The thing missing from this analysis is that on data bus lines like SPI, I2c and others there are "passive" components like resistors and capacitors used for signal conditioning and line bias. If one of these components were replaced with a highly integrated IC, it would be possible to mimic the function of the passive component most of the time, but sometimes hijack existing data streams as a MITM.
This can be used to insert alternative boot code, firmware, microcode, or even FPGA structures.
Note that the component would not have to generate its own signal source, because by merely inserting a lower than normal resistance or capacitance it could alter an existing bitstream to reflect the desired payload.
It would require a high degree of integration and power management finesse, but is certainly doable to replace an existing SMT resistor or Capacitor with such a device.
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..... Such a device would not be detectable by xray (micrographic xray maybe?) , ultrasound, visual inspection, circuit analysis, or signal analysis until it was activated by a particular bitstream, possibly as part of a firmware update released by the manufacturer.
This can be used to insert alternative boot code, firmware, microcode, or even FPGA structures.
Note that the component would not have to generate its own signal source, because by merely inserting a lower than normal resistance or capacitance it could alter an existing bitstream to reflect the desired payload.
It would require a high degree of integration and power management finesse, but is certainly doable to replace an existing SMT resistor or Capacitor with such a device. .