source: http://www.securityfocus.com/bid/1416/info

The Check Point Firewall-1 SMTP Security Server in Firewall-1 4.0 and 4.1 on Windows NT is vulnerable to a simple network-based attack which can increase the firewall's CPU utilization to 100%. Sending a stream of binary zeros (or other invalid SMTP commands) to the SMTP port on the firewall raises the target system's load to 100% while the load on the attacker's machine remains relatively low. According to Check Point Software this only disables mail relay while allowing other firewall operations to continue normally. 


This can easily be reproduced from a Linux system using netcat with an input of /dev/zero, with a command such as "nc firewall 25 < /dev/zero".