Is ci/sessions safe to use?

[eluser]walrus_lt[/eluser]
Hi. Is ci/sessions safe to use? I heard that ci/session saves information in user's cookie. So...

If i do:

Code:

$this->session->set_userdata(array('id'=>$id, 'nick'=>$nick))

In another place:

Code:

if ($this->session->userdata('id') == '') die('NOT LOGED IN')

Can it be hacked if hacker creates cookie in his browser with id=1?