A Trojan recently made the rounds for Mac that appeared to be an update for Flash, but was actually a piece of malicious software called Flashback.A. Apple has since updated XProtect to block this dangerous piece of software from infecting your Mac, but the bad kind of hackers will not be stopped, and they brought out an even more malicious piece of software that actually manages to prevent XProtect from updating, thus stopping future viruses from being detected. This new Trojan is called Flashback.C.
Basically, this Trojan overwrites XProtectUpdater files preventing the program from updating and finding new threats that need to be handled. This opens your Mac to a wide of range of possibly threatening software and attacks.
Right now it isn’t clear what this Trojan is actually trying to do, but it is able to connect to a remote host and run other pieces of code. It seems more like it is setting up for future attacks, and not doing the attacking itself. Either way, this cannot end well if it ends up on your system.
Please, make sure to only update your software from a source you trust, or better still, from the official site of the software developer. This practice applies to pretty much any piece of software, but it applies especially to flash now.