Howard University was forced to cancel all of its online and hybrid undergraduate classes on Tuesday and Wednesday after a ransomware attack compromised its networks and rendered the school Wi-Fi network unusable. The attack, which came just as universities around the country were gearing up for the start of the fall semester, was a reminder of all the ways that online classes’ benefits—offering schools a way to deal with some less high-tech threats like snowstorms and pandemics—come with a vulnerability to disruption.