Update: I also posted this fix on Instructables which somehow made it to the front page as featured instructable! Go me!
For lens with wide zoom range, it’s not unusual that lens creep will happen sometime in its life. This phenomenon occurs as zoom ring loses friction and cannot hold the weight of the large front element. Canon EF 28-135mm f/3.5-5.6 IS USM is one of those lenses that have this problem, and I just found perhaps the most basic solution.