Seems like shutil._unpack_tarfile() is affected. I guess it could at least do with one of those warnings in the documentation for make_archive().

The patch for this bug looks a bit over enthusiastic, for example skip_prefixes("blaua../stuff") would incorrectly strip the first bit and just return "stuff".

It seems there might already be plenty of existing code to check for bad paths. Examples that come to mind:

* http.server.SimpleHTTPRequestHandler.translate_path()
* zipfile.ZipFile._extract_member()
* shutil._unpack_zipfile()

This code either ignores the bad path elements, or ignores the whole path. Perhaps some of it could be recycled into a common function somewhere, rather than implementing it all over again for tar files.

I have written my own function joinpath() to do this sort of checking, which you are welcome to use:

https://bitbucket.org/vadmium/pyrescene/src/34264f6/rescene/utility.py#cl-217

You would call it with something like joinpath(tarpath.split("/"), osdir).