pydicom: Path traversal in FileSet/DICOMDIR ReferencedFileID allows file access outside the File-set root

pydicom is a pure Python package for working with DICOM files. Versions 2.0.0-rc.1 through 3.0.1 are vulnerable to Path Traversal through a maliciously crafted DICOMDIR ReferencedFileID when it is set to a path outside the File-set root. pydicom resolves the path only to confirm that it exists, but does not verify that the resolved path remains under the File-set root. Subsequent public FileSet operations such as copy(), write(), and remove()+write(use_existing=True) use that unchecked path in file I/O operations. This allows arbitrary file read/copy and, in some flows, move/delete outside the File-set root. This issue has been fixed in version 3.0.2.