According to Russian antivirus firm Dr. Web, over 600,000 Macs worldwide are infected with the Mac flashback trojan. The trojan can be installed if you visit a malicious website, and it will attempt to connect your Mac to a botnet.
Fifty-seven percent of infected machines are located in the US and 20 percent are in Canada. There are even 24 infected machines supposedly connected to the botnet from Apple's Cupertino campus. This trojan targets a Java vulnerability in Mac OS X that was recently patched.
It should be noted that in OS X 10.7 Lion, Java isn't included by default; only those who have deliberately installed it are potentially vulnerable to this exploit (or those running Snow Leopard or earlier OS X versions). If you installed it at some point but no longer have a reason to run Java, you should probably turn it off completely or at a minimum disable it in Safari.
F-Secure has provided a set of diagnostics that'll let you know if you have been infected. If you have the malware on your machine, F-Secure's page can walk you through the steps to remove the infection.
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[Via The Loop]