Docker Security: 14 Best Practices You Should Know

Docker has revolutionized the way developers build, package, and deploy applications. Its lightweight containerization technology has become an integral part of modern software development and deployment pipelines.

However, as with any powerful and widespread tool, Docker has its own set of security challenges. Ensuring the security of Docker containers is not just a matter of fortifying the application but involves a comprehensive approach encompassing the entire ecosystem.

This article delves into the best practices for Docker security, offering a guide to safeguarding your containerized environments. Our aim is to provide you with a detailed guide to not only understand the potential security pitfalls in Docker but also to equip you with the knowledge and tools to address them effectively.

By implementing these best practices, you will ensure that your Docker deployments are robust, resilient, and ready to meet the challenges of securing modern applications.

Understanding Docker's security model

Before you can effectively secure your containerized applications, you need to have a good grasp of Docker's security model. Docker's approach to security is distinct from traditional virtualization methods, primarily due to its reliance on the host OS kernel.

Here's an overview of the key elements of its security model:

1. Kernel Namespaces: Docker leverages kernel namespaces for process isolation. This means each container operates in its own namespace, isolating its processes and system resources (like network interfaces and mount points) from other containers and the host.

2. Control Groups (Cgroups): Cgroups play an important role in managing and limiting container resource usage. They help prevent denial-of-service attacks by allocating resources fairly among containers, ensuring that excessive resource use by one container doesn't impact others.

3. Docker Daemon Security: The Docker daemon requires root privileges by default, making it necessary to restrict access to trusted users. Docker's REST API endpoint, used for communication, has evolved to enhance security, utilizing either a UNIX socket or HTTPS with certificate-based security

4. Linux Kernel Capabilities: Docker restricts the capabilities of containers, meaning even root-level access within a container is less privileged than on the host system. This capability system allows specific privileges without full root access, reducing the risk of significant damage if an attacker gains root access within a container.

5. Docker Content Trust Signature Verification: Docker Engine can be configured to run only signed images, enhancing security through image signature verification. This feature, set up in the Docker configuration file (daemon.json), gives you control over enforcing security policies related to image usage.

6. User Namespaces: Docker's User Namespaces feature maps the container's root user to a non-root user on the host, mitigating the risk of container breakout. While not enabled by default, this feature is beneficial for containers that need root privileges, offering an additional layer of security.

Docker security best practices: Impact vs. Difficulty

Now that you have a foundational understanding of Docker's security model, let's explore some best practices to fortify your Docker environment.

The following table offers a comparative analysis of the impact and implementation difficulty for each practice. Following the table, we will elaborate on each practice and provide guidance for integrating them into your Docker setup.

1. Use official images

Utilizing official Docker images is critical for maintaining security, as these images are regularly updated and patched by reliable entities. This approach significantly lowers the risk of deploying containers with existing vulnerabilities or malicious code.

The use of official images brings several key benefits. They are regularly updated for security, originate from verified sources, and are trusted widely within the Docker community. Their extensive usage and the large community actively examining them contribute to the quick discovery and rectification of any potential security issues.

Implementing this practice is straightforward, as it mainly involving the selection of official images from Docker Hub or other trusted sources for your container builds.

2. Pin Docker image versions

To mitigate risks in your Docker containers, it's important to use specific image versions instead of the latest tag. Unspecified versions can lead to unintentionally deploying containers with known vulnerabilities.

Pinning specific versions allows for predictable builds, controlled updates, and a more thorough evaluation of security patches. For instance, in your Dockerfile or when running containers, explicitly specify the version:

FROM nginx:1.21.3

Or when running a container:

docker run -d --name my-container nginx:1.21.3

This approach guarantees the use of the Nginx 1.21.3 image, unlike the latest tag, which may not always be the newest version and can lead to unpredictability.

Regularly updating pinned versions requires a systematic approach, including testing new versions for compatibility and security. Tools like Diun for notification of base image updates and Watchtower for automated container updates can streamline this process.

3. Keep Docker and its host up to date

Regularly updating Docker and its components is necessary for maintaining security. These updates address vulnerabilities in Docker, providing essential security patches and bug fixes to protect containerized applications from known threats.

Since containers share the kernel with the host (unlike in virtual machines), updating the host system is also important to protect against kernel-level exploits that could compromise the host when executed inside a container.

4. Minimize image size and layers

Smaller Docker images mean a smaller attack surface, reducing potential vulnerability exploitation. They also allow for quicker security scanning, speeding up vulnerability detection and remediation. Additionally, these images use resources more efficiently, helping guard against resource exhaustion attacks.

