If it is, it shouldn't be.

Really, illegal? I need to find one as I'm often away for work for a couple months, even longer.

I don't own a car. I don't own a parking space. In other words, I have no skin in this issue but I do think it is completely nuts that people can't rent out an unused parking space if they own one.

It is illegal to rent driveways or spaces in building's garage for a few reasons - one being liability (homeowners does not cover that), security and generally, people living on residential streets do not want to live next to "businesses" where unknown people are coming and going (think shared driveway). Also, many people collect and don't report the rental income and park their own cars (stickers) on the street, which puts more stress on their neighbors that don't have off street parking.

City Hall – DR designou esta questão a Traffic - KC (Permits)

People that don't own cars ride in cars and still have things delivered in cars and have visitors that arrive in cars and legitimately use Visitor passes. All of those cars have to be parked somewhere and often they are idling and parking in resident parking when waiting for next ride or delivery call.

Thank you for reporting your concern to City of Cambridge via Commonwealth Connect. We have reviewed the information you provided our staff and will be investigating this issue further.