The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-08:05.openssh.asc

Upgrade your system to the appropriate stable release or security branch dated after the correction date https://secure1.securityspace.com/smysecure/catid.html?in=FreeBSD-SA-08:05.openssh.asc

OpenSSH is an implementation of the SSH protocol suite, providing an encrypted and authenticated transport for a variety of services, including remote shell access. The OpenSSH server daemon (sshd) provides support for the X11 protocol by binding to a port on the server and forwarding any connections which are made to that port. When logging in via SSH with X11-forwarding enabled, sshd(8) fails to correctly handle the case where it fails to bind to an IPv4 port but successfully binds to an IPv6 port. In this case, applications which use X11 will connect to the IPv4 port, even though it had not been bound by sshd(8) and is therefore not being securely forwarded.