Summary

A path traversal vulnerability in PackageIndex was fixed in setuptools version 78.1.1

Details

    def _download_url(self, url, tmpdir):
        # Determine download filename
        #
        name, _fragment = egg_info_for_url(url)
        if name:
            while '..' in name:
                name = name.replace('..', '.').replace('\\', '_')
        else:
            name = "__downloaded__"  # default if URL has no path contents

        if name.endswith('.[egg.zip](http://egg.zip/)'):
            name = name[:-4]  # strip the extra .zip before download

 -->       filename = os.path.join(tmpdir, name)

Here: https://github.com/pypa/setuptools/blob/6ead555c5fb29bc57fe6105b1bffc163f56fd558/setuptools/package_index.py#L810C1-L825C88

os.path.join() discards the first argument tmpdir if the second begins with a slash or drive letter.
name is derived from a URL without sufficient sanitization. While there is some attempt to sanitize by replacing instances of '..' with '.', it is insufficient.

Risk Assessment

As easy_install and package_index are deprecated, the exploitation surface is reduced.
However, it seems this could be exploited in a similar fashion like GHSA-r9hx-vwmv-q579, and as described by POC 4 in GHSA-cx63-2mw6-8hw5 report: via malicious URLs present on the pages of a package index.

Impact

An attacker would be allowed to write files to arbitrary locations on the filesystem with the permissions of the process running the Python code, which could escalate to RCE depending on the context.

References

https://huntr.com/bounties/d6362117-ad57-4e83-951f-b8141c6e7ca5
#4946