In FreeBSD before 11.2-STABLE(r338986), 11.2-RELEASE-p4, 11.1-RELEASE-p15, 10.4-STABLE(r338985), and 10.4-RELEASE-p13, due to improper maintenance of IPv6 protocol control block flags through various failure paths, an unprivileged authenticated local user may be able to cause a NULL pointer dereference causing the kernel to crash.