GitHub Security Advisory: GHSA-6r62-w2q3-48hf
Package: bentoml (PIP)
Severity: HIGH (CVSS 7.4)
CWE: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Vulnerable versions: < 1.4.34
Patched in: 1.4.34

Summary: BentoML has a Path Traversal via Bentofile Configuration

Details:
### Summary

BentoML's `bentofile.yaml` configuration allows path traversal attacks through multiple file path fields (`description`, `docker.setup_script`, `docker.dockerfile_template`, `conda.environment_yml`). An attacker can craft a malicious bentofile that, when built by a victim, exfiltrates arbitrary files from the filesystem into the bento archive. This enables supply chain attacks where sensitive files (SSH keys, credentials, environment variables) are silently embedded in bentos and exposed when pushed to registries or deployed.

### Details

The vulnerability exists in how BentoML resolves user-provided file paths without validating that they remain within the build context directory.

**Vulnerable function** in `src/bentoml/_internal/utils/filesystem.py:114-131`:

```python
def resolve_user_filepath(filepath: str, ctx: t.Optional[str]) -> str:
    _path = os.path.expanduser(os.path.expandvars(filepath))
    if not os.path.isabs(_path) and ctx:
        _path = os.path.expanduser(os.path.join(ctx, filepath))
    if os.path.exists(_path):
        return os.path.realpath(_path)  # No path containment check
    raise FileNotFoundError(f"file {filepath} not found")
```

**Vulnerable code** in `src/bentoml/_internal/bento/bento.py:348-355`:

```python
if build_config.description.startswith("file:"):
    file_name = build_config.description[5:].strip()
    if not ctx_path.joinpath(file_name).exists():
        raise InvalidArgument(f"File {file_name} does not exist.")
    shutil.copy(ctx_path.joinpath(file_name), bento_readme)  # Path traversal
```

All four vulnerable fields:
- `description: "file:../../../etc/passwd"` → copied to `README.md`
- `docker.setup_script: "../../../etc/passwd"` → copied to `env/docker/setup_script`
- `docker.dockerfile_template: "../../../secret"` → copied to `env/docker/Dockerfile.template`
- `conda.environment_yml: "../../../etc/hosts"` → copied to `env/conda/environment.yml`

**Multiple path formats are supported, making exploitation trivial:**

| Format | `description` | `setup_script` | `dockerfile_template` | `environment_yml` |
|--------|---------------|----------------|----------------------|-------------------|
| Absolute paths (`/etc/passwd`) | Yes | Yes | Yes | Yes |
| Tilde expansion (`~/.ssh/id_rsa`) | No | Yes | Yes | Yes |
| Env vars (`$HOME/.aws/credentials`) | No | Yes | Yes | Yes |
| Relative traversal (`../../../etc/passwd`) | Yes | Yes | Yes | Yes |
| Proc filesystem (`/proc/self/environ`) | Yes | Yes | Yes | Yes |

The `description` field uses `pathlib.Path.joinpath()` directly, while other fields use `resolve_user_filepath()` which calls `os.path.expanduser()` and `os.path.expandvars()`.

The `/proc/self/environ` vector is particularly dangerous in CI/CD pipelines where secrets are commonly passed as environment variables (`AWS_SECRET_ACCESS_KEY`, `GITHUB_TOKEN`, `DATABASE_PASSWORD`, etc.).

### PoC

1. Create a minimal service:

```python
# service.py
import bentoml

@bentoml.service
class Te