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Date: Wed, 30 May 2012 14:16:40 -0600
From: Kurt Seifried <kseifried@...hat.com>
To: oss-security@...ts.openwall.com
CC: John Haxby <john.haxby@...cle.com>
Subject: Re: CVE Request -- kernel: tcp: drop SYN+FIN messages

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On 05/30/2012 02:07 PM, Kurt Seifried wrote:
> On 05/30/2012 12:48 PM, John Haxby wrote:
> 
>> On 30 May 2012, at 19:25, Florian Weimer wrote:
> 
>>> * John Haxby:
>>> 
>>>> Recently we have a couple of queries relating to a Nessus 
>>>> "TCP/IP SYN+FIN Packet Filtering Weakness".   This has not
>>>> been helped by the fact that [1] actually points (indrectly)
>>>> to CVE-2002-2438 which is actually a SYN+RST problem.
>>> 
>>> Reading the discussion here,
>>> 
>>> <http://comments.gmane.org/gmane.linux.network/213981>
>>> 
>>> it seems to me that this is just a performance optimization 
>>> which could be bypassed by using different flags, so I don't 
>>> think there's a vulnerability or fix here, except the general 
>>> lack of source IP address validation in IP networks.
> 
>> That's the same thread that I referred to but I didn't reach the 
>> same conclusion that you did.   It is possible to block SYN+FIN
>> in iptables, but the distros I'm aware of don't have that kind
>> of check in place so people will be vulnerable to this kind of
>> DoS.
> 
>> The conclusion from the thread was that SYN+FIN is not a
>> legitimate packet so the kernel should drop it.   The nessus
>> people seem to think the same thing: they have a test for this
>> (although they refer to the SYN+RST fix from a decade ago).    If
>> there's a consensus that we don't need a CVE then we can go to
>> nessus and have them fix, remove or update their test.
> 
>> One could argue that if SYN+FIN doesn't need a CVE then SYN+RST 
>> didn't either since it can be blocked by the same, or very
>> similar, iptables rule.
> 
>> jch
> 
> No this definitely gets a CVE (see previous email), it directly 
> bypasses a security mechanism that is documented (man iptables,
> --syn section), and other parts of iptables do handle it correctly
> as far as I can tell (e.g. --state NEW). It allows bypass of
> firewall rules as documented, so if that doesn't get a CVE then
> nothing the world has gone upside down =).

Sorry I got that backwards (getting over a bad head cold), any ways
the point is packets with an invalid set of flags should generally not
get treated as legitimate (and over the years there have been many
efforts to block these types of attacks/etc., witness OpenBSD's pf's
normalizing and so on).


- -- 
Kurt Seifried Red Hat Security Response Team (SRT)
PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993

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