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...