It's difficult to tell from just one picture. But the light has entered from the bottom of the film, just about parallell to the film plane. At the bottom of the image (remember the film is upside down in the camera), there is a reflection from the top of the film holder.

Odds are good that it's a leak in the tape on the flap of the film holder. Camera leaks usually aren't that parallell to the film plane.

The leak I has was in the seal between camera back and film holder - it showed some of the same characteristics, but stayed a fixed distance from the edge of the film.