Not exactly what you're asking, but multiple CVEs have been found in Intel's Management Engine (ME) which have been used in spyware.
It might not be an intentional backdoor, but it very much seems designed with out-of-band access in mind, with the AMT remote management features and the fact that the network controller has DMA (this enables packet interception).
It might not be an intentional backdoor, but it very much seems designed with out-of-band access in mind, with the AMT remote management features and the fact that the network controller has DMA (this enables packet interception).