Sure:

7.1.5.1/4: Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior.

const_cast is not undefined behavior, but modifying a const object is.

const_cast is undefined behavior if the actual object is const. If it's a mutable object that just happened to be passed as a reference you can const_cast it. Or that's how I understand it.