Juniper Junos 10.4 before 10.4S15 11.4 before 11.4R9 11.4X27 before 11.4X27.44 12.1 before 12.1R7 12.1X44 before 12.1X44-D20 12.1X45 before 12.1X45-D15 12.2 before 12.2R6 12.3 before 12.3R3 13.1 before 13.1R3 and 13.2 before 13.2R1 when Proxy ARP is enabled on an unnumbered interface allows remote attackers to perform ARP poisoning attacks and possibly obtain sensitive information via a crafted ARP message.