cfg80211 in net/wireless/scan.c in the Linux kernel 2.6.30-rc1 and other versions before 2.6.31-rc6 allows remote attackers to cause a denial of service (crash) via a sequence of beacon frames in which one frame omits an SSID Information Element (IE) and the subsequent frame contains an SSID IE, which triggers a NULL pointer dereference in the cmp_ies function.  NOTE: a potential weakness in the is_mesh function was also addressed, but the relevant condition did not exist in the code, so it is not a vulnerability.