A spate of hacked Instagram accounts. A $220 million lawsuit against AT&T. A bustling underground crime ring. They all have roots in an old problem that has lately found new urgency: SIM card swaps, a scam in which hackers steal your mobile identity—and use it to upend your life.

At its most basic level, a SIM swap is when someone convinces your carrier to switch your phone number over to a SIM card they own. They’re not doing it for prank call cover, or to rack up long-distance charges. By diverting your incoming messages, scammers can easily complete the text-based two-factor authentication checks that protect your most sensitive accounts. Or, if you don’t have two-factor set up in the first place, they can use your phone number to trick services into coughing up your passwords.