Safer tar extraction

safer_tar_extract.py

# SPDX-FileCopyrightText: 2023 mara004
# SPDX-License-Identifier: CC-BY-4.0 OR Apache-2.0 OR BSD-3-Clause

# Safer tar extraction (hopefully) preventing CVE-2007-4559 etc.
# Tries to use the most elegant strategy available in the caller's python version (>= 3.6)

__all__ = ["safer_tar_unpack"]

import sys
if sys.version_info >= (3, 11, 4):  # PEP 706
    import shutil
    def safer_tar_unpack(archive_path, dest_dir):
        shutil.unpack_archive(archive_path, dest_dir, format="tar", filter="data")

else:  # workaround
    import tarfile
    from pathlib import Path
    
    def safer_tar_unpack(archive_path, dest_dir):
        dest_dir = Path(dest_dir).resolve()
        with tarfile.open(archive_path) as tar:
            for m in tar.getmembers():
                if not ((m.isfile() or m.isdir()) and dest_dir in (dest_dir/m.name).resolve().parents):
                    raise RuntimeError("Path traversal, symlink or non-file member in tar archive (probably malicious).")
            tar.extractall(dest_dir)

Just tested with Jakub Wilk's traversal archives.
Results good. Workaround code correctly blocks all invalid samples.
Same for PEP 706 code, except that it automatically converts absolute to relative paths, i.e. they are extracted into dest_dir. This is safe, only a behavioral difference.

Also confirmed with all strategies that we prevent traversal even if a directory of same prefix exists in the directory the archive attempts to traverse to, while invalid archives get through with the < (3, 9) code passage when changing commonpath() to commonprefix(). This proofs the Trellix patch and pip's code are incorrect (they use commonprefix() as of this writing).

I just realized that we could simplify the is_within_dir() check to dir in path.parents.
It'd be easier than both current strategies (no version checks), and strictly speaking also more correct, because it does not permit the case that path == dir.

Also note, a known limitation of the above code is that existing files in dir may be overwritten by the archive. But this is expected.

Re Comment 2: I updated the code accordingly, however the latest revision is now untested.