Googling for ICMP tunnel I found some interesting results: http://en.wikipedia.org/wiki/ICMP_tunnel http://www.cs.uit.no/~daniels/PingTunnel/ http://thomer.com/icmptx/ There is at least a trojan using ICMP to transmit sensitive data (link provided by egemen) http://www.websense.com/securitylabs/alerts/alert.php?AlertID=570 If I understand correctly the idea is put data in the data section of ICMP Echo request packets and the other computer will reply with ICMP echo replies containing data as well. How about this scenario?: A downloader (forget HIPS, Anti-executable, LUA, DEP, etc for the moment) connects to a malicious server using echo requests and the malicious server sends malware using echo replies. Which firewalls can block outgoing ICMP for a specific application without blocking ICMP for other program? I know that Comodo and OA Free can but how about other firewalls?