Kleissner previously developed a proof-of-concept 'bootkit' called Stoned[PDF] capable of attacking Windows platforms ranging from XP to 7. It seems that this work has now been extended to include Windows 8. The source code to Stoned is available for download from Kleissner's website.
That means we can expect this vulnerability to be fixed before Windows 8 sees light of day.
Given Kleissner's background, I have no reason to doubt his claims at this stage. We'll know more about the attack when his paper is released on Saturday.
Kleissner is tentatively expected to present a paper at the MalCon security conference in Mumbai, India, later this month (he has yet to be granted a visa). Additionally, he is set to appear in court on December 15 on charges related to his Stoned 'bootkit' malware.