123456789123456789 is simply a placeholder on the changebasic and editbasic pages for the password field so that it appears populated; the changebasic and editbasic script pages ignore the password field when this value is present.
if ($ChosenPassword != "123456789123456789") {
$changepassword = 1;
$ChosenPassword = md5($ChosenPassword);
}
It is odd that the system allowed the page to process the password change to this value as it's specifically coded to ignore that value.