In this article “When testing some x-frame-options behavior, I noticed that sameorigin cares about the top location, but not the parent location. At first I thought I discovered this, but turns out it’s not a brand new issue, and others have talked about it at least here and here.
It wasn’t immediately clear to me what sites were impacted, but two conditions need to be present: 1) somewhere on the victim domain, they need to allow you to frame stuff and 2) the victim domain uses x-frame-options sameorigin as a mitigation against clickjacking. With number two, most websites tend to not protect against clickjacking at all (which is worse, of course). However, there are a few places where these two conditions seem pretty pervasive. Google is the first thing that comes to mind because it’s common for Google to protect from clickjacking with x-frame-options: sameorigin, but then they allow external framing on the same domain. Using this and a little social engineering you can do some pretty nasty things, like an unprivileged Google Apps user taking over a domain.”