On Broadcom BCM4355C0 Wi-Fi chips 9.44.78.27.0.1.56 and other chips, properly crafted malicious over-the-air Fast Transition frames can potentially trigger internal Wi-Fi firmware heap and/or stack overflows, leading to denial of service or other effects, aka B-V2017061205.
https://support.apple.com/en-us/HT208113
https://support.apple.com/en-us/HT208112
https://support.apple.com/HT208113
https://support.apple.com/HT208112
https://source.android.com/security/bulletin/2017-09-01
https://lists.apple.com/archives/security-announce/2017/Sep/msg00009.html
https://lists.apple.com/archives/security-announce/2017/Sep/msg00007.html