By manipulating the TCP packet and changing the source address one is able to spoof the IP. As I understand it, you will not be able to set up a full handshake by doing this, as you will never receive the returning packets.
Does anyone know how this poses as a security risk today?
- SYN flooding from an IP that is not filtered.
- Connection hijacking by learning the next sequence number
- Bypass firewall and other defenses by acting as a legit source
- IDLE scan
- Smurf attack
- DNS Cache Poisoning
Update: Im editing this post with information from the answers