Re: flow:established still broken in 2.9.0.5? (was:FP shows snort-2.9.0.3 confused over packets and sessions)

[Joel Esler <jesler () sourcefire com>]

Just a couple thoughts initially, I'll fwd this over to devel for them to look at as well.

Are you dropping packets?  I am wondering that, because maybe Snort tagged this as a midstream pickup or something.
Do you have a pcap?


As a rule writing note, "isset" flowbit checks generally should come before content.  I have no idea what this rule 
does though, but I'd want the flowbit check before the content in this case, as it's only a two byte match.  

J

On Jun 29, 2011, at 4:49 AM, Jason Haar wrote:

> Hi there
>
> We're still seeing the problem under 2.9.0.5 where snort misclassified a packet in the middle of a TCP stream  as 
> being the first packet and matches against that.
>
> e.g. we just had the following FP
>
> alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET TROJAN Win32.Inject.ajq Initial Checkin to CnC packet 
> 2"; flow:established,to_server; content:"|07|F"; depth:2; flowbits:isset,ET.inj.ajq.1; classtype:trojan-activity; 
> reference:url,doc.emergingthreats.net/2008056; 
> reference:url,www.emergingthreats.net/cgi-bin/cvsweb.cgi/sigs/VIRUS/TROJAN_Win32.Inject; sid:2008056; rev:4;) 
>
>
> It has "flow:established" and 'content:"|07|F"; depth:2'. So that should mean it can only alert IFF the *first two 
> bytes* of the tcp stream are '|07|F'. However, we had it trigger in the middle of a HTTPS session (via a proxy on 
> port 3128 - which we've  defined as HTTP_PORTS). The packet it matched on was 1260 bytes in size and indeed began 
> with those two bytes.
>
> We've seen this in earlier releases as well as 2.9.0.5. Is this a known problem? I didn't get any feedback last time 
> I brought this up
>
> Thanks
>
> Jason
>
>
> On 12/05/11 13:50, Jason Haar wrote:
> > On 10/05/11 19:42, rmkml wrote:
> > > Hi Jason,
> > > I suggest replace `depth:4;` to `http_method;`.
> > > Replace it's work on my test.
> > > I have another suggest, replace `isdataat:200,relative;` to
> > > `isdataat:200,relative; content:!"|0A|"; within:200;`.
> > > I have another another suggest, on pcre, replace `(?!\n)` to `(?!\r?\n)`.
> > I think your suggested changes make a lot of sense, but that wasn't
> > really my point. Why did a "depth:4" rule match *inside* a stream
> > instead of the *beginning* of a stream?
> >
> > > Please upgrade to snort v2.9.0.5.
> > Is there a stream5 bug in 2.9.0.3 that caused this? Changelog doesn't
> > show anything. My understanding of how snort merges packets into streams
> > is contradicted by this event: either my understanding is incorrect, or
> > there's a bug(?)
> >
> >  
>
> -- 
> Cheers
>
> Jason Haar
> Information Security Manager, Trimble Navigation Ltd.
> Phone: +64 3 9635 377 Fax: +64 3 9635 417
> PGP Fingerprint: 7A2E 0407 C9A6 CAF6 2B9F 8422 C063 5EBB FE1D 66D1
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All of the data generated in your IT infrastructure is seriously valuable.
Why? It contains a definitive record of application performance, security 
threats, fraudulent activity, and more. Splunk takes this data and makes 
sense of it. IT sense. And common sense.
http://p.sf.net/sfu/splunk-d2d-c2

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