[Snort-sigs] Question about Content-Disposition, Content-Type, etc. and http_header buffer

Mike Cox mike.cox52 at ...2420...
Wed Oct 17 13:04:28 EDT 2012

Hi Joel,

I appreciate your timely response and feedback.

I believe the issue here is that HTTP headers normally seen in the initial
HTTP data are being sent in subsequent packets (as a result of
multipart/form-data POSTS) and formatted differently from the standard HTTP
header.  I hope I made this clear in previous emails but some simple Google
searches would also help illuminate what I'm talking about.  I'd rather not
get in to a RFC argument with you (if you really want to we can but,
apparently this is a standard format and accepted by both modern browsers
and servers).

My concern here is Snort rules that are looking for data normally in the
HTTP headers (using the http_header buffer) do not work in the case of some
multipart/form data POSTS.  This means that snort rules are not alerting as
intended (and introduces an IDS bypass avenue, albeit somewhat paltry in
terms of eloquence but nonetheless functional).

Does Sourcefire/VRT,etc. intend to account for this in future http-inspect
capabilities or should we modify our current rules to do inefficient,
global content matches?  Or is there another solution that I am missing
here?  What can we do *now*?


-Mike Cox

On Wed, Oct 17, 2012 at 10:51 AM, Joel Esler <jesler at ...435...> wrote:

> Well, from the looks of the "dump" (or whatever you sent below) it looks
> like there are multiple carriage returns in there.
> If the "header" of an http session has |0d 0a 0d 0a|, according to the RFC
> which states that character sequence indicates the end of the header, and
> the start of the packet data.  In which case the http_inspect preprocessor
> will move onto the http_client_data buffer.
> Hence why I asked for the pcap, to see if this is true.  We can create a
> pcap that looks like yours below, but I know the exact behavior of the
> preprocessor in this case.
> If you wanted to test it, you could do a
> content:"Content-Type"; http_header; content:"Content-Disposition";
> http_header; distance:0;
> which you indicated doesn't work.
> then test by doing:
> content:"Content-Type"; http_header; content:"Content-Disposition";
> http_client_body;
> and if that fires, then there's your problem.  Broken RFC adherence by
> whatever program is generating that traffic.
> --
> Joel Esler
> Senior Research Engineer, VRT
> OpenSource Community Manager
> Sourcefire
> On Oct 17, 2012, at 11:45 AM, Mike Cox <mike.cox52 at ...2420...> wrote:
> Hey Joel, thanks for the quick response.  Right now I don't have a pcap
> ... the info I got was reproduced from what I saw on a screen while helping
> our guys/gals track down an issue/RCA in the "Temptest" (SOC) room and I
> can't get any data out of there and if I could, it would be breaking many
> laws (well, internal "laws" but there could be international intel impact
> and NDA lawsuits if you know what I mean :)
> However, I think that the information provided is sufficient to illustrate
> this issue and it should be trivial for VRT to create necessary pcaps for
> their testing, if that is what you want.
> I can cook up a pcap but as earlier implied, I don't think it is is
> necessary for me to provide this at this point and I certainly do not want
> to insult VRT by spoon-feeding them data that they can easily and more
> completely create on their own with minimal effort.
> I believe that I have outlined the issue and provided the necessary data
> to accurately describe the problem.  Even without a pcap, I think the info
> provided is sufficient for a good discussion.  Has anyone else experienced
> this challenge with their snort installs?  If so, how do you deal with this
> nuance of HTTP and the current state of http-inspect?
> Please let me know if you are unclear on anything or need more information
> regarding this issue.
> Thanks all.
> -Mike Cox
> On Tue, Oct 16, 2012 at 4:51 PM, Joel Esler <jesler at ...435...> wrote:
>> Can you send me a pcap?
>> --
>> Joel Esler
>> Senior Research Engineer, VRT
>> OpenSource Community Manager
>> Sourcefire
>> On Oct 16, 2012, at 5:18 PM, Mike Cox <mike.cox52 at ...2420...> wrote:
>> I've noticed that in some multipart/form-data POSTs, data that is
>> normally in the HTTP header gets sent in the body of the message and not
>> parsed by http-inspect as part of the http_header buffer.  Specifically,
>> the headers "Content-Type", "Content-Disposition", and
>> "Content-Transfer-Encoding", although there could be others.  For example:
>> POST /blackhole/safe.php HTTP/1.1
>> Host: snort.org
>> Content-Type: multipart/form-data, boundary=---dG91Y2hteXNub3J0
>> Content-Length: 8675309
>> ---dG91Y2hteXNub3J0
>> Content-Disposition: form-data; name="name"
>> Joshua
>> ---dG91Y2hteXNub3J0
>> Content-Disposition: form-data; name="play_a_game"
>> True
>> ---dG91Y2hteXNub3J0
>> Content-Disposition: form-data; name="file";
>> Content-Type: application/pdf
>> Content-Transfer-Encoding: binary
>> ...
>> So a snort rule looking for a specific filename in a Content-Disposition
>> header wouldn't match if it were written as you would expect it to be
>> written.  For example, this wouldn't match the above:
>> alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Bad PDF File
>> Upload"; flow:established,to_server; content:"Content-Disposition";
>> http_header; content:"filename="; distance:0; http_header; content:".pdf";
>> distance:0; within:100; http_header; sid:1234567;)
>> What is the best way to match this and not incur the overhead of using
>> global content matches?  Is there a plan for the http-inspect pre-processor
>> to account for this?
>> Thanks.
>> -Mike Cox
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