Row hammer is a well-publicized target for cyberattacks on DRAM, and there have been attempts to stop these attacks in DDR4 and DDR5, but with mixed results. The problem is that as density increases, distance decreases, making it more likely that flipped bit cell in one row can disturb a bit cell in another, and that bits flipped across an entire row can flip another row. Steven Woo, fellow and distinguished inventor at Rambus, talks with Semiconductor Engineering about why this has become such a serious problem, why it pass virtually unnoticed by error correction technology, and how to stop it with no impact on performance or power.