Description

pyload-ng: Incomplete Tar Path Traversal Fix in UnTar._safe_extractall via os.path.commonprefix Bypass

Summary

The _safe_extractall() function in src/pyload/plugins/extractors/UnTar.py uses os.path.commonprefix() for its path traversal check, which performs character-level string comparison rather than path-level comparison. This allows a specially crafted tar archive to write files outside the intended extraction directory. The correct function os.path.commonpath() was added to the codebase in the GHSA-7g4m-8hx2-4qh3 fix (commit 5f4f0fa) but was never applied to _safe_extractall(), making this an incomplete fix.

Details

The GHSA-7g4m-8hx2-4qh3 fix (commit 5f4f0fa) added a correct is_within_directory() function to src/pyload/core/utils/fs.py:384-391 using os.path.commonpath():

# fs.py:384 — CORRECT implementation
def is_within_directory(base_dir, target_dir):
    real_base = os.path.realpath(base_dir)
    real_target = os.path.realpath(target_dir)
    return os.path.commonpath([real_base, real_target]) == real_base

However, the _safe_extractall() function in UnTar.py:10-22 was left unchanged with the broken os.path.commonprefix():

# UnTar.py:10-22 — VULNERABLE implementation
def _safe_extractall(tar, path=".", members=None, *, numeric_owner=False):
    def _is_within_directory(directory, target):
        abs_directory = os.path.abspath(directory)
        abs_target = os.path.abspath(target)
        prefix = os.path.commonprefix([abs_directory, abs_target])  # BUG: line 14
        return prefix == abs_directory
...
Show more

os.path.commonprefix() is a string operation, not a path operation. For extraction destination /downloads/pkg and a malicious member ../pkg_evil/payload (resolving to /downloads/pkg_evil/payload):

commonprefix(['/downloads/pkg', '/downloads/pkg_evil/payload']) → '/downloads/pkg' — equals the directory, check passes

commonpath(['/downloads/pkg', '/downloads/pkg_evil/payload']) → '/downloads' — does NOT equal the directory, check correctly fails

The extraction path is reached via: ExtractArchive.package_finished() (line 182) → extract_queued() → UnTar.extract() (line 76) → _safe_extractall(t, self.dest) (line 81).

PoC

Self-contained proof of concept demonstrating the bypass:

import tarfile, io, os, shutil

dest = '/tmp/test_extraction_dir'
shutil.rmtree(dest, ignore_errors=True)
shutil.rmtree('/tmp/test_extraction_dir_pwned', ignore_errors=True)
os.makedirs(dest, exist_ok=True)

# Step 1: Create malicious tar with member that escapes via prefix trick...
Show more

Output:

Member: ../test_extraction_dir_pwned/evil.txt
Resolved: /tmp/test_extraction_dir_pwned/evil.txt
Check passes (should be False): True
File escaped to: /tmp/test_extraction_dir_pwned/evil.txt
Content: escaped the sandbox!

Impact

An attacker who hosts a malicious .tar.gz archive on a file hosting service can write files to arbitrary sibling directories of the extraction path when a pyLoad user downloads and extracts the archive. This enables:

Writing files outside the intended extraction directory into adjacent directories

Overwriting other users' downloads

Planting malicious files in predictable locations on disk

If combined with other primitives (e.g., writing a .bashrc, cron job, or plugin file), this could lead to code execution

The attack requires the victim to download a malicious archive (either manually or via the pyLoad API with ADD permission) and have the ExtractArchive addon enabled.

Recommended Fix

Replace the broken inline _is_within_directory with the correct is_within_directory from pyload.core.utils.fs:

import os
import sys
import tarfile

from pyload.core.utils.fs import is_within_directory, safejoin
from pyload.plugins.base.extractor import ArchiveError, BaseExtractor, CRCError

...
Show more

This removes the broken inline function and uses the already-existing correct implementation that was added in the GHSA-7g4m-8hx2-4qh3 fix.

Mitigation

Aliases

References