The incident took place at about 5.30am in The Gullet when the victim was pushed over by an Asian man in a black hooded top before being sexually assaulted.
The attacker, who ran away, is described as being in his 20s and has a moustache.
No arrests have been made.
Call 101 or Crimestoppers anonymously on 0800 555111.
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The victim of an assault in Cippenham was flown to hospital by air ambulance this afternoon.
A rapid response vehicle, an ambulance officer, an air ambulance and an ambulance crew were all called to an address in Earls Lane at 2.10pm.