One year ago, two Australian hackers found themselves on an eight-hour flight to Singapore to attend a live hacking competition sponsored by Dropbox. At 30,000 feet, with nothing but a slow internet connection, they decided to get a head start by hacking Zoom, a videoconferencing service that they knew was used by many Dropbox employees.

The hackers soon uncovered a major security vulnerability in Zoom’s software that could have allowed attackers to covertly control certain users’ Mac computers. It was precisely the type of bug that security engineers at Dropbox had come to dread from Zoom, according to three former Dropbox engineers.

Now Zoom’s videoconferencing service has become the preferred communications platform for hundreds of millions of people sheltering at home, and reports of its privacy and security troubles have proliferated.

Zoom’s defenders, including big-name Silicon Valley venture capitalists, say the onslaught of criticism is unfair. They argue that Zoom, originally designed for businesses, could not have anticipated a pandemic that would send legions of consumers flocking to its service in the span of a few weeks and using it for purposes — like elementary school classes and family celebrations — for which it was never intended.