Likely the story of the day. Bloomberg posted an extensive article where they claim that the Chinese government tried to infiltrate into US companies by adding chips on server motherboards. 

First, off this, the companies involved and the Chinese government is denying the story, but it seems well investigated, and if true the implications would be enormous as server/mobo provider SuperMicro is involved. And adding a chip into a PCB, is not something you do without corporation of the server motherboard manufacturer. 

So the story is that basically some core logic was added onto the motherboards, chips the sized as a single grain of rice that had it's own IO, a networking interface and even a micro CPU. The discovery was made at Amazon who researched Elemental Technologies, a maker of software for servers that the American company Super Micro Computer, or Supermicro, uses. During that investigation, Amazon stumbled on the extra chip on the motherboard. Initially sized slightly smaller than a fingernail and later on an even smaller version. This chip would be able to contact servers over the web and receive instructions, and it's claimed it would be able to modify the server software, a backdoor. 

The stories get weird here; Apple and Amazon are denying any existence of the chip. "Apple has never found malicious chips, hardware manipulations or vulnerabilities that have been deliberately placed on a server, Apple has never had contact with the FBI or any other service about such an incident," Apple says it has 2000 servers from Supermicro, but denies that it has found the chips. Amazon says in its denial that it found four problems with the purchase of Elemental, a takeover that took place in 2015. None of those were in the hardware.

Have a read here, at bloomberg.

Bloomberg: China broke into US companies by adding chip on server motherboards