The big tech news last month was the FBI’s alert to iPhone and Android users to stop texting after Chinese hackers attacked U.S. networks. Sending SMS messages is a blanket no—it’s woefully insecure.

The French and U.K. equivalents to the U.S. government’s cyber defense agency have issued new advice for users. “Deactivate the automatic reception of MMS messages,” ANSSI warns, “to avoid the ...