Message Sender can read Receiver's message

Sender sends message and can read it using link from Receiver's notification email.

Two users - 'Sender' & 'Receiver' are necessary for test.

1. 'Sender' login in forum,
2. 'Sender' sends private message to 'Receiver',
3. 'Sender' logout,
4. 'Receiver' receives notification email,
5. 'Receiver' clicks on link to received private message; page 'sign in' is shown,
6. login as 'Sender'(NOT 'Receiver');

Actual Result: received message is shown.

Expected result:'Access Denied' page.