This is called "Session Fixation". Most modern frameworks prevent it, which is a reason to use your framework's session functionality rather than re-inventing it.
Session fixation used to be a common problem. There were lots of J2EE applications which were not only fix-able, but which would allow an attacker to fix a session ID with a carefully crafted GET URL. It's much rarer now.
Session fixation used to be a common problem. There were lots of J2EE applications which were not only fix-able, but which would allow an attacker to fix a session ID with a carefully crafted GET URL. It's much rarer now.