If you have a recent Pentium 4 or Xeon-based server running in a multi-user environment, watch out: there is current talk that Hyper-Threading, a technology inherent in Intel Pentium 4 systems that allows multiple parallel threads to execute on a processor to speed up operations, suffers from a serious security flaw:
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.
Generally, the average user with a single desktop at home won’t have much to worry about, but workplaces with multi-user environments will want to take heed.
More information on the announcement from FreeBSD guru Colin Percival can be found here.