/usr/include/CGAL/Polynomial/internal/Kernel/ is in libcgal-dev 4.2-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/CGAL/Polynomial/internal/Kernel/Is_even_multiplicity.h
/usr/include/CGAL/Polynomial/internal/Kernel/Is_rational.h
/usr/include/CGAL/Polynomial/internal/Kernel/Isolating_interval.h
/usr/include/CGAL/Polynomial/internal/Kernel/Lower_bound_root.h
/usr/include/CGAL/Polynomial/internal/Kernel/Multiplicity.h
/usr/include/CGAL/Polynomial/internal/Kernel/Rational_between_roots.h
/usr/include/CGAL/Polynomial/internal/Kernel/Root_container.h
/usr/include/CGAL/Polynomial/internal/Kernel/Sign_above.h
/usr/include/CGAL/Polynomial/internal/Kernel/Sign_at.h
/usr/include/CGAL/Polynomial/internal/Kernel/Sign_below.h
/usr/include/CGAL/Polynomial/internal/Kernel/Sign_between_roots.h
/usr/include/CGAL/Polynomial/internal/Kernel/To_rational.h
/usr/include/CGAL/Polynomial/internal/Kernel/predicate_template.h