Docker Security Best Practices: How to Secure Your Containers

Docker Security Best Practices: How to Secure Your Containers. Docker is a popular platform for organizations that want to streamline build and deployment processes. A containerized platform where users build, share, and run applications. Basically, containers are standardized units for developing, shipping, and deploying code. Lightweight and standalone solution containing everything needed to run a container i.e system libraries, code, settings, runtime, and system tools.

Moreover, Docker containers have become popular in recent years due to their efficiency. But they have security risks. Therefore, it’s necessary to harden containers security wise to safeguard data. 

This article discusses 15 Docker Security Best Practices: How to Secure Your Containers. Let’s start.

Docker Security Best Practices: How to Secure Your Containers

1. Secure Containers at Runtime

Securing Docker containers at runtime involves securing your workloads, such that no drift occurs once the container is running. In case a malicious action is encountered, it’s immediately blocked.

One of the key aspects of runtime security is least privilege, which is about granting a container the capabilities that are absolutely necessary to function correctly. For example, if a container doesn’t require access to system-level processes, then those permissions should not be granted.

2. Segregate Container Networks

Normally, Docker containers require a network layer for communication. All docker hosts have the default network, unless you specify a different network for your containers. All new containers connect to the default network bridge, and it’s necessary to segregate them. Segregating container networks means isolating the network communication of different containers. This is done to prevent a potential security breach in one container from spreading to others through the network. 

It’s not advisable to rely on the network bridge, but rather custom bridge networks to dictate how containers communicate. Create unlimited number of networks and assign them to different containers. Therefore, containers within the same network communicate freely, but they can’t access containers on other networks unless explicitly allowed.

Always ensure that containers connect to each other when it’s necessary. Also, avoid connecting containers with sensitive resources to public-facing networks.

3. Scan and Verify Container Images

Next puzzle of Docker Security Best Practices: How to Secure Your Containers is scanning and verifying container images. All in all that helps to maintain the security of your Docker environment. Well, container images form the basis of your containers. Any security vulnerabilities in these images directly impacts your deployed applications. Scanning Docker containers involves analyzing any misconfigurations, content, and composition of images. When building an Image from the CI pipeline it’s imperative to scan it before running through the build. In case an image comes out unsafe, it should be pushed to the container registry.

When choosing an Docker image scanner, ensure it supports the language used by the components of the image. By consistently scanning and verifying container images, you ensure that your containers have a strong security foundation.

4. Always Monitor Docker Containers

Additionally, monitoring Docker containers helps to maintain a proper security posture. Since containerized environments are dynamic, continuous monitoring helps understand your runtime environment, as well as any anomalies. Well, the monitoring involves keeping track of the behaviour of containers, such as resource usage, performance metrics, and application health. Through monitoring, you detect unusual or unexpected behaviour in real time, which might indicate a potential security issue.

A container image has multiple instances running. If new images are deployed quickly, any issues within them propagate across containers and applications. What is more, monitoring also allows you to identify trends and patterns in the container’s operation. Implement monitoring solutions for the following components:

  • Workloads running in containers.
  • Docker hosts.
  • Networks.
  • Master nodes (when running Kubernetes).
  • Container engines.

Finally, monitoring also extends to logging. 

5. Secure Container Registries

Additionally, container registries are central repositories for storing and distributing container images. Please configure the registry to allow access by authenticated or authorized users only. This prevents unauthorized access.

One of the most ideal solutions is to use a private registry deployed behind your own firewall to reduce the risk of malicious actors manipulating it. Besides, implement role based access control to restrict which users upload or download its contents.

Another solution is to secure transmission of images from the registry to the host. The connection should be encrypted to prevent any possibility of the images being intercepted in transit.

