[Snort-sigs] Looking for HTTP POST's over 1mb in size

evilghost at ...3397... evilghost at ...3397...
Thu Apr 8 23:07:01 EDT 2010


Colon doesn't need to be escaped in a PCRE, even in a Snort PCRE. It
does in a content match. :)

-evilghost

Matt Olney wrote:
> Actually (don't ask me why)...they both work:
>
> alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"PCRE with
> colon"; pcre:"/User-Agent:/H"; classtype: attempted-admin; sid: 33333;)
> alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"PCRE with
> colon escaped"; pcre:"/User-Agent\:/H"; classtype: attempted-admin;
> sid: 33334;)
>
> Alerts:
> 1:33333:0 PCRE with colon Alerts: 2
> 1:33334:0 PCRE with colon escaped Alerts: 2
>
> [HTTP_HEADER BUFFER DATA (0x8ac90a0)]:
> 55 73 65 72 2d 41 67 65 6e 74 3a 20 43 42 4e 65 User-Agent: CBNe
> 74 44 61 74 61 53 65 74 0d 0a 48 6f 73 74 3a 20 tDataSet..Host:
> 73 65 67 6d 65 6e 74 2e 70 77 30 38 2e 69 63 69 segment.pw08.ici
> 62 61 2e 63 6f 6d 0d 0a 43 61 63 68 65 2d 43 6f ba.com..Cache-Co
> 6e 74 72 6f 6c 3a 20 6d 61 78 2d 61 67 65 3d 32 ntrol: max-age=2
> 35 39 32 30 30 0d 0a 56 69 61 3a 20 31 2e 30 20 59200..Via: 1.0
> 50 52 4f 58 59 0d 0a 43 6f 6e 6e 65 63 74 69 6f PROXY..Connectio
> 6e 3a 20 63 6c 6f 73 65 0d 0a 0d n: close...
>
> Matt
> (Who has been stupid busy, but is still listening)
>
> 2010/4/8 L0rd Ch0de1m0rt <l0rdch0de1m0rt at ...2420...
> <mailto:l0rdch0de1m0rt at ...2420...>>
>
>     I disagree. Unless snort is not PCRE compatible (which it seems they
>     should be based on the acronym), you don't have to escape the colon in
>     this context for a pcre check.
>
>     Cheers,
>
>     -L0rd Ch0de1m0rt
>
>     On Thu, Apr 8, 2010 at 7:46 PM, 김무성 <kimms at ...3282...
>     <mailto:kimms at ...3282...>> wrote:
>     > Missed escape : \
>     >
>     > You have to write this
>     >
>     > pcre:"/^Content-Length\:\s*[0-9]{7,}$/i";
>     > or
>     > pcre:"/^Content-Length\x3a\s*[0-9]{7,}$/i";
>     >
>     > -----Original Message-----
>     > From: evilghost at ...3397... <mailto:evilghost at ...3397...>
>     [mailto:evilghost at ...3397... <mailto:evilghost at ...3397...>]
>     > Sent: Friday, April 09, 2010 2:01 AM
>     > To: JOSH RIVEL, BLOOMBERG/ 731 LEXIN
>     > Cc: SNORT-SIGS at ...3473...
>     <mailto:SNORT-SIGS at ...3473...>
>     > Subject: Re: [Snort-sigs] Looking for HTTP POST's over 1mb in size
>     >
>     > Glad to help Josh, also drop the '/s', I meant to write the PCRE as:
>     >
>     > pcre:"/^Content-Length:\s*[0-9]{7,}$/i";
>     >
>     >
>     > -evilghost
>     >
>     > JOSH RIVEL, BLOOMBERG/ 731 LEXIN wrote:
>     >> evilghost-
>     >> Yeah my PCRE skills are pretty weak. I'll try your change and
>     let you know how it works out (I also change the source from "any"
>     to $HOME_NET as well)
>     >> Thanks!!
>     >> Josh
>     >>
>     >> ----- Original Message -----
>     >> From: Evilghost at ...3475... <evilghost at ...3397...
>     <mailto:evilghost at ...3397...>>
>     >> To: JOSH RIVEL (BLOOMBERG/ 731 LEXIN)
>     >> Cc: SNORT-SIGS at ...3473...
>     <mailto:SNORT-SIGS at ...3473...>
>     >> At: 4/08 12:49:17
>     >>
>     >> Hey Josh, isn't the root issue here 10[1-9] in the PCRE OR
>     match since
>     >> it'll match on on 101, 102, 103, etc?
>     >>
>     >> What about:
>     >>
>     >> pcre:"/^Content-Length:\s*[0-9]{7,}$/si";
>     >>
>     >> It'll still match against 1,000,000 bytes which is close enough
>     to 1Mb for me. Also, note sure why you need the other PCRE flags.
>     >>
>     >> -evilghost
>     >>
>     >>
>     >>
>     >>
>     >> JOSH RIVEL, BLOOMBERG/ 731 LEXIN wrote:
>     >>
>     >>> So I wrote a signature to detect HTTP POST's over 1mb in size,
>     but I think that my pcre logic is flawed. Can someone take a look
>     and let me know if this is OK? (It does work, but will trigger on
>     file sizes < 1mb based on the Content-Length: header)
>     >>> (We have some stuff in there to ignore posts to certain sites
>     due to too many false positives)
>     >>> The rule is:
>     >>> alert tcp any !20 -> $EXTERNAL_NET !25
>     (flow:established,to_server; priority:1; content:"POST"; nocase;
>     http_method; content:!"Shockwave"; nocase; http_header;
>     content:!"x-flash-version"; nocase; content:!"Host\: live.com
>     <http://live.com>"; nocase; http_header; content:!"Host\:
>     mail.google.com <http://mail.google.com>"; nocase; http_header;
>     content:!"Host\: mail.yahoo.com <http://mail.yahoo.com>"; nocase;
>     content:!"Host\: webmail.aol.com <http://webmail.aol.com>";
>     nocase; http_header; content:!"Host\: webmail.juno.com
>     <http://webmail.juno.com>"; nocase; http_header; content:!"Host\:
>     webmailb.juno.com <http://webmailb.juno.com>"; nocase;
>     http_header; content:"multipart/"; nocase;
>     content:"Content-Length\:"; nocase; http_header;
>     pcre:"/^Content-Length:\s*([1-9][0-9]{6,}|10[1-9])/smix";
>     pcre:!"/^Host:\s.*[\.live.com <http://live.com>]$/smi";
>     msg:"http-post-pcre-jr"; classtype:policy-violation; sid:1000060;
>     gid:1; rev:4; )
>     >>>
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>     >> >
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