Remote Web Server Text File Viewing Vulnerability in WebLibs 1.0

[John Bissell <monkey321_1 () hotmail com>]

Advisory:
=========
 Remote Web Server Text File Viewing Vulnerability in WebLibs 1.0

Author:
=======
 John Bissell A.K.A. HighT1mes

Vendor Homepage:
================
 http://awsd.com/

Date:
=====
 12, 07, 2004

Severity:
=========
 High

Overview:
=========
WebLibs is a pretty popular simple little Perl script made avaiable from  Darryl C. Burgdorf on his site at 
http://awsd.com/scripts/weblibs/index.shtml. This script is very old and I'm surprised no one else came upon this 
vulnerability a while ago. People can install the script on there server so they can invite visitors to thier site to 
have some fun and create there own mad libs called web libs to amuse at...

Problem Details:
================
The security vulnerability as whoever is reading this might guess is caused by a lack of proper input validation when 
the weblibs.pl script reads in a static piece of sensitive data hidden in a hidden <INPUT> tag embedded inside the 
dynamically generated web page. This lack of input validation leads to a typical directory traversal attack (sometimes 
directory traversal isn't even needed if the web enviroment is setup in a particular way) that can be leveraged to read 
in non-executable text file from the vulnerable web server that the web server has permission. The line of Perl code 
that causes this vulnerability is...

open (FILE,"$FORM{'TextFile'}");

There may be a way to get local files to execute on the system or get the output from commands like ls -la. I haven't 
done any through testing so I will leave this open for the computer security community to look into if they wish.

Explotation:
============
To exploit this vulnerability the attacker/pen tester needs to first save a copy of the initial generated WebLibs page 
that asks for the users input so the script can generate the results for the user. Next after making a local copy of 
that page you need to look for the HTML <INPUT> tag that has a name="TextFile" attribute. For instance <INPUT 
type=hidden value="/some/path/to/the/weblibs/input.txt" name="TextFile"> 

Now simply edit the line to something other non executable server file like /etc/passwd or /etc/hosts etc to read 
sensitive private data from the vulnerable web server which may give the the person attacking a chance at gaining shell 
access to the web server. For example edit the HTML line mentioned above to something like <INPUT type=hidden 
value="/../../../../../etc/passwd" name="TextFile"> then save the file and load it up in your web browser, fill in all 
the form input fields hit the submit button and the vulnerable web server will return the contents of the text file 
requested if allowed.

Solution:
=========
 One possible solution is to not use the hidden <INPUT> tag in the generated pages from weblibs.pl and instead define 
$TextFile as a user installation variable that needs to be filled in only in the beginning by the script installer so 
the attacker can't easily manipulate it.

Vendor Status:
==============
 The author of this script has been notified by email of this issue...