Does your aperture plate or film holder properly mask the negative? Are you using the correct film holder for the negative you are using?
Are you cropping in on the image, thus leaving some of the image (and some of the enlarger's light beam) outside the borders of the four-bladed easel you are using?
'Cause here's what I see in the image.. I see that you are using a piece of paper that's larger than the blades as you have them set on your four-blade easel and some of that paper that's peeking out between the gaps in the blades is getting exposed by the "excess" light that comes from the lens because you are cropping too tight.
Use smaller paper or readjust the blades of the easel so that they don't let some of it peek out.
OR... Place a piece of cardboard masking on the easel around the blades where the paper peeks out.
OR... Readjust and re-crop your picture so that light doesn't spill out.
OR... Check to be sure you have the right aperture and/or film holder in place to match the aspect ratio and/or format of film you are using.