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Re: Proposed Sirefef (was Re: Late in the day...bet this could be sig'd)
From: Joel Esler <jesler () sourcefire com>
Date: Mon, 6 May 2013 17:18:22 -0400

On May 6, 2013, at 5:04 PM, waldo kitty <wkitty42 () windstream net> wrote:

On 5/6/2013 13:37, Joel Esler wrote:
On May 3, 2013, at 8:54 PM, lists () packetmail net <mailto:lists () packetmail net>
wrote:
alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"ET
CURRENT_EVENTS/VRT_COMMUNITY Potential Sirefef hostile executable served from
compromised or malicious WordPress site"; flow:established,from_server;
content:"/wp-content/"; http_uri; content:".exe|20|HTTP/1."; fast_pattern:only;
pcre:"/\/\d+\.exe$/U"; classtype:trojan-activity;
reference:url,blog.avast.com/2013/05/03/regents-of-louisiana-spreading-sirefef-malware
<http://blog.avast.com/2013/05/03/regents-of-louisiana-spreading-sirefef-malware>;
sid:x; rev:1;)

Nathan,

Looking at what you are intending here, I think you mean it the other way
(HOME_NET -> $EXTERNAL_NET)

ok... now i'm officially confused... the flow in the rule is "from_server"... 
with that specified, does it really matter if HOME_NET or EXTERNAL_NET come first?

then there's the situation of not only detecting this coming into a network from 
an external server, but also of detecting this going out of a network that runs 
servers feeding the public on the outside...

does the '->' really make any difference?

should it instead have been '<-' if the rule writer really wanted HOME_NET to be 
first?

There is no such thing as "<-".  

The way that Nathan wrote the rule above says we are looking for a URI to be returned from a server external to our 
network to a client that initiated the connection.  This wouldn't work.  Which is why I said we need to reverse it to 
look for HOME_NET -> $EXTERNAL_NET and "to_server" in the flow.  That way we are alerting on someone making an outbound 
request for a file with the exe extension in the /wp-content/ directory on the server.


would using '<->' or '<>' (if either is allowed) detect the traffic no matter 
which way the traffic was going (internal server to external client or external 
server to internal client) no matter where the server is located??


<> is allowed, but isn't very descriptive from an alert point of view.  Plus, coupled with flow "to_server" you'd want 
to make sure that your msg was reflective of what you were trying to do in the rule.

http://blog.snort.org/2011/09/flow-matters.html

--
Joel Esler
Senior Research Engineer, VRT
OpenSource Community Manager
Sourcefire

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