Directories management bypassing in Goahead webserver <= 2.1.8

[Luigi Auriemma <aluigi () altervista org>]

#######################################################################

                             Luigi Auriemma

Application:  Goahead webserver
              http://www.goahead.com/webserver/webserver.htm
Versions:     <= 2.1.8
Platforms:    multiplatform
Bug:          bypassing of special directories management with the
              effect of downloading cgi-bin files and more
Risk:         medium/high
Exploitation: remote with browser
Date:         19 Jan 2004
Author:       Luigi Auriemma
              e-mail: aluigi () altervista org
              web:    http://aluigi.altervista.org


#######################################################################


1) Introduction
2) Bug
3) The Code
4) Fix


#######################################################################

===============
1) Introduction
===============


Goahead webserver is an embedded OpenSource server that can be build on
a lot of systems (CE, Ecos, GNU/Linux, Lynx, MacOS, NW, QNX4, VXWORKS,
Win32 and others).
It is supported by a lot of companies that use it for their projects
and it is also used like "base" for other webservers, furthermore it
has been developed for be very tiny and to run on embedded systems.


#######################################################################

======
2) Bug
======


Goahead webserver has an internal problem that lets it to accept also
HTTP requests that don't start with the slash or that contain
backslashes (both \ and %5c) after or at the same place of the initial
slash.
So we have that a right request as "GET /file HTTP/1.0" is accepted
but is also accepted a bad request as "GET file HTTP/1.0" or
"GET \file HTTP/1.0", "GET /\\%5cfile HTTP/1.0" and so on.

This bug leads to the bypassing of the management of the "special"
directories (as cgi-bin) and to their usage as normal "unmanaged"
directories.

In fact the server uses a function called websUrlHandlerDefine()
letting the admin to specify how to manage each directory he wants.
For example by default we have

        websUrlHandlerDefine(T("/goform"), NULL, 0, websFormHandler, 0);

used to execute the built-in functions written by the same admin or

        websUrlHandlerDefine(T("/cgi-bin"), NULL, 0, websCgiHandler, 0);

used just to manage the cgi-bin directory.

So if an attacker uses a HTTP request as one of those seen previously
he will easily bypass the management decided by the admin for a
specific directory (function websUrlHandlerRequest()).

The most common and useful effect of this bug is the download and the
viewing of any file in the cgi-bin directory.


#######################################################################

===========
3) The Code
===========


Some examples for reading the files in the cgi-bin directory:

   http://server/\cgi-bin/cgitest.c
   http://server/\\\cgi-bin/cgitest.c
   http://server/%5ccgi-bin/cgitest.c

   "GET cgi-bin/cgitest.c HTTP/1.0"
   "GET \cgi-bin/cgitest.c HTTP/1.0"
   "GET %5ccgi-bin/cgitest.c HTTP/1.0"


#######################################################################

======
4) Fix
======


No official fix.

A simple patch can be the adding of the following 3 lines of code at
about the line 265 of the file handler.c in the function
"int websUrlHandlerRequest(webs_t wp)":

   if((wp->path[0] != '/') || strchr(wp->path, '\\')) {
       websError(wp, 400, T("Bad request"));
       return(0);
   }


#######################################################################


--- 
Luigi Auriemma
http://aluigi.altervista.org