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.
All you need is an elastic hair band. The type required for this fix is the flat one as regular rubber band is too thick. The one I use is Goody black elastics. For aesthetic, I'd suggest using black one as it blends in with the lens.