tl;dr: Researcher Christopher Domas discovered a hardware backdoor in VIA C3 x86 CPUs: a hidden non-x86 coprocessor that, when activated, lets ring 3 userland code bypass memory protections and modify ring 0 kernel data. While the backdoor is supposed to require kernel access to enable, it's turned on by default on some systems, allowing trivial privilege escalation. The issue is limited to the C3 family (used in industrial, POS, ATM, and some consumer hardware), and tools are provided to detect and disable it.