[Snort-users] help with flow:established

Felix Erlacher felix.erlacher at ...17726...
Mon Jan 9 10:30:11 EST 2017

Thanks Albert for pointing that out. I did not know the "-k none"
switch, this solved my issue.

Just another thought: couldn't this be exploited by an attacker by using
bad checksums in an attack?

Or is it then the victim/receivers fault, because it is his (or his
servers) duty to check the checksums...

On 09/01/17 16:15, Al Lewis (allewi) wrote:
> That sounds like correct behavior. If one of the packets needed to establish the session has a bad checksum it is ignored (if you don’t have the -k none added when running snort).
> Albert Lewis
> SOURCEfire, Inc. now part of Cisco
> Email: allewi at ...589... 
> On 1/9/17, 9:55 AM, "Felix Erlacher" <felix.erlacher at ...17726...> wrote:
>> Hi all,
>> I encountered a strange behavior in snort
>> I wanted to trigger rule with sid:2010054 from the current
>> emerging-threat ruleset (emerging-all.rules)[1].
>> I created a very simple traffic dump with an ARP request/response, TCP 3
>> way handshake and a HTTP GET request containing the content the rule is
>> looking for, and a 404 answer from my http server.
>> If I run snort with only this rule no alarm is triggered.
>> Now, if I remove the option "established" from the "flow:" keyword,
>> leaving only "flow:to_server" left in the rule than snort triggers an
>> alarm for this rule.
>> There is only one thing that imho could be blamed for this behavior: the
>> second segment in the tcp 3whs coming from the server has a wrong tcp
>> checksum.
>> Is it possible that the preprocessor (and thus snort) does not consider
>> a TCP connection "established" if there is checksum error in the 3whs?
>> [1] alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET TROJAN
>> Likely TDSS Download (codec.exe)"; flow:established,to_server;
>> content:"GET"; nocase; http_method; content:"/codec.exe"; nocase;
>> http_uri; reference:url,doc.emergingthreats.net/2010054;
>> classtype:trojan-activity; sid:2010054; rev:6;)
>> -- 
>> Felix Erlacher
>> Key-ID:4EAC0959

Felix Erlacher


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