Internet Explorer for Mac 5.2.3, Internet Explorer 6 on Windows XP, and possibly other versions, 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
http://secunia.com/multiple_browsers_frame_injection_vulnerability_test/