Having written several times — and taken a lot of heat from PC users — about the relative security of Apple’s (AAPL) operating systems (See Why are there no Mac viruses), I feel obliged to report that Mac OS X is under what appears to be the most serious malware attack to date.

According to a report posted Wednesday by Dr. Web, a Russian anti-virus vendor that may have a stick in this fire, the security of more than 550,000 Macs around the world have been compromised by the Flashback trojan.

Dr. Web, which sells an antidote for the versions of Flashback that run on Microsoft (MSFT) Windows machines, describes the Mac variant like this:

JavaScript code is used to load a Java-applet containing an exploit… The exploit saves an executable file onto the hard drive of the infected Mac machine. The file is used to download malicious payload from a remote server and to launch it…It may get and run any executable specified in a directive received from a server.

Oracle (ORCL), which assumed responsibility for the the Java programming language when it acquired Sun Microsystems in 2010, released a fix for the vulnerability in February. According to Ars Technica‘s Jacqui Cheng, “Apple didn’t send out a fix until earlier this week, after news began to spread about the latest Flashback variant.”

The fix is part of the OS X software update called Java for OS X 2012-001. You’ll find it in Software Update in System Preferences. If you think one of your Macs is infected, F-Secure has instructions on how to use the Terminal application in your Utilities folder to find out:

UPDATE: According to an update posted on Twitter Wednesday afternoon by a Dr. Web malware analyst, the number of infected Macs has reached 600,000.