Yes it does, through exhaust fumes from visiting spacecraft. The Russians did an EVA summer 2015 to clean the windows.

Russian cosmonauts clean ISS' windows in a six-hour spacewalk

It hit the news because the Russians said they had found microbes on the windows. Leading to the question, how did they get there? Microbial life found living on the exterior of the International Space and German Space Agency Chimes In on Alleged Discovery of Sea Plankton on ISS

I’m not sure what the latest is on that. Microbial life has been found high in the upper atmosphere of Earth. If it did somehow get to the height of the ISS and impacted on the windows, you’d expect it to be thoroughly sterilized by then by the unfiltered UV radiation from the sun and then the nearly 8 km / sec impact on the ISS (as that’s the speed with which it is traveling through the very very tenuous hard vacuum “atmosphere” at that height).

(Microbial life can survive in space in a dormant state, sheltered from UV light by other microbes in a biofilm, or in a minute crack or shadow on the surface of a spacecraft, or just beneath the surface of a meteorite. UV light is just a form of light and a thin layer of anything that casts a shadow will block it. However, if not protected like that, even the most hardy microbes., the ones with UV shielding pigments and ability to repair their own DNA after damage, can’t survive direct UV for more than a few hours if exposed to it directly without the filtering effect of our atmosphere).

So doesn’t seem too likely it’s alive if they did find evidence of life, most likely dead - it could also be contamination from Earth that somehow got onto the windows from visiting rockets say. If anyone reading this knows the latest on this, do say.