CGI Script Source Code Disclosure Vulnerability in Apache for Windows

Advisory Content :

ADVISORY NAME:

CGI Script Source Code Disclosure Vulnerability in Apache for Windows

VULNERABLE SYSTEMS:

The vulnerability has been verified on Apache 2.2.2 running on Microsoft
Windows XP, Version 2002, Service Pack 2.

FOUND BY:

Susam Pal

FOUND ON:

8th August, 2007

VULNERABILITY TYPE:

Information Disclosure

SYSTEM DESCRIPTION:

Apache HTTPD is a web server that can run on many platforms to provide
web-service. The basic server configuration is controlled by the file
'httpd.conf'. The 'DocumentRoot' directive controls which directory is
considered to be root for serving documents. For instance:-

DocumentRoot "/home/webmaster/site/docroot/"

In the above example, a request to 'http://[target]/foo.html' would fetch
the 'foo.html' page from '/home/webmaster/site/docroot/' directory of the
server.

The 'ScriptAlias' directive controls which directory contains server
scripts. The following is an example of a typical 'ScriptAlias'
directive:-

ScriptAlias /cgi-bin/ "/home/webmaster/site/docroot/cgi-bin"

If a user makes a direct request to 'http://[target]/cgi-bin/foo' where
'cgi-bin' is the scripts' directory and 'foo' is the script, the user gets
the output of the 'foo' script. In a secure system, the user is not
supposed to view the source-code of 'foo' by making an HTTP GET request.

VULNERABILITY DESCRIPTION:

Usually the following directives in 'httpd.conf' file can be considered
safe for Unix/Linux (assuming that other directives haven't been insanely
edited):-

# Sample Safe Configuration for Unix/Linux

DocumentRoot "/home/webmaster/site/docroot/"

ScriptAlias /cgi-bin/ "/home/webmaster/site/docroot/cgi-bin"

But a similar configuration isn't safe in Windows. For instance:-

# Sample Unsafe Configuration for Windows

DocumentRoot "C:/Documents and Settings/webmaster/site/docroot"

ScriptAlias /cgi-bin/ "C:/Documents and
Settings/webmaster/site/docroot/cgi-bin/"

If the scripts' directory (represented by 'ScriptAlias') lies inside the
document-root directory (represented by 'DocumentRoot') and the name of the
script-alias is same as that of the directory containing the scripts then
the attacker can obtain the source code of the CGI scripts by making a
direct request to 'http://[target]/CGI-BIN/foo'.

Apache web-server checks for the exact case mentioned in the 'ScriptAlias'
directive before deciding whether the directory mentioned in the HTTP GET
request is a scripts' directory or not. So, when Apache web-server receives
a request for a file in 'CGI-BIN' directory, it finds it to be different
from 'cgi-bin' mentioned in the 'ScriptAlias' directive. So, it concludes
that it is not a script-alias. Then it checks for 'CGI-BIN' directory in
the document-root directory and finds it since file-names and
directory-names are not case-sensitive on Windows. So, it simply sends the
content of the 'foo' file as the HTTP response. It doesn't execute the
'foo' script because it isn't found in a directory pointed by
script-alias.

EXPLOIT:

The vulnerability can be exploited by making a direct request to
http://[target]/CGI-BIN/foo

PREVENTION:

1. Choosing a name for the 'ScriptAlias' different from the name of the
actual directory will reduce the risk. For instance,

# Sample Configuration for Reducing Risk

DocumentRoot "C:/Documents and Settings/webmaster/site/docroot"

ScriptAlias /cgi-bin/ "C:/Documents and
Settings/webmaster/site/docroot/sdy1x9y/"

The attacker can still get the source code by making a direct request to
'http://[target]/sdy1x9y/foo' if the attacker can somehow determine that
the 'ScriptAlias /cgi-bin/' refers to the 'sdy1x9y' directory.

2. A more secure preventive measure would be to place the scripts folder
outside the 'DocumentRoot' directory and then form a 'ScriptAlias' to it.
For instance,

# Sample Configuration for Increased Security

DocumentRoot "C:/Documents and Settings/webmaster/site/docroot"

ScriptAlias /cgi-bin/ "C:/Documents and Settings/webmaster/site/cgi-bin"

DISCLAIMER:

The information, codes and exploits in this advisory should be used for
research, experimentation, bug-fixes and patch-releases only. The author
shall not be liable in any event of any damages, incidental or
consequential, in connection with, or arising out of this advisory, or its
codes and exploits.

CONTACT INFORMATION:

For more information, please contact:-

Susam Pal

Infosys Technologies Ltd.

Survey No. 210, Manikonda Village

Lingampally, Rangareddy District

Hyderabad, PIN 500019

India

Phone No.: +91-9985259521

Email: susam.pal (at) gmail (dot) com [email concealed]

http://susampal.blogspot.com/

http://securecoding.blogspot.com/

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CGI Script Source Code Disclosure Vulnerability in Apache for Windows