Roof leak around metal chimney

Hi there-

I have a leak coming in somewhere around my chimney (water dripping down the chimney pipe). The house has a flat rubber (I asume EPDM) roof. You can see in the picture that when the chimney was installed, it probably wasn’t the greatest roofing job. I think regular roofing cement type material was just used around the edges. What would be the best fix for this?

https://imgur.com/a/wfuCXAd