Hyper-Threading considered harmful

Hyper-Threading, as currently implemented on Intel Pentium Extreme Edition, Pentium 4, Mobile Pentium 4, and Xeon processors, suffers from a serious security flaw. This flaw permits local information disclosure, including allowing an unprivileged user to steal an RSA private key being used on the same machine. Administrators of multi-user systems are strongly advised to take action to disable Hyper-Threading immediately; single-user systems (i.e., desktop computers) are not affected.

12-page paper, Cache Missing for Fun and Profit, discussing this flaw and related problems, both realized and theoretical.