Prioritizing Container Image Security

Container images make application deployment easy and convenient. But alongside that ease and convenience, you also need container security. Security issues are very real when working with container images. This is why container image security should be a priority when you migrate to Docker.

Docker’s popularity is largely due to the fact that when using container images, anyone can package code and dependencies into an image and easily publish to a registry. From there, anyone can download the image and run containers from the image. This has brought portability of code across teams and quickened the application lifecycle.

Yet, containers can inadvertently expose vulnerabilities if you don’t take the required security measures. This is especially true when working with Docker containers that are shared between users and organizations.

So, let’s discuss how to execute effective Docker security solutions for your container images.

Verify the source of images

Container images are downloaded from registries like Docker Hub or third-party registries like Quay. These registries host container images from organizations and individuals alike. There are official repositories from most IT vendors, and many unauthorized ones as well–which can lead to some serious security concerns. Across the application lifecycle, developers, QA and IT will download many images for different needs. It’s important to monitor these container images, and perform checks before they are installed in order to ensure container security.

To do this, you can enable Docker Content Trust, which integrates with third-party registries to verify digital signatures for container images downloaded from Docker Hub. Docker Content Trust helps you to whitelist official repositories from authorized, trusted sources. This is an excellent way to implement image signing.

If you need to work with unverified images from partners and vendors, for example, you could consider upgrading your image scanning to more robust container security tools like Twistlock. It not only scans images, but also lets you set up custom alerts whenever anyone attempts to install a suspicious image.

Implement robust access controls

The source of container images and the way official images are used can leave images compromised. This is why access control is extremely important for container images.

By default, all users are assigned root privileges inside a container. However, this is not a good practice from a Docker security point of view. Whenever a user is created, you need to change their access level to non-root user. Certain users may genuinely need root access to complete certain tasks, but these exceptions should be made only within those containers that perform the task, and only as long as is necessary. This task-centric access control ensures that even if one user account is compromised, attackers can’t inflict much damage on the rest of the system (thus helping to reduce the effective attack surface).

It’s not possible to manually change the status of users for every container every time. This task needs to be automated. A platform like Twistlock enables role-based access control (RBAC) for container images and lets you assign privileges to users based on their job function. You can configure RBAC based on complex rules and ensure that all users have the necessary privileges to carry out their tasks—no less, and no more. These types of security policies, including options for privilege escalation, are very important.

Keep containers lightweight

Developers and DevOps teams are attracted to Docker containers because of how much lighter they are compared to virtual machines (VMs). When running containers, it’s possible to load too many packages on a container so that it becomes bloated to more than 100 MB. The ideal container size should be just tens of MBs.

When selecting an operating system for your image’s base layer, look for a minimalist option. There are a couple of good options like BusyBox, Alpine Linux, and RancherOS. Additionally, install only the packages that are required for a container to perform its task. This improves the performance of containers, and, more importantly, reduces the attack surface area.

Keep images healthy

Once you’ve followed all security best practices to set up your Docker images the right way, it’s important to monitor their health during runtime. This requires routine “health checks” on the containers. If Docker Engine finds containers that aren’t working, it can automatically replace them. This way you can keep the system healthy even if individual containers are found to be vulnerable.

Another important practice to ensure the good health of your containers is to keep container images updated with the latest version and apply security patches to them frequently. You need to be able to scan the container images during runtime to find vulnerabilities and patch them promptly.

Detecting vulnerabilities is not an easy task, as your system could run tens of thousands of Docker containers. At this scale, you require a threat detection tool like Twistlock that monitors container runtime with the help of machine learning algorithms. Twistlock is able to spot concerning patterns and alert you of their impact. This job of finding the needle in the haystack is not possible through the manual study of log data and metrics. It takes intelligent algorithms and a modern threat detection platform like Twistlock to achieve this level of security when running containers.

Handle confidential data with care

Despite assigning read-only access to users, you still need to watch what data you store in your Docker containers. For example, you should never store secrets like passwords, tokens, keys, and confidential user information inside dockerfiles. Even if deleted later, this data can be retrieved from the image’s history. That’s where security tools come into play.

Use the secrets management security feature that comes with both Kubernetes and Docker Swarm. Each has strong defaults to ensure secrets are properly encrypted, stored in an encrypted format, and, when retrieved, can be decrypted only by authorized users. This is very important for achieving container security.

Container images may be the most fun part of the Docker hub experience, yet they can also be the most dangerous from a container security standpoint. By understanding the various nuances to Docker security, you can ensure your cloud-native apps are even more secure than your legacy apps ever were. Taking the time to verify the sources of Docker container images, make wise use of access control, minimize the attack surface by keeping the containers lightweight, keeping images healthy when running containers, and using the secrets management feature that comes with both Docker Swarm and Kubernetes, you will be well on your way to implementing a robust Docker security solution.

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