Tips for image size and layer reduction

1. Choose a minimal base image: Opt for the smallest base image possible, like Alpine Linux for compactness and security.

2. Implement multi-stage builds: Multi-stage builds separate the build and runtime environments, producing smaller final images by excluding unnecessary files.

3. Utilize .dockerignore: Use .dockerignore to exclude non-essential files from the Docker build context, further reducing image size.

Consider this Dockerfile for a Go application:

# Builder stage: building the executable
FROM golang:1.20.8-alpine as builder

WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -o myapp

# Final stage: build the container to run
FROM scratch as final

COPY --from=builder /app/myapp /

This setup involves two stages:

1. Builder stage: Compiles the Go app into a statically linked binary using an official Alpine-based Go image.

2. Final stage: Scratch is an empty image, so it can be used for statically linked binaries that do not require libc.

This method results in a compact, secure final image with just the necessary binary, embodying Docker's best practices for security and efficiency.

5. Run containers with the least privileges

Operating Docker containers in privileged mode introduces considerable security risks due to the nearly unrestricted access they gain to the host system. This access heightens the likelihood of privilege escalation, enlarges the attack surface, and poses risks to data integrity.

While privileged mode is useful for scenarios such as legacy application compatibility, hardware interfacing, or kernel operations, its significant security implications suggest that it should be employed only when absolutely essential.

In addition to avoiding privileged mode, implement the following practices to tighten the security of your Docker containers.

• Set filesystem and volumes to read-only: Set your container's filesystem to read-only to prevent runtime modifications. Use the --tmpfs flag for selective write access in areas that require it, balancing security with functional needs.

  docker run --read-only --tmpfs /tmp ubuntu sh -c 'echo "whatever" > /tmp/file'

You can also mount volumes as read-only by appending :ro to the -v like this:

  docker run -v volume-name:/path/in/container:ro ubuntu

• Prevent privilege escalation: Include measures to hinder privilege escalation within the container. For example, you can use --security-opt=no-new-privileges in order to prevent using setuid or setgid to escalate privileges.

• Limit Linux capabilities: You can minimize the Linux capabilities assigned to the root user in the Docker container by dropping all privileges with --cap-drop=all, and only explicitly adding the capabilities you need using --cap-add. See this article for more details.

6. Implement network segmentation

In Docker, the default network setting permits unrestricted communication between containers (using the default bridge network). Implementing network segmentation is necessary to mitigate these risks and control the communication flow within your containerized infrastructure.

Network segmentation ensures isolation of container communications, reducing attack vectors, protecting sensitive workloads, and enforcing access controls to thwart unauthorized interactions. Even if a container is compromised, attacker's activities will be confined to it thereby protecting the larger system.

To effectively implement network segmentation in Docker, follow these practices:

• Use custom bridge networks to isolate and apply network policies to specific containers.

• Connect each container to the intended network to control their communication pathways.

  docker run --network mynetwork mycontainer

• Employ network isolation techniques, like configuring iptables rules, to restrict container interactions and shield them from unauthorized external access.

• For more advanced features, consider third-party solutions like Calico, which provide comprehensive network security and management capabilities, particularly beneficial in complex Docker setups.

Through these strategies, network segmentation in Docker becomes a powerful tool to enhance the overall security posture of your containerized systems.

7. Keep sensitive data secure

Docker offers a secrets management system in Swarm mode, enabling secure storage and handling of sensitive data. However, this feature is exclusive to swarm services and not applicable to standalone containers.

When not using Docker Swarm, here are alternative approaches for managing sensitive data in your containerized environment:

1. Environment variables: For Docker Compose users, sensitive data can be stored in environment files and loaded when running services. Ensure these files are excluded from your build context with .dockerigonre.

2. External secrets management tools: Tools like HashiCorp Vault or AWS Secrets Manager offer advanced features for storing and retrieving secrets, which can be integrated with Docker containers.

3. Configuration management systems: Using systems like Ansible, Puppet, or Chef, sensitive data can be managed in encrypted configuration files, then deployed and managed on containers. To ensure the security of credentials, tools like Deepfence SecretScanner can be used to detect unprotected secrets in images.

8. Linting your Dockerfiles at build time

Adhering to best practices while crafting the Dockerfile can avert numerous issues. Incorporating a security linter into the build pipeline is a highly effective step in preventing potential complications down the line.

