Hot on the heels of the news about a reshare bug on Google+, we’ve detected another flaw that should be of real concern to users
The Financial Times already reported on the important privacy issue regarding the reshare feature that cannot be disabled by default, and this related flaw presents similar problems.
One of the main features of Google+ is Circles, the possibility to easily share the right content to the right people. However, once the content has been shared to a Circle, anyone can share it by default to other Circles. It boils down to the fact that the tagging feature can be bypassed by using the reshare option. Let's say user “A” shares a picture only to their “Close Friends” circle, and disables resharing. All it takes is for someone from that "Close friend" circle to tag a person from outside this circle in the picture. Once this has been done, that person can share the picture to anyone, in any way.
Source: BitDefender Blog