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CGI Script Source Code Disclosure Vulnerability in Apache for Windows
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SUMMARY
Apache <http://www.apache.org/> HTTPD is "a web server that can run on
many platforms to provide web-service". A vulnerability in Apache running
on Windows allows remote attackers to disclose the content of files
located on the remote machine.
DETAILS
Vulnerable Systems:
* Apache 2.2.2 running on Microsoft Windows XP, Version 2002, Service
Pack 2
Background:
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"
ADDITIONAL INFORMATION
The information has been provided by <mailto:susam.pal@gmail.com> Susam
Pal.
The original article can be found at: <http://susampal.blogspot.com/>
http://susampal.blogspot.com/
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