Check Point Firewall-1 Internal Address Leakage Vulnerability
It has been reported that a service pack has been released by Checkpoint to address this problem. For FW-1 4.0, this is SP5. It is available at their website.
A suitable solution may be to establish egress filtering. Information can be found on this subject at: http://www.sans.org/y2k/egress.htm
Check Point Software Firewall-1 4.0