I’ve reproduced this behavior by logging in to Google Book search using a Gmail account in Internet Explorer. I then closed IE, switched to Firefox, and logged out of my Google account in Firefox. I then opened the URL with the “auth” field, and found myself being automatically logged into the account – the password of which I didn’t enter in Firefox.
Now from what I understand and tested, you will not be able to switch into other Google account applications (like Gmail) this way, and you will not be able to hack anyone else’s account this way – all you do is see the other person’s Gmail name. So the only thing you can do with this information is to be careful to remove the “auth” field from Google Book search URLs you pass on to others.
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