Well, this isn't good. Quicktime Player version 7.13 and below has memory buffer overflow vulnerability that can allow a malicious user to issue instructions to the target computer, effectively taking it over. The interesting thing about this one is that the buffer overflow apparently works on both the Apple OS X and Windows version of the application which makes it a vulnerability that affects both Apple's and Microsoft's operating systems. Quicktime Player is rather ubiquitous so, if this vulnerability was exploited, it could affect a wide range of computers. No word on a fix yet, but I'm sure one will be soon forthcoming.
Some links:
An article about the vulnerabilityAn article detailing how the vulnerability is exploited