Get Wi-Fi password with only text message to the router

Recently, German security researcher Jan Hörsch found that there was a magical vulnerability in the router, sending a text message to the router, and it would reply the administrator password and the Wi-Fi password as a text message.
What is this message?

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The vulnerable router is TP-Link M5350, a portable 3G router. As a 3G router, TP-Link M5350 supports SMS messaging, but in the SMS function location is not well filtered, so that a loophole.
Similar loopholes in the Internet products are more common, but can cause such effects are very rare, visual inspection is TP-Link internal security process failed to notice.