A set of significant vulnerabilities have been disclosed in the encryption of Wifi networks (specifically the WPA2 protocol). An attacker who is within range to connect to a Wifi network can exploit these vulnerabilities to completely decrypt traffic as well as manipulate or inject data. These vulnerabilities impact nearly every vendor of Wifi client software. The impact on Linux and Android devices is particularly severe.
How to mitigate:
The best way to mitigate these vulnerabilities is to install patches. The vulnerabilities impact multiple vendors, so CERT/CC is hosting a webpage with links to security advisory and patch information for each affected vendor. This page will be updated over time as new patches are released: http://www.kb.cert.org/vuls/id/228519
Deploying a second layer of encryption can be a useful mitigation while patches are unavailable. The simplest way to achieve this is to require users on Wifi networks to employ their corporate VPN clients while connected to Wifi. An ACL or firewall rule could be used to block traffic destined from the Wifi network to every destination other than the VPN.
Switching your Wifi network from WPA2 to WEP encryption is not advised as WEP has more significant security problems.
A detailed description of the vulnerabilities and the research surrounding them is available at this link: https://www.krackattacks.com
Briefly, the vulnerability impacts the WPA2 protocol. Part of the handshake for that protocol can be replayed to a client, causing the client to reuse an old encryption key. This key reuse can lead to effective cryptanalysis and decryption. In the case of Linux and Android devices, the encryption key can be reset to an all-zero key, with catastrophic consequences.