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.