Research and Innovation
There are only a handful of cutting edge manufacturing facilities across the world, and microelectronic designs from world over are sent to these facilities for manufacturing. This provides an excellent opportunity to introduce malice to create a foothold for future attacks. A few well placed employees at these foundries can modify designs before they are even fabricated.
A mitigating factor against these attacks is that editing a taped out design is like walking on eggshells: while possible, but one has be very careful not to break the original design while introducing alterations. The easiest effects to achieve is causing the device to break prematurely. Deeper understanding and more effort is needed to introduce stray wires that act as antenna to exfiltrate secrets, or bias a random number generator to produce low entropy outputs.
Split manufacturing or the use of FPGAs can mitigate the problem depending on the type of FPGA used: some FPGAs in the market have more third-party IP than even general-purpose processors.
These attacks do not require a lot of planning: a few employees in the foundry are sufficient to change the design, but the effects that can be achieved are fairly limited.