendor Dr. Web.

Here's how victims are getting infected: the Trojan is redirecting Mac users to a bogus site from a compromised resource or via a traffic distribution system. Then, Java code is used to load a Java applet that contains the exploit, Dr. Web explained. News reports indicate links to more than 4 million compromised pages were on Google at the end of March.

Target On Your Mac?

We asked Graham Cluley, senior security Relevant Products/Services analyst at Sophos, about his thoughts on the latest attack Relevant Products/Services against the Apple platform.

"Undoubtedly more and more Mac users will be targeted by malware," Cluley said. "Although there is much more malware for Windows than Mac, that's not to say that Mac malware doesn't exist."

Why so much focus on Mac when they have so little market share? Because, Cluley says, Mac market share is growing and, unfortunately, many Mac users have been far too laid-back when it comes to security, incorrectly believing that they're somehow immune to attacks.

"Inevitably, some cybercriminals will target Mac users because they are a 'soft target' which hasn't protected itself as well as Windows users," Cluley said. "Run an anti-virus, keep your Mac patched with the latest security updates. Luckily there are free anti-virus products for Mac home users." Sophos has a free anti-virus product on its Web site.

Apple has issued a patch that fixes the vulnerability for Snow Leopard and Lion users. But Cluley is pondering whether it's really worth it for Mac users to have Java installed on their computer. As he sees it, having Java on a PC or Mac may let consumers run some archaic applications, but it can also dramatically widen the attack surface hackers can exploit.

"My advice is that if you have no real need for Java, remove it," Cluley said. "The latest version of Mac OS X -- known as Lion -- unlike earlier editions, does not include Java by default, meaning users are not at risk unless they have subsequently installed the software."

From Apple to Twitter

In other security news this week, Symantec is exposing abusive tweets. Symantec reports that scammers are taking advantage of tweets in a way that is similar to how they rely on Internet search engines -- they create tweets that include keywords in them.

This is somewhat similar to how they design Web sites in order to gain visibility for their sites. In order for these sites to appear in search results, scammers need to make the effort to boost visibility, which is known as search engine optimization," Joji Hamada of Symantec wrote in the company blog.

"But tweets may require less effort by the scammers to get users to come across them: new tweets should appear at the top in the search results list for the most recent tweets. However, accounts engaging in automation, spam, and other violations of the Twitter Rules may be investigated for abuse."