Blocking nimda probes with a content-layer switch

Wed Sep 19 21:24:19 UTC 2001

If you're using snort for network intrusion monitoring, you can do something
like this:

alert tcp any any -> any 80 (content: "cmd.exe";msg: "cmd.exe access in HTTP!!";react: block;)
alert tcp any any -> any 80 (content: "root.exe";msg: "root.exe access in HTTP!!";react: block;)

If the snort box is on the same subnet as your webservers, the webserver will
still get the request, but will be sent a RST and won't respond.  Not as good
as dropping the packets at the front door, but better than your webserver
responding.

If you have the snort box in the data path, then you can actually drop the
packets instead of just sent the RSTs.

On Tue, 18 Sep 2001, Lincoln Dale wrote:

>
> At 12:32 AM 19/09/2001 -0400, Joe Abley wrote:
> >Has anybody tried to block nimda HTTP GET probes using URL
> >pattern matches in a "layer-4-7"[1] switch?
>
> note that this worm uses multiple methods for spreading - email and
> HTTP.  these methods are only addressing the HTTP side of things:
>
>
> NBAR (Network Based Application Recognition) in IOS:
>          existing CodeRed NBAR settings will block the HTTP probes.
>          see http://www.cisco.com/warp/public/63/nbar_acl_codered.shtml
>
> Cisco Content Engine:
>          rule enable
>          rule block url-regex http://.*/cmd\.exe
>          rule block url-regex http://.*/root\.exe
>
> Cisco CSS11K (Content Services switch):
>          service dummy
>            ip address 10.1.1.1
>            keepalive type none
>            active
>          !
>          header-field-group cmd.exe
>            header-field cmd.exe request-line contain "cmd.exe"
>          header-field-group root.exe
>           header-field root.exe request-line contain "root.exe"
>          !
>          content block_cmd.exe
>           protocol tcp
>           port 80
>           url "/*"
>           header-field-rule cmd.exe
>           add service dummy
>           active
>          content block_root.exe
>           protocol tcp
>           port 80
>           url "/*"
>           header-field-rule root.exe
>           add service dummy
>           active
>
> >The ideal result is to prevent nimda GET probes from ever
> >reaching the destination address, but causing the session
> >to be reset towards the server after the open handshake but
> >before the GET can be sent to the server would be acceptably
> >useful.
>
> note that only the CSS and the CE in the above will actually prevent the
> 'tcp syn' from propogating, and they'll establish the tcp 3-way handshake,
> receive the HTTP headers and then match the request to be an "attack".
> NBAR will only match the headers on connections that successfully establish
> a 3-way handshake.  (NBAR won't cause the router to spoof the TCP setup -
> so-called delayed-binding).
>
>
> cheers,
>
> lincoln.
>