@shiftsrl:
I do not know if you've resolved the issue yet or not, but today I was contacted by the administrator of a joomla site running on our server. He reported the exact same hack as you described (modified mod_mainmenu.php to include iframe to some site).
I searched the logfiles for his site and found the following lines:
Code:
82.207.99.8 - - [04/Aug/2006:11:06:51 +0200] "GET //administrator/components/com_classifieds/popups/ImageManager/images.php HTTP/1.0" 200 14196 "-" "Mozilla/4.0"
82.207.99.8 - - [04/Aug/2006:11:06:52 +0200] "GET //administrator/components/com_classifieds/popups/insert_image.php HTTP/1.0" 200 18502 "-" "Mozilla/4.0"
82.207.99.8 - - [04/Aug/2006:11:21:50 +0200] "POST /administrator/components/com_classifieds/popups/ImageManager/images.php HTTP/1.0" 200 14198 "-" "Mozilla/4.0 (compatible; Synapse)"
82.207.99.8 - - [04/Aug/2006:11:21:52 +0200] "GET /components/com_classifieds/adimages/config.php HTTP/1.0" 200 5 "-" "Mozilla/4.0"
This looked suspicious to me. A direct link to images.php? From an IP address in the Ukraine? I assume there aren't too many Ukranians who speak dutch (it's a dutch website).
I found that the direct URL does indeed allow direct execution of images.php, bypassing normal joomla security. That file, as well as a few other php files in that folder are missing some of the standard joomla security checks (like "defined( '_VALID_MOS' ) or die( 'Restricted access' );" for example). I'm not a joomla expert but I assume this is the cause of the vulnerability.
I still need the do several more checks to see if this is indeed the cause of the problem, but at the moment it sure looks like it. Since you posted that you're also running com_classifieds the same vulnerability may also be the cause of your problem.
HTH
Cheers,
Aspegic