Bug report

Bug description:

Summary

I found a Windows-specific issue in shutil.unpack_archive() when extracting ZIP files.

In Lib/shutil.py, the private helper _unpack_zipfile() skips names that start with / or contain .., but it does not reject or sanitize Windows drive-prefixed names such as D:/path/file.

On Windows, such names are joined into a drive-qualified path and can escape the intended extraction directory. As a result, a crafted ZIP archive can cause files to be written outside extract_dir.

Affected component

• Lib/shutil.py
• _unpack_zipfile()

Impact

A crafted ZIP archive can cause an arbitrary file write outside extract_dir on Windows.

Trigger condition

• Platform: Windows
• Archive format: ZIP
• A ZIP entry name contains a drive prefix such as D:/...
• extract_dir is on a different drive

Tested environment

I reproduced this on Windows with Python 3.12.8.

Minimal reproduction

I attached a minimal repro script:

• repro_shutil_unpack_zip_windows_drive_path_min.py

The repro creates a ZIP archive containing an entry like:

D:/shutil_outside_min.txt

and then calls:

shutil.unpack_archive(str(zip_path), str(extract_dir))
With extract_dir on another drive, the file is written outside the intended extraction directory.

Root cause
The validation in _unpack_zipfile() appears incomplete for Windows path semantics. Checking only for leading / and .. is not sufficient, because drive-prefixed paths such as D:/... are still treated as rooted or drive-qualified paths by Windows path handling.

For comparison, zipfile's own extraction logic strips drive information before extraction, but shutil._unpack_zipfile() does not.

Expected behavior
ZIP entries that are absolute, drive-prefixed, or otherwise resolve outside extract_dir on Windows should be rejected or normalized so that extraction always remains within extract_dir.

Actual behavior
A crafted ZIP entry with a drive prefix can escape extract_dir and be written to another location.

Additional context
I previously reported this privately to the Python Security Response Team on March 28, 2026, but I have not received a response yet, so I am opening this issue for tracking and triage.

I also attached a short write-up:

vuln_shutil_unpack_archive_zip_windows_drive_path.md

shutil_unpack_archive_zip_windows_drive_path_min.zip

CPython versions tested on:

CPython main branch, 3.14

Operating systems tested on:

Windows

Linked PRs

• gh-146581: Use ZipFile.extractall() in shutil for secure ZIP extraction #146588
• gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows #146591
• [3.14] gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows (GH-146591) #149064
• [3.13] gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows (GH-146591) #149065
• [3.12] gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows (GH-146591) #149066
• [3.11] gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows (GH-146591) #149071
• gh-146581: Update docs for dangerous filenames in ZIP files #149994
• [3.15] gh-146581: Update docs for dangerous filenames in ZIP files (GH-149994) #150064
• [3.14] gh-146581: Update docs for dangerous filenames in ZIP files (GH-149994) #150065
• [3.13] gh-146581: Update docs for dangerous filenames in ZIP files (GH-149994) #150066
• [3.10] gh-146581: Fix vulnerability in shutil.unpack_archive() for ZIP files on Windows (GH-146591) (GH-149071) #155440