CORS (Cross-Origin Resource Sharing) allows web pages to retrieve the contents of pages from other sites, with their permission, as they would appear for the current user. When requests are made in this way, the browser should only allow the page content to be retrieved if the target site sends the correct headers that give permission for their contents to be used in this way. Specially crafted requests may trick Opera into thinking that the target site has given permission when it had not done so. This can result in the contents of any target page being revealed to untrusted sites, including any sensitive information or session IDs contained within the source of those pages.
Opera Software has released Opera 12.10, where this issue has been fixed.