Amazon, Cisco etc no Angels
I can see Amazon not wanting to push this publicly other t might get mentioned how they work with the CIA and allow parcels on route to be opened, motherboards with backdoors placed in to packages in place of legit hardware, then sent on the rest of the journey.
It's highly possible super micro has made a few modified boards for Chinese intelligence to be used in a handful of places, in a similar way the US works with Intel, Cisco, Apple, Microsoft etc to backdoor devices and software in a small number of cases.
Finding evidence when only a few are out there would be difficult. At the same time, without evidence anyone can com e along and claim x, y and z and damage a companies credentials. The US have a habit of trying to damage foreign competitors of its companies. Look at all the angles they attacked Kaspersky with (even signing US gov malware with kasperskys digital keys) in a hope of damaging the company beyond repair. THey have upped their game on these attacks since the Sony attack.
Biting the hand that feeds IT
