There exists a serious vulnerability in FreeBSD's implementation of packet filtering for IPv4 and IPv6.
The vulnerability exists in situations where a filtering rule permits packets through if they are part of an established connection.
It is possible for packets that are not part of an established connection to be allowed through. These packets must have the ECE flag set, which is in the TCP reserved options field.
Exploitation of this vulnerability may allow for unauthorized remote access to otherwise protected services.