Claroline contains a flaw that may allow a remote attacker to execute arbitrary commands. The issue is due to claro_init_header.inc.php not properly sanitizing user supplied input. This may allow an attacker to include a file from a remote host that contains arbitrary commands which will be executed by the vulnerable script.
Upgrade to version 1.5.4, 1.6 final or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.