Date: Fri, 08 Jun 2012 09:35:13 +0100 From: John Haxby <john.haxby@...cle.com> To: Kurt Seifried <kseifried@...hat.com> CC: oss-security@...ts.openwall.com Subject: Re: CVE Request -- kernel: tcp: drop SYN+FIN messages On 07/06/12 19:37, Kurt Seifried wrote: > On 06/07/2012 01:31 AM, John Haxby wrote: > > > On 01/06/12 20:12, Kurt Seifried wrote: > >> In my limited testing with iptables on RHEL 6.2 it appears that > >> --state NEW works properly, and won't allow SYN+FIN to create > >> connections (I used hping3 and the SYN+FIN Packets were > >> blocked). > > >> So the default ruleset: > > >> -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A INPUT > >> -m state --state NEW -m tcp -p tcp --dport 22 -j DROP -A INPUT -j > >> REJECT --reject-with icmp-host-prohibited > > >> should work, so you could do you clever --syn bits first and then > >> have that set to protect stuff from SYN+FIN. > > > What happens if you have "-j ACCEPT" instead of "-j DROP"? I > > would expect that sshd wouldn't see the connection but you would > > get all the unpleasant side effects that made T/TCP deprecated. > > Ooops yeah typo, that DROP should have been ACCEPT. So to summarize > properly: > > -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT > -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT > -A INPUT -j REJECT --reject-with icmp-host-prohibited > > results in ICMP unreachable (the -F -S bypasses the "--dport 22 -j > ACCEPT" but gets caught in the final "icmp-host-prohibited" rule) with: > > hping3 -c 3 -n -S -F -p 22 192.168.51.195 > > with: > > -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT > -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j DROP > -A INPUT -j REJECT --reject-with icmp-host-prohibited > > with hping -F -S the packets bypass the "--dport 22 -j DROP" and get > caught by the icmp-host-prohibited > > with hping -S the packets get caught by "the "--dport 22 -j DROP"" as > expected. > > So basically --state new works fine and dandy. > > > > jch > > Good. That makes the kernel change just hardening then. If you're not using iptables you're leaving yourself open to all kinds of abuse anyway so I don't think we need a CVE for the kernel. jch
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