[Snort-sigs] http_header issues, Snort 2.8.5.3

Steven Sturges steve.sturges at ...435...
Thu Apr 1 11:33:22 EDT 2010


That separation is all about a performance optimization for the
fast pattern matcher.  If you also notice in the documentation,
it mentions that the cookie cannot be used in conjunction with
fast_pattern.  The cookie is not searched with fast pattern contents.

If you have a 2000 byte HTTP Cookie, there is little use searching
all of that data for content with the fast pattern matcher, which
is used to select rules for further inspection.

In 2.8.6, there is a configuration option that allows you to disable
cookie extract, thereby keeping HTTP cookies with the rest of the
HTTP headers.

Typically, you won't find something in a cookie header field that
you would also want to find in another header -- Host, Content-Type,
etc.  When the extraction of various pieces of the HTTP requests was
added in 2.8.3 -- a good 2+ years ago -- the development team
discussed with VRT and we came to the conclusion that for best
performance and flexibility, it was easiest to separate the two.

There is nothing preventing a rule writer from using BOTH http_cookie
and http_header as modifiers to the same content, aside from other
limitations on http_cookie.

Cheers.
-steve

Mike Cox wrote:
> Agreed, I'm shocked that the http_header buffer doesn't include the
> Cookie header.  It doesn't make sense.  According to the manual, "The
> http header keyword is a content modifier that restricts the search to
> the extracted Header fields of an HTTP client
> request." (as an aside, note the incorrect use of 'an' instead of 'a')
>  Why is the Cookie header a second class citizen in the HTTP headers
> world?  I understand having a separate http_cookie buffer but it
> doesn't mean Cookies are not headers anymore....
> 
> -Mike Cox
> 
> On 4/1/10, evilghost at ...3397... <evilghost at ...3397...> wrote:
>> Thanks Will for the speedy response, I apologize for not have read your
>> response to the list earlier.  I agree with you regarding this and it's
>> counter-intuitive to have the Cookie removed from the http_header buffer.
>>
>> -evilghost
>>
>> Will Metcalf wrote:
>>> That because the cookie isn't included in the normalized header
>>> buffer, you can only get to via http_cookie modifer.  I know it
>>> doesn't make any sense to me either.  I sent the following e-mail to
>>> snort-devel on 3/17.
>>>
>>> "This is just my 2 cents, but I don't think the following behavior
>>> makes sense.  I think that even though you are providing http_cookie
>>> as a separate buffer to match on it should still be included in the
>>> http_header buffer, well because it is part of the headers.
>>>
>>> You can still match using the raw buffer but then you have to add
>>> additional checks to try and differentiate between the headers and the
>>> body which is why I'm guessing these modifiers were created in the
>>> first place.  I realize that in most cases header order doesn't matter
>>> but there may be instances where you can fingerprint a piece of
>>> automated code (read malware) using a rule similar to sid 69 below.
>>>
>>> Regards,
>>>
>>> Will"
>>>
>>> #this matches but I loose the performance/accuracy benefit of only
>>> matching within the buffer containing http_headers.
>>> alert tcp any any -> any any (msg:"http_cookie + ";
>>> content:"Cookie|3A|
>>> e6504ae48c99f09df7f58996aacbb6b0=120e494ce857d6ceeef89f9678d4d703|0d
>>> 0a|Content-Type|3A| application"; classtype:bad-unknown; sid:68;
>>> rev:1;)
>>>
>>> #this fails to match as the cookie is not part of the http_header
>>> buffer but is part of the real http headers.
>>> alert tcp any any -> any any (msg:"http_cookie + ";
>>> content:"Cookie|3A|
>>> e6504ae48c99f09df7f58996aacbb6b0=120e494ce857d6ceeef89f9678d4d703|0d
>>> 0a|Content-Type|3A| application"; http_header; classtype:bad-unknown;
>>> sid:69; rev:1;)
>>>
>>>
>>>
>>> On Thu, Apr 1, 2010 at 9:22 AM, evilghost at ...3397...
>>> <evilghost at ...3397...> wrote:
>>>
>>>> Hello, I am running Snort 2.8.5.3 and it appears that either
>>>> http_header; is not working correctly, does not work with a relative
>>>> keyword, or I do not understand http_header; correctly.  I am attempting
>>>> to constrain a content match to the http_header for performance reasons.
>>>>
>>>> Note, no need to recommend isdataat, I know there is data within 1024
>>>> bytes past the previous content match.
>>>>
>>>> Does NOT work:
>>>>    uricontent:"/login.php"; fast_pattern; content:"|0d 0a|Cookie\: ";
>>>> nocase; http_header; content:"ieatbugs="; within:1024;
>>>>
>>>> Does work:
>>>>    uricontent:"/login.php"; fast_pattern; content:"|0d 0a|Cookie\: ";
>>>> nocase; content:"ieatbugs="; within:1024;
>>>>
>>>> Comments/insight appreciated.
>>>>
>>>> -evilghost
>>>>
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> 
> 




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