X11Forwarding needs xorg-x11-xauth
In this example on a RHEL 5 system, X11Forwarding is enabled in /etc/ssh/sshd_config, but cookie-based access will not work until the xorg-x11-xauth package and its dependency is installed.
$ grep X11Forwarding /etc/ssh/sshd_config X11Forwarding yes
$ echo $DISPLAY # yum -y install xorg-x11-xauth
/usr/bin/xauth: creating new authority file /root/.Xauthority $ echo $DISPLAY localhost:10.0