Hardening Your Multistage Builds: Security Considerations from Base Image to Final Artifact
Published: 2024-06-14 13:35:15

Multistage builds are a powerful tool in Docker, allowing developers to create efficient and optimized Docker images by breaking the build process into multiple stages. However, this complexity also introduces potential security risks if not properly managed. In this article, we will delve into the security considerations for multistage builds, from the base image to the final artifact, and provide practical guidance on how to harden your builds.

### Base Image Selection

The base image is the foundation of your Docker image, and its security is crucial. When selecting a base image, consider the following factors:

- **Official Images**: Use official images from Docker Hub or other trusted sources. These images are regularly updated and maintained, ensuring you have the latest security patches.
- **Vulnerability Scanning**: Perform vulnerability scanning on the base image to identify potential security issues. Tools like Clair or Anchore can help with this process.

### Stage Isolation

Multistage builds allow you to isolate stages, reducing the attack surface. Ensure that each stage has the minimum required permissions and access to resources.

```dockerfile
# Stage 1: Build
FROM python:3.9-slim as build
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .

# Stage 2: Runtime
FROM python:3.9-slim
WORKDIR /app
COPY --from=build /app .
CMD ["python", "app.py"]
```

In this example, the build stage has access to the `requirements.txt` file and the application code, while the runtime stage only has access to the built application.

### Least Privilege Principle

Apply the least privilege principle to each stage by limiting the user permissions and access to resources.

```dockerfile
# Stage 1: Build
FROM python:3.9-slim as build
USER nobody
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .

# Stage 2: Runtime
FROM python:3.9-slim
USER nobody
WORKDIR /app
COPY --from=build /app .
CMD ["python", "app.py"]
```

By setting the user to `nobody`, we limit the permissions of each stage, reducing the potential damage in case of a security breach.

### Secure Environment Variables

Environment variables can contain sensitive information such as database credentials or API keys. Ensure that these variables are not exposed in the final image.

```dockerfile
# Stage 1: Build
FROM python:3.9-slim as build
ENV DB_USER=myuser
ENV DB_PASSWORD=mypassword
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .

# Stage 2: Runtime
FROM python:3.9-slim
WORKDIR /app
COPY --from=build /app .
CMD ["python", "app.py"]
```

In this example, the environment variables `DB_USER` and `DB_PASSWORD` are set in the build stage but are not carried over to the runtime stage, ensuring they are not exposed in the final image.

### Secure File Permissions

Ensure that file permissions are set correctly to prevent unauthorized access.

```dockerfile
# Stage 1: Build
FROM python:3.9-slim as build
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
RUN chmod 755 /app

# Stage 2: Runtime
FROM python:3.9-slim
WORKDIR /app
COPY --from=build /app .
CMD ["python", "app.py"]
```

By setting the permissions to `755`, we ensure that the application files are executable only by the owner and not by others.

### [Platform Engineering](www.platformengineers.io) and Continuous Integration/Continuous Deployment (CI/CD)

Integrate your multistage builds with your CI/CD pipeline to automate the build and deployment process. This ensures that security checks are performed consistently and efficiently.

### Conclusion

[Multistage builds](https://platformengineers.io/blog/multi-stage-build-for-ci-cd-pipeline-using-dockerfile/) offer a powerful way to optimize Docker images, but they also introduce security risks if not properly managed. By selecting secure base images, isolating stages, applying the least privilege principle, securing environment variables, and setting correct file permissions, you can harden your multistage builds and ensure the security of your final artifact.