An Incorrect Behavior Order vulnerability in the Packet Forwarding Engine (PFE) of Juniper Networks Junos OS on EX4300 Series allows an unauthenticated, network-based attacker to cause an integrity impact to networks downstream of the vulnerable device. When an output firewall filter is applied to an interface it doesn't recognize matching packets but permits any traffic. This issue affects Junos OS 21.4 releases from 21.4R1 earlier than 21.4R3-S6. This issue does not affect Junos OS releases earlier than 21.4R1.