https://motherboard.vice.com/en_us/article/3kaxz3/krack-wifi-hack-attack-guide-explainer

For years security experts have advised users to only connect to password-protected wireless networks to prevent traffic snooping attempts that could expose their sensitive data. Due to new vulnerabilities announced today in WPA2, the most widely used Wi-Fi security protocol, virtually all devices are now vulnerable to such attacks.

The flaws, discovered by University of Leuven researcher Mathy Vanhoef, allow attackers in the vicinity of a legitimate WPA2-protected wireless network—like the one you have at home, at your favorite coffee shop or in your company—to intercept and decrypt traffic from connecting devices. In some cases it's even possible to inject malicious data into the traffic.