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So, you thought Google is perfect? Well, they are definitely the best, but that doesn't mean they never do anything wrong. Not too long ago, there was a report on security loophole on Google custom domains, where the new feature in Blogger was easily exploited to make it appear to be hosted on Google, and that actually allow any experienced hacker to insert some javascript code to steal their Google cookies and access into your Google account and abuse the information that you have. And the good news is, it have already been fixed. And I would say, we can rest assured this problem won't occured again in future. So, there's no need to worry about it.
It was first discovered by Tony. The details of what actually he discovered can be read on Outer-Court blog. There was a similar security loophole with the Google Public search service previously. And the page has been down since then.

Source: Details of Google’s Latest Security Hole