You've got to feel sorry for Apple over this, which comes on top of other, as yet unfixed critical vulnerabilities in Quicktime. Some of them require only a visit to a specially crafted web site, so they are really serious problems.
For right now, I would simply uninstall any Quicktime versions on my machine and do without until I knew for sure it was safe. Apple is apparently working frantically on the problems; they released four fixes already. Still, the application is not yet safe.