APACHE HTTPD SERVER (current version 2.0.47):
##########################################################
How to return files in a Apache Deny All directory.
The Directives controlling host access may be bypassed even 
if they have not permission to be override.

11 Jan 2004


DESCRIPTION

Apache Web Server allows manage configurations via the main 
httpd.conf file, and via the other configuration files placed 
inside the children web trees, may owned by the user accounts, 
named .htaccess by default. The server administrator further 
controls what Directives may be placed in .htaccess files by 
configuring the AllowOverride Directive in the main httpd.conf 
files.

If the server admin sets the Deny Directive to All (for example), 
and does not allow the user accounts to modify this Directive in 
their .htaccess file by setting the AllowOverride values without 
the Limit type, his/her users are still able to bypass the Deny 
option by using the ErrorDocument Directive.

Tested in Apache 2.0.47/RH-Linux/WinXP.


EXAMPLE

##########################################################
# In the main httpd.conf file:
#
<Directory />
    AllowOverride FileInfo
    Deny From All
</Directory> 
##########################################################

##########################################################
# In the user's .htaccess file placed in a child directory:
 
ErrorDocument 403 /child/dir/fetch.php

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

/********************************************************/
<?php
// In the fetch.php placed in the same directory:

$url = parse_url( $_SERVER['REQUEST_URI'] );
@include basename( $url['path'] );
 
?>
/********************************************************/


In this example, assuming the web server can execute PHP script 
or some scripts/server-includes. By modifying some codes you can 
return other mime file types such as image/gif etc.

Event if the server does not allow any file parsed (Deny From All), 
the script file fetch.php will still be executed, and it includes 
again and parses any other files in a same directory, which 
indecated by the query variables, to the web client. 

Looking in the source code, I think the missing auth checking is in 
the function ap_process_request_internal() in the file request.c. 
One of the major changes in Apache 2.0 is to the internal redirect 
mechanism. To prevent the code from falling out of sync again in the
current directory, the configuration comparing between two config 
directories may bypass the authentication checking again. That 
explains why this vulnerability (if any?) can only done if the 
redirect ErrorDocument file is placed in the same request directory.



FIX

Do not skip auth checker even if the per_dir_config member value 
doesn't change in the ap_process_request_internal() function.


NOTE 

I post this issue in the public mailing list, because I think this 
vuln is not exploitable by a remote attacker. If something were 
wrong, drop a line to me.


Vietnamese Security Group
Trung - caothuvolam - trungonly@yahoo.com
http://www.security.com.vn

31 Nui Truc st. Ba Dinh dist. Ha Noi . Vietnamese
Phone : 84.4.8465701 / Fax: 84.4.8465701