This is from Will Konsider
What is Flashback?
Flashback is the name for a malicious software program discovered in September 2011 that tried to trick users into installing it by masquerading as an installer for Adobe Flash. (Antivirus vendor Intego believes Flashback was created by the same people behind the MacDefender attack that hit last year.) While the original version of Flashback and its initial variants relied on users to install them, this new form is what’s called in the security business a drive-by download: Rather than needing a user to install it, Flashback uses an unpatched Java vulnerability to install itself.
If you visit a malicious (or unwillingly infected) website hosting Flashback, the program attempts to display a specially crafted Java applet. (We don’t yet know how many websites host Flashback.) If you have a vulnerable version of Java installed and enabled in your Web browser, the malicious code will infect your system and then install a series of components. Since Apple did not release an update for that vulnerable version of Java until April 3rd, many users were and are still susceptible.
After initial infection, Flashback pops open a Software Update window to try and obtain your administrative password, but it does so only to embed itself more deeply into your Mac. Even if you aren’t fooled at this point, you are still infected.
Once it succeeds in infecting your Mac, Flashback inserts itself into Safari and (according to F-Secure) appears to harvest information from your Web browsing activities, including usernames and passwords. It then sends this information to command-and-control servers on the Internet.
The significant thing is that, unlike almost all other Mac malware we’ve seen, Flashback can insinuate itself into your system if you merely visit an infected webpage and are using vulnerable software. You do not need to enter your administrative password or to manually install anything.
Am I at risk?
You are at risk if you meet four criteria:
1. You have Java installed on your Mac. One way to find out: Open Terminal and type java -version at the prompt. If you do have Java installed, you'll get a version number. It is installed by default on OS X 10.6 Snow Leopard, but not by OS X 10.7 Lion. (But is installed the first time you need to run it, which means most Macs likely have it).
2. You do not have the Java for OS X Lion 2012-001 (if you're running OS X Lion) or Java for Mac OS X 10.6 Update 7 installed (if you're running Snow Leopard) or you were infected before either of them was installed. Both of those updates install Java version 1.6.0_31; running that java -version command above will tell you if that's what you've got.
3. You allow Java applets to display in your browser. In Safari, go to Preferences > Security > Web Content and see if the Enable Java option is checked. You can turn that option off by unchecking it.
4. You do not have certain security tools installed on your Mac that Flashback checks for, including Little Snitch, Xcode, and a few anti-malware tools.
Antivirus vendors do not appear to have detected this particular version of Flashback for a few days after it appeared in the wild, though some vendors—including Intego—protected users with updates in late March. Malware often shares bits of code from earlier versions that may be detectable by antivirus products before those products have been specifically updated to catch newer versions, but such protection is hit-or-miss.
Silence is the scammer's best friend; knowledge is the scammer's worst enemy. 沈黙は詐欺師のよき友達、知識は詐欺師の天敵。Think globally, act globally.