6. Run Containers in Isolation

After all, container isolation means separating the runtime environment from the host OS and any other process running on the host. There are multiple ways to isolate containers and their host OS, such as:

  • PID/User namespace
  • seccomp filters
  • cgroups

Well, docker’s User Namespace support allows you to map a user inside a container to a different user on the host. This feature prevents a process from obtaining more privileges than it should have. Therefore, you’re essentially isolating system users and preventing potential attackers‘ ability to modify files owned by the root user on the host.

Also enforce container isolation by using Docker’s security profiles or seccomp. Seccomp is a system call implemented in the Linux kernel that means “operate on Secure Computing state of the process”. Certainly, Docker uses seccomp to limit the set of system calls applications inside containers make, reducing their attack surface. By using a default or customized security profile, you filter out system calls that your application doesn’t need. This limits the capabilities of the processes running within the container.

Consequently, containers also employ Control Groups (cgroups). Another key Linux feature that helps limit the resources a container consumes. Cgroups allow Docker to share available hardware resources to containers and, if required, set up limits and constraints. This means a single container cannot use up all the host’s resources. This isolates the impact of one container’s resource usage on others.

7. Limit Container Privileges

Keep on reading about Docker Security Best Practices: How to Secure Your Containers. Interestingly, there is a  privileged mode which allows you to run containers as root of the local machine. When you run a container in root mode, the host has the following capabilities:

  • Install a new Docker instances using the host’s kernel capabilities.
  • Tamper with Linux security modules such as SELinux and AppArmor
  • Has Root access to all devices.

This means that, if a malicious user or corrupted software infiltrates the container, it potentially gains access to the host system,  leading to infiltration of other containers and host processes. Eventually it illegitimates the application.

Using Docker containers as root also lead to operational issues due to inappropriate permissions and ownership settings. A root user inside a container could create or modify files owned by root on the host’s file system. This leads to other services on the host not being able to access or modify these files due to insufficient privileges.  Result is application errors or system instability.

Evidently, to run containers as non-rooter, specify the user when starting the container. Let’s say you are starting a container that you want to assign root user ID 255. Use this command to run the container as non-user:

					docker run --user 255 my_container

Besides, create a new user and configure the necessary permissions to specifically run containers.

8. Use Docker's Build Secrets

Docker Secrets help manage and secure sensitive data in a Docker Swarm environment. Basically, a Docker Secret is a file that contains sensitive data such as passwords, SSL certificates, or SSH keys. Explicitly, Docker Secrets uses a built-in orchestration feature of Docker Swarm to securely distribute these secrets to services that require them. Secrets are encrypted during transit and at rest in a Docker swarm. A given secret is only accessible to those services which have been granted explicit access to it, and it is only available while that service is running.

To configure Docker secrets, all you need is a series of Docker CLI commands. Here is how to create a Docker Secret:

					docker secret create my_secret

Once created, grant a service access to a secret on a need-to-know basis using the –secret option when defining the service. Here is the command syntax:

					docker service create --name my_service --secret my_secret my_image

In this case, my_service is the name of the service and my_secret is the name of the secret. my_image is the Docker image the service is based on.

To remove a Docker secret, first remove the service access using this command:

					docker service update --secret-rm my_secret my_service

After removing all the secret from all services, delete it using this command:

					docker secret rm my_secret

The secrets are encrypted during transit and at rest. Besides, they are only accessible to services within the same Docker swarm with explicit access.

9. Avoid Exposing the Docker Daemon Socket

Exposing Docker daemon socket leads to serious security risks. This is because it gives the container the root access of the host machine. Basically, the Docker daemon runs with root privileges. This means anyone controls the entire host system provided they interact with the daemon. For example, they create containers with malicious capabilities, manipulate existing containers, and much more.

A malicious user with access to the Docker socket modify their privileges on the host machine. Furthermore, they use the Docker API to run containers with malicious code. Other risks include overwriting the Docker engine binary or other critical files on the host system. 

Use alternatives to expose Docker services. There are several built-in methods to securely expose Docker APIs. These include the use of TLS for encrypting communication or implementing role-based access controls.

