Some Intel CPUs have JTAG over USB 3 debugging built-in. This could be good for debugging but is really bad for computer security.
Having the DCI interface enabled is sufficient to make target computers vulnerable. Researchers found that on many computers, DCI is enabled out-of-the-box and not blocked by default.
This mechanism in Intel CPUs could lead to a whole new class of Bad USB-like attacks. Fortunately this mechanism can be exploited only on Intel U-series processors (used on laptops and NUC) at the moment.