The DHCPv6 server in ISC DHCP 4.0.x and 4.1.x before 4.1.2-P1, 4.0-ESV and 4.1-ESV before 4.1-ESV-R1, and 4.2.x before 4.2.1b1 allows remote attackers to cause a denial of service (assertion failure and daemon crash) by sending a message over IPv6 for a declined and abandoned address.