In pre-Vista versions of 64bit Windows it seems that not only has a vulnerability using BrowserWriteErrorLogEntry() and the mrxsmb.sys driver be discovered, it is already out there in the wild.  As The Register is quick to point out, in this particular case browser refers to how older versions of Windows discovered workgroups, domains, and the computers belonging to them, as opposed to being an IE flaw.

Another Zero Day Flaw to worry about - General Tech  1

So while the vulnerability exists, the amount of PCs that could be affected are much less than if it was able to attack 32bit versions as well; though not much comfort for the geeks running 64bit WinXP Professional.

“Vupen, which rates the vulnerability as critical, has confirmed the bug in Windows Server 2003 SP2 and Windows XP SP3. Secunia rates it as moderately critical, the third most serious notch in its five-tier system.”

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