I've had this problem in the past. From what other forester owners told me is that the rubber doesn't fully seal it from water, there's a drainage system with a single port on the left and right corners of the system. They're just small holes with a tube under the roof liner that drain outside. A few solutions I was told to do was to pull the head liner down and see if the tue is attached to the drain or that those drains may be plugged. Hope that helped.
And others correct me if I'm wrong, I'm going by memory on this.

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Welcome, hope we can help you out.

There is a TSB (safe) here http://www.subaruforester.org/vbulletin/f106/tsb-15-134-08r-sunroof-does-not-close-properly-44728/ that provides the part number for a shim kit that will help and as you can see there are numerous people who have had issues over the years going by this thread although not all related to your problem. http://www.subaruforester.org/vbulletin/f88/forester-moonroof-problems-merged-thread-3967/. From pages 34 and on there is some discussion regarding this shim fix and the availability of two different thickness of shims and part numbers are mentioned to make it work.

And there is also this video http://www.subaruforester.org/vbulletin/f80/video-sunroof-moonroof-fix-83587/

Hope this helps, let us know back here.

Cheers
Doug