SAN FRANCISCO — When hackers took over the Twitter account of Twitter’s chief executive, Jack Dorsey, last week, they used an increasingly common and hard-to-stop technique that can give them complete access to a wide array of the most sensitive digital accounts, including social media, email and financial accounts.

Called SIM swapping, it allows hackers to take control of a victim’s phone number. In recent months, SIM swapping has been used to hijack the online personas of politicians, celebrities and notables like Mr. Dorsey, to steal money all over the world and to simply harass regular people.

Victims, no matter how prominent or technically sophisticated, have been unable to protect themselves, even after they have been hit again and again.

“I’ve been looking at the criminal underground for a long time, and SIM swapping bothers me more than anything I’ve seen,” said Allison Nixon, the director of research at the security firm Flashpoint. “It requires no skill, and there is literally nothing the average person can do to stop it.”