Once such linter is hadolint. It can scan your local Dockerfiles and integrated into your CI pipeline to ensure consistent checks at build time.

docker run --rm -i hadolint/hadolint < Dockerfile

9. Don't expose the Docker daemon socket

Securing the Docker daemon socket is essential for the safety of your Docker environment. The Docker daemon usually communicates through a non-networked UNIX socket, but it can also use SSH or a TLS (HTTPS) socket, which offers enhanced security.

Exposing the Docker daemon socket to running containers allows you to invoke the Docker API from your container to start/stop/build images/containers as if calling those commands on the host.

docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock docker sh

However, it introduces significant security risks and becomes a potential gateway for attackers to access and control your Docker host. This could lead to unauthorized command execution, compromising sensitive data like Docker secrets and API keys.

In production settings, you need to protect the Docker daemon socket and have your containers communicate with it over either SSH or HTTPS.

10. Limit container resource usage

Limiting container resource usage is a key practice for preventing resource exhaustion, Denial of Service (DoS) attacks, and maintaining fair resource distribution across containers.

You can limit memory, CPU, maximum number of restarts, maximum number of file descriptors and maximum number of processes accordingly using various methods depending on your setup and tools.

When running containers using the docker run command, there are several flags available to establish resource constraints right at the container's startup.

Similarly, when using Docker Compose, resource limits can be defined directly within the docker-compose.yml file, and container orchestrators like Kubernetes also let you specify resource limits within their deployment configurations.

11. Run Docker in rootless mode

Docker's rootless mode is a notable security enhancement that graduated from experimental status in Docker Engine v20.10.

It significantly reduces the potential attack surface area by ensuring that the Docker daemon and containers are running as an unprivileged user. This means that even if an attacker breaches the container's security, they would not possess root access on the host system.

Keep in mind that using rootless mode comes with some limitations. You can read more about them, and find the installation and usage instructions on the Docker documentation page.

12. Scan images for vulnerabilities

Scanning Docker images using static analysis tools is useful for detecting containers with known vulnerabilities. Once issues are found, they can be addressed by updating dependencies, applying patches, or switching to safer base images.

Here are some highly-regarded tools you can explore for this purpose:

• Clair
• ThreatMapper
• Snyk
• Docker Bench for Security
• SecretScanner

13. Use Docker Content Trust to verify image authenticity

Docker Content Trust (DCT) is a security feature in Docker that ensures the authenticity and integrity of container images using digital signatures. It associates these signatures with image tags, allowing users to verify both the integrity and the publisher of images from Docker registries.

Some key aspects of DCT include:

• Image tag signatures: DCT focuses on the "TAG" part of an image identifier, with publishers using keys to sign specific image tags. A repository can have a mix of signed and unsigned tags, offering flexibility for publishers.

• Consumer usage: For consumers, enabling DCT means that docker CLI commands that operate on tagged images (like build, create, pull, or run) will require content signatures or explicit content hashes.

• Key management: Managing trust keys involves securing the offline root key, using repository keys for tag signing, and server-managed keys for additional security.

• Trust verification: Users can inspect trust data with docker trust inspect to see signature details for tags or repositories.

In summary, DCT not only secures the transfer and use of Docker images but also aids in preventing the deployment of tampered or malicious images, significantly bolstering the overall security posture.

14. Collect and monitor Docker logs

Collecting Docker daemon logs is instrumental in identifying and responding to security incidents. These logs offer comprehensive insights into Docker's system-level operations, encompassing critical aspects such as container lifecycle events, network configurations, image management, and incoming API requests.

Monitoring these logs helps with detecting unusual activities or changes in containers and networks, which could indicate unauthorized access or potential security breaches. With Better Stack, you can get alerted to such issues quickly aiding in prompt recovery before it gets out of hand.

Learn more: A Complete Guide to Logging in Docker

Final thoughts

In this guide, we covered several essential Docker best practices to help you secure your containerized environment. These practices not only improve security but also the efficiency and reliability of Docker-based solutions.

Remember, implementing these practices is just the beginning of your security journey. Continually updating your security knowledge, staying alert to new vulnerabilities, and prioritizing security in your development process are crucial steps to maintaining robust protection.

For further exploration, see the official Docker security docs, this list of Docker security resources, and our other Docker guides.

Thanks for reading!

Article by

Muhammed Ali

Muhammed is a Software Developer with a passion for technical writing and open-source contribution. His areas of expertise are full-stack web development and DevOps.

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