There was a thread here a while ago :
http://www.clubsnap.com/forums/threads/144692-Help-solve-light-leak-on-Kiev-88...
The Kievaholic page might be useful if you need more info.
http://www.kievaholic.com/
No expert and don't have this camera. but my few cents,
1. Try looking at the inside of the camera with the camera under a very bright lamp to see if the light leaks in.
For the front, take off the film back and look in. For the rear, put on the film back and look from front with mirror up (or something similar).
2. Tape up with black electrical tape the areas that are suspicious (eg. film back)
3. For #2, you might want to tape all, shoot 1 frame, remove 1 tape > shoot 1 frame, etc. All the time noting which frame correlates to which tape taken off. This will let you identify where the leak is coming from.
Good luck.