AMD Zen 1 through Zen 4 CPUs use an insecure hash function in the signature validation for microcode updates; researchers released a proof of concept update which makes the RDRAND instruction return 4

for readers missing the significance of the number 4 in the proof of concept: to demonstrate this vulnerability the researchers created a microcode update which replaces the "hardware" random number generator behind the RDRAND instruction with an implementation of xkcd#221 😭