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This iPhone bug kills your phone's Wi-Fi — what you need to know

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Look out — there's a new iOS bug that can kill your iPhone or iPad's Wi-Fi functions if you connect to a hotspot with a very unusual name, or SSID. 

You won't be able to connect to another hotspot, and rebooting the iDevice doesn't fix the problem. But there is a way to get out of the hole without completely factory-resetting your iPhone or iPad.

"After joining my personal WiFi with the SSID "%p%s%s%s%s%n", my iPhone permanently disabled its WiFi functionality," wrote Danish hacker Carl Schou on Twitter on Friday (June 18). "Neither rebooting nor changing SSID fixes it :~)"

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When Schou tried to turn Wi-Fi back on again manually, the iPhone switched it off immediately.

Schou told Bleeping Computer that he had found this flaw on an iPhone XS running iOS 14.4.2. The website reproduced it on an iPhone running iOS 14.6, the most recently released version of Apple's mobile operating system. Android phones do not seem to be affected.

"In some tests, connecting to the SSID would fail, but we could no longer access our regular wireless network," wrote Bleeping Computer's Ax Sharma. "Other tests led to the behavior described by Schou, where the iPhones Wi-Fi setting would be disabled, and we could no longer enable it again."

Malicious actors could weaponize this flaw by setting up hotspots in public places without passcodes, leading data-hungry iPhones and iPads with Wi-Fi turned on to try to connect to it. Connecting to the hotspots would kill the Wi-Fi functions until the user took steps to fix it.

How to get your Wi-Fi back

Schou was at a loss on how to regain Wi-Fi functionality on his iPhone without having to factory-reset the whole thing, but fortunately another Twitter users showed him that an easy solution was at hand. 

All you need to do is go into Settings, then select General, then go to Reset. Don't "Reset All Settings" or "Erase All Content and Settings." Instead, scroll down a bit and then tap "Reset Network Settings.

Your iPhone or iPad will then reboot normally, and you'll be able to reconnect to other Wi-Fi hotspots, though you may have to enter in the hotspot passwords manually.

Silly strings

Chinese iPhone hacker ChiChou, aka CodeColorist, put up a blog post that dissected the flaw and explained that it's a format-string bug, something that is "rarely seen nowadays." 

Bleeping Computer said that iOS seems to be interpreting the "%n" string in the SSID name as a command variable in the C programming language, rather than as just plain text.

"%p%s%s%s%s%n" is not an ordinary Wi-Fi network name, to say the least. Schou told Bleeping Computer that "all my devices are named after format strings to f*** with poorly developed devices."

Paul Wagenseil
Paul Wagenseil is a senior editor at Tom's Guide focused on security and privacy. That's all he's going to tell you unless you meet him in person.