FreeBSD published a security advisory FreeBSD-EN-09:05.null due to a security vulnerability in the FreeBSD kernel. A malicious process that has mapped code or data at address 0 may be able to execute arbitary code with kernel privileges when invoking a NULL pointer.
This is an issue that was also already discovered in the Linux kernel a few days before.
After updating your kernel you should add the following line to either /boot/loader.conf or /etc/sysctl.conf:
Don't forget to reboot! ;-) ■