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.

Code
	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.