10. Lint the Dockerfile at Build Time

Dockerfile Linting in AWS

Furthermore, Docker provides a set of best practices on some of the best practices for writing Docker files. Linting the Dockerfile is a practice that helps maintain good code quality. A Dockerfile linter is a tool for analyzing and parsing the Dockerfile. It warns developers when the file doesn’t match the best practices or guidelines and to detect potential security vulnerabilities before they integrate into the Docker image. The most popular Dockerfile linter is Hadolint that parses the Dockerfile into an abstract syntax tree (AST) and applies rules to check its conformity. 

Easily integrate Hadolint into your Docker file creation process as well as CI/CD pipeline. Install Hadolint using package managers or download a static binary from Hadolint’s GitHub repository. 

11. Limit the Number of Open Ports on a Container

Minimizing the number of open ports on Docker containers helps improve your Docker environment security. Open ports on a container present possible access points for malicious actors that increase the application’s risk of cyber threats. Therefore, reduce the number of exposed ports as it consequently minimizes the attack surface. It makes it difficult for potential hackers to exploit vulnerabilities or gain unauthorized entry into the container.

When setting up Docker containers, expose only those ports absolutely necessary for the application’s operations. Do this while creating the container by using various commands that allow you to specify which ports to expose. By exposing only the essential ports, you maintain functionality while improving container security.

12. Use Fixed Tags for Immutability

Next with Docker Security Best Practices: How to Secure Your Containers. is to use fixed tags. A Docker tag is a unique label used for identifying a Docker image. This tag allows you to deploy a single image with multiple tags associated with it. Once a Docker image has a new version, all its tags also get updated.

In a standard Docker setup, image tags are mutable, meaning they can be overwritten with a different image. The mutable nature of tags potentially exposes your system to supply chain attacks, where malicious code is injected into a Docker image that your application depends on. Use fixed tags, where each image and version has a unique identifier that is immutable, significantly reducing the risk of such attacks.

One way to establish immutability is by using content-based identifiers, such as the image’s SHA-256 hash, as the tag. This approach ensures that each tag is inherently tied to the image’s content, promoting traceability and accountability.

13. Build Containerized Applications in Stages

To build containerized applications consistently, it’s imperative to use multi-stage builds. This approach provides security and operational advantages. When building applications in stages, you first create a container with all the tools you need to compile or generate the final artifact. In the final stage, only generated artifacts are copied to the final image. There are no temporary build files or development dependencies. This strategy helps minimize the attack surface. 

14. Set Filesystem and Volumes to Read-Only

One of the most effective methods to secure contains is to set volumes and filesystem to read only. When the filesystem is in read-only mode, it’s difficult for an attacker to modify the configurations or deploy malware on the container. To set the root filesystem at container runtime, use the –read-only option. A Docker volume allows data to persist beyong the container’s lifecycle. 

15. Keep Docker and Host Up to Date

It’s best to always run the latest Docker version and keep the underlying host operating system up to date. Each update contains various security patches to potential vulnerabilities, and it’s essential to update your versions immediately. Whichever host system you are using, be it Windows or Linux, always keep it up to date. If you do not update the host system, it could potentially lead to security breaches.

Thank you for reading Docker Security Best Practices: How to Secure Your Containers. We shall conclude the article. 

Docker Security Best Practices: How to Secure Your Containers Conclusion

Finally, Docker is a complex platform when it comes to security fixes. Securing your containers is a continuous process that revolves not only around the container but also the underlying host system, networks, and user controls. These practices are just some of the ways to secure Docker containers, but it doesn’t end there. Take a proactive approach and ensure your team stays informed on how to approach container security.

Avatar for Dennis Muvaa
Dennis Muvaa

Dennis is an expert content writer and SEO strategist in cloud technologies such as AWS, Azure, and GCP. He's also experienced in cybersecurity, big data, and AI.

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