Apple released QuickTime version 7.2 yesterday. The update includes eight important security fixes in which viewing a maliciously crafted H.264 movie/movie/.m4v/SMIL file or visiting a malicious website may lead to arbitrary code execution. Apple's website has additional details.
The QuickTime update is available from Apple's Software Download for both Mac OS X and Windows. If you have iTunes or Apple Software Update installed, then you can just install iTunes 7.3.1 and QuickTime 7.2 will be included. If you only have QuickTime installed, perhaps on a corporate network, then you'll need to manually download the update.
It's important to update. Why? Because of stuff like MPack.
MPack is a PHP based malware kit that's sold as if it were commercial software. It includes updates, support, and additional modules can be purchased. It's very successful at the moment.
The kit uses compromised passwords to hack web servers and to insert an IFrame. If you visit a web page with such an IFrame, MPack's PHP script will be run and it will attempt to infect your computer. The PHP script is structured so that OS and browser versions are identified. The IFrame redirects to other PHP scripts depending on the details. These various scripts are easily updated by MPack's authors. Among the list of exploits it tries is one for QuickTime.
This new update may fix some of the QuickTime flaws known to malware authors. And it may also tip them off to new exploits. Apple's iTunes and therefore QuickTime is a very popular application. If everyone updates sooner than later it will shorten the window of opportunity for the bad guys. Patch your applications as well as your operating system.