Yes. This is entirely legal. (It is dangerous, but it is legal.) If you (attempt to) modify a an object declared const, then the behaviour is undefined.
From n4659 (which is the last draft of C++17), section 10.1.7.1 [dcl.type.cv] para 4:
Except that any class member declared mutable (10.1.1) can be modified, any attempt to modify a const object during its lifetime (6.8) results in undefined behavior
My emphasis. That is from C++17, but this has been true of all versions of C++.
If you look at the section on const_cast there is a note that
[ Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer
to data member resulting from a const_cast that casts away a const-qualifier76 may produce undefined
behavior (10.1.7.1). — end note ]
Notes are not normative, but this strongly implies that obtaining a non-const reference or pointer to a const object is legal. It is the write that is not allowed.
New references for C++20:
From N4680 (which is the last draft of C++20), section 9.2.8.1 [dcl.type.cv] para 4:
Any attempt to modify (7.6.19, 7.6.1.5, 7.6.2.2) a const object (6.8.3) during its lifetime (6.7.3) results in
undefined behavior.
If you look at the section on const_cast, section 7.6.1.10 [expr.const.cast] para 6 linking to above statement:
[Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier may produce undefined
behavior (9.2.8.1).]