Diligence Security Advisory Issue: Checkpoint's Firewall-1 has a "feature" that can allow an external intruder to pass through the firewall and attack machines, unihibited, on the protected side. Details: When Firewall-1 is installed there is an implicit rule: ANY (Source), ANY (Destination), ANY (Service) and ACTION (drop). This means, in theory, that all IP based packets, whether incoming or outgoing should be dropped. However, Firewall-1, out of the box, allows certain "core" network protocols through - these being RIP (UDP port 520), DNS (UDP and TCP port 53) and all ICMP except Redirects. These are allowed through, from ANY (source) to ANY (Destination), without being logged, before the rule base is referenced. Consequently, DNS cache poisoning aside, if an attacker has managed to place a trojan or another "backdoor" on a host on the protected side, through whatever method, and set it listening on TCP or UDP port 53, they will be able to access this host transparently, through the firewall. No logging will take place. The firewall host itself is reachable by this method, even if a 'stealth' rule has been placed in the rule-base to protect it. During our lab tests we set an NT Server listening on TCP port 53 using netcat and on connection spawned a command prompt (cmd.exe). On telnetting to this server, through the firewall, we were able to attack all other machines on the "protected" side. We also installed the cDc's Back Orifice on a Windows 95 client listening on UDP port 53 and could access this machine through the firewall. When listening on UDP 520 (RIP) the we could not access the 95 client, indicating that firewall-1 checks the validity of traffic sent over the RIP port. Versions tested: Firewall-1 v3.0b on NT server 4.0 with Service Pack 3 Fix: From the Firewall-1 Security Policy Window choose Properties from the Policy Menu. Uncheck the "Accept Domain Name Queries (UDP)" and "Accept Domain Name Download (TCP)". This will disable DNS which, of course, will cause problems. In order to avoid this you will need to create a specific rule in the rule base to allow these core protocols to function. The exact nature of this rule will vary depending on the configuration of DNS within your own network and the above steps should only be taken after consulting with in-house DNS administrators. Diligence accepts no responsibility for any problems caused by the disabling of these default settings. For further information see: http://www.diligence.co.uk
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