Juniper Junos OS before 13.2X51-D36, 14.1X53 before 14.1X53-D25, and 15.2 before 15.2R1 on EX4300 series switches allow remote attackers to cause a denial of service (network loop and bandwidth consumption) via unspecified vectors related to Spanning Tree Protocol (STP) traffic.
http://www.securitytracker.com/id/1035106
http://kb.juniper.net/InfoCenter/index?page=content&id=JSA10719