For the example QR-Code:
http://www.example.com/User/login?username=simon&password=12345
In Controller use this in login-action:
if ($this->request->is('get'))
{
$user = $this->User->find('first', array(
'conditions' => array(
'User.name' => $this->request->query['username'],
'User.password' => $this->request->query['password'] // don´t forget to hash password here
)));
if ($user != null)
{
$this->Auth->login($user);
$this->redirect('/admin/index');
}
}