According to Auto Trader less than 1000kg (500 for the older one, 750 for the later models). I know I looked at them when I had a Legacy and the towing weight of the Forester was less than the Legacy.
I'm not entirely sure the Auto Trader numbers are accurate (they were way out on my old 4x4), they tend to go on the unbraked weight not the braked weight like a horse trailer.
There seems to be a lot of variation between models and years from 1,000 to 1,700 kg towing limit so you'd need to know the exact model, ideally taking a look at the vehicle plate.