bootp in Apple iOS before 8.4.1 and OS X before 10.10.5 allows remote attackers to obtain potentially sensitive information about MAC addresses seen in previous Wi-Fi sessions by sniffing an 802.11 network for DNAv4 broadcast traffic.
http://lists.apple.com/archives/security-announce/2015/Aug/msg00001.html
http://lists.apple.com/archives/security-announce/2015/Aug/msg00002.html
https://support.apple.com/kb/HT205030