SAN FRANCISCO — Earlier this year, the Federal Trade Commission brought a potentially groundbreaking case alleging that selling connected devices like routers and video cameras with known security weaknesses was an unfair and deceptive business practice.

In filing the suit against Taiwanese device manufacturer D-Link Systems Inc., the regulator was grabbing hold of an emerging theory in litigation that makes tech companies tremble: that manufacturers can be held liable if their products don't provide a minimum level of security.

This week, that theory took a bit of a beating.