Opera 7.51 for Windows and 7.50 for Linux does not properly prevent a frame in one domain from injecting content into a frame that belongs to another domain, which facilitates web site spoofing and other attacks, aka the frame injection vulnerability.
https://exchange.xforce.ibmcloud.com/vulnerabilities/1598
https://euvd.enisa.europa.eu/vulnerability/EUVD-2004-0715
http://secunia.com/multiple_browsers_frame_injection_vulnerability_test/