One purported benefit of 5G is that it protects phone identifiers, like your device’s “international mobile subscriber identity,” to help prevent tracking or targeted attacks. But downgrade attacks like the ones the researchers found can bump your device down to 4G, or put it into limited service mode, then force it to send its IMSI number unencrypted. Increasingly, networks use an alternative ID called a Temporary Mobile Subscriber Identity that refreshes periodically to stymie tracking. But the researchers also found flaws that could allow them to override TMSI resets, or correlate a device’s old and new TMSI, to track devices. Mounting those attacks takes only software-defined radios that cost a few hundred dollars. The 5GReasoner tool also found issues with the part of the 5G standard that governs things like initial device registration, deregistration, and paging, which notifies your phone about incoming calls and texts. Depending on how a carrier implements the standard, attackers could mount “replay” attacks to run up a target’s mobile bill by repeatedly sending the same message or command. It’s an instance of vague wording in the 5G standard that could cause carriers to implement it weakly.