Oracle issued a patch outside its standard patch schedule as this vulnerability was rated as critical risk. Port 1521 could be successfully exploited by a remote user to impact the confidentiality, integrity and availability of systems. Our advice: Start patching as soon as possible, especially in those cases, where port 1521 can be reached via the internet. We expect a rise in port scans aming at TCP 1521. Have a close eye to your firewall logs, SIEM or any other intrusion detection tool you have implemented to know when you're hit.
Links:
1. Disclosure of this finding by Joxean Koret (Technical Details)
2. Oracle Scurity Alert CVE-2012-1675