Attackers used an army of hijacked security cameras and video recorders to launch several massive internet attacks last week, prompting fresh concern about the vulnerability of millions of “smart” devices​in homes and businesses connected to the internet.

The assaults raised eyebrows among security experts both for their size and for the machines that made them happen. The attackers used as many as one million Chinese-made security cameras, digital video recorders and other infected devices to generate webpage requests and...