Zend-Session session validation vulnerability

Summary

`Zend\Session` session validators do not work as expected if set prior to the start of a session. For instance, the following test case fails (where $this->manager is an instance of `Zend\Session\SessionManager`): ``` $this ->manager ->getValidatorChain() ->attach('session.validate', array(new RemoteAddr(), 'isValid')); $this->manager->start(); $this->assertSame( array( 'Zend\Session\Validator\RemoteAddr' =3D> '', ), $_SESSION['__ZF']['_VALID'] ); ``` The implication is that subsequent calls to `Zend\Session\SessionManager#start()` (in later requests, assuming a session was created) will not have any validator metadata attached, which causes any validator metadata to be re-built from scratch, thus marking the session as valid. An attacker is thus able to simply ignore session validators such as `RemoteAddr` or `HttpUserAgent`, since the "signature" that these validators check against is not being stored in the session.