[Snort-users] Trying to use snort with TALOS-2016-0219

Patrick Mullen pmullen at ...1935...
Tue Nov 29 15:04:49 EST 2016

What you're describing isn't a snort problem.  It's the Operating System.
Look up "disabling TCP Checksum Offloading" for how to do it with your OS.
It's what happens when you run your capturing software on either the
generating or receiving device, as opposed to a third party, using modern
networking hardware and operating systems.

If you want, for testing purposes, you can run snort with "-k none" to
ignore tcp checksums (keeping in mind that you don't want to do this for
actual packet captures as it opens you up to evasions as described before),
or you can fix the checksums in the pcap after the fact using tcprewrite,
which is what I do and is The Right Thing To Do (TM).

My tcpfix.sh script fixes tcp and udp checksums and stores the updated pcap
to the a new pcap with "-fixed" in the name.

$ cat ~/bin/tcpfix.sh
NEWFILE=`echo -n $FILE | sed 's/\.pcap$/-fixed.pcap/'`
tcprewrite --fixcsum -i $FILE -o $NEWFILE



On Mon, Nov 28, 2016 at 8:51 PM, Yuri Niyazov <yuri at ...17684...> wrote:

> Thank you very much for your help. After I included the correct filter, it
> still didn't work. I then noticed that all the outgoing packets in the new
> capture file had bad checksums; I disabled NIC-level checksumming ("ethtool
> -K eth0 tx-checksum-ip-generic off") and did another capture, which worked
> fine without disabling snort-level checksumming.
> Is there a way to tell snort, while capturing, to properly calculate
> checksums for packets that are outgoing from the machine? "-k all" didn't
> seem to do it.
> On Mon, Nov 28, 2016 at 1:03 PM, Patrick Mullen <pmullen at ...1935...>
> wrote:
>> Yuri,
>> By "asymmetric," Joel means your pcap is broken.  :)  Your "works" config
>> modified snort such that it ignores the fact that it can only see half of
>> the traffic.  Please note that while this configuration means you did alert
>> in this situation, in the long run it opens you up to evasions and other
>> issues so it's not an ideal solution.
>> I suspect that when you created your pcap, you probably did a filter like
>> "dst host and port 11211", which filtered out return traffic
>> from that host.  If you instead used the filter "host and
>> port 11211", the host filter would be bidirectional (the ip could be the
>> dst or src) so you should see the entire set of traffic and snort should
>> alert for you without needing to change the configuration.
>> Thanks,
>> ~Patrick
>> On Mon, Nov 28, 2016 at 11:17 AM, Joel Esler (jesler) <jesler at ...589...>
>> wrote:
>>> Your traffic is “asymmetric”.  This is why turning off stream makes it
>>> “work”.
>>> Snort needs both sides of the traffic flow in order to process the
>>> traffic correctly.
>>> *--*
>>> *Joel Esler *| *Talos:* Manager | jesler at ...589...
>>> On Nov 25, 2016, at 10:41 PM, Yuri Niyazov <yuri at ...17684...> wrote:
>>> Hi everyone,
>>>   Snort newbie here. I am trying to detect the latest memcache
>>> vulnerabilities, http://www.talosintelligence.c
>>> om/reports/TALOS-2016-0219/
>>> Output of snort -V, as requested in the instructions for posting reports
>>> to this list:
>>>    ,,_     -*> Snort! <*-
>>>   o"  )~   Version GRE (Build 383)
>>>    ''''    By Martin Roesch & The Snort Team:
>>> http://www.snort.org/contact#team
>>>            Copyright (C) 2014-2015 Cisco and/or its affiliates. All
>>> rights reserved.
>>>            Copyright (C) 1998-2013 Sourcefire, Inc., et al.
>>>            Using libpcap version 1.5.3
>>>            Using PCRE version: 8.31 2012-07-06
>>>            Using ZLIB version: 1.2.8
>>> So, I have a packet capture that is the proof-of-concept exploit (code
>>> copy-pasted from the vulnerability announcement). That packet capture is
>>> attached. It is detected when I run "snort -c etc/works.conf -r
>>> /var/log/snort/memcachedump.1480128874", I get the text below in
>>> /var/log/snort/alert:
>>> [**] [3:40474:2] SERVER-OTHER TRUFFLEHUNTER TALOS-CAN-0219 attack
>>> attempt [**]
>>> [Classification: Attempted Administrator Privilege Gain] [Priority: 1]
>>> 11/26-02:54:44.674785 ->
>>> TCP TTL:63 TOS:0x0 ID:47627 IpLen:20 DgmLen:1100 DF
>>> ***AP*** Seq: 0xF7EF58B0  Ack: 0x1E0819C9  Win: 0x1C9  TcpLen: 32
>>> TCP Options (3) => NOP NOP TS: 3334822 5964160
>>> [Xref => http://www.talosintelligence.com/reports/TALOS-2016-0219]
>>> However, when I run "snort -c etc/broken.conf -r
>>> /var/log/snort/memcachedump.1480128874" the alert doesn't happen
>>> The difference between works.conf and broken.conf is that broken.conf
>>> includes the stream5_global, stream5_tcp and stream5_udp preprocessors as
>>> they are configured in the latest downloadable ruleset (these aren't the
>>> files I will end up using, these are just the smallest difference I was
>>> able to isolate between "working" and "not working").
>>> Now, if I understand things correctly, the streaming preprocessor
>>> provides important functionality that shouldn't just be turned off blindly,
>>> so, the question is: what in that preprocessor configuration could be
>>> masking the memcached exploit?
>> --
>> Patrick Mullen
>> Response Research Manager
>> Cisco TALOS

Patrick Mullen
Response Research Manager
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