A CLA only covers the things you wrote, and while you ensure in the CLA that you only contribute things you wrote and are allowed to license to them, it is poss

A CLA only covers the things you wrote, and while you ensure in the CLA that you only contribute things you wrote and are allowed to license to them, it is possible that you lie, so there is the danger (although a small one), that contributors might upload proprietary code written by others. IANAL