Your Guide to Container Security

Gray Calendar Icon Published: March 29, 2017
Gray Calendar Icon Updated: January 26, 2021

Your storage system should be locked down with all security and access control tools available to you as well. That is true whether the storage serves containers or any other type of application environment. How do you secure containers? That may sound like a simple question, but it actually has a six- or seven-part answer. That’s because securing containers doesn’t involve just deploying one tool or paying careful attention to one area where vulnerabilities can exist. Because a containerized software stack involves so many different components, you need to secure many different layers. The tools designed to help you harden one part of your environment won’t protect other segments. Commercial security tools do exist, and are designed to provide relatively comprehensive security or container environments. They are good tools, and they can certainly be useful parts of a container security strategy, but they have their limitations. To be truly secure, you need to analyze each of the layers in your stack, and be sure that they are covered adequately by the security tools or processes you put in place. This post helps you plan a complete container security strategy by outlining all of the layers you need to secure, and explaining the primary considerations to keep in mind when securing each one.

Understanding the Layers

When planning your approach to container security, you should begin by identifying all of the different layers of the software stack that you have to secure. Those layers include:

Your image registry. This is the part of your stack that hosts your images. Security vulnerabilities here could allow attackers to add malicious images to your environment, or steal private data. The orchestrator. Your orchestrator is the brains of your container cluster. If it’s not secured, an attacker could use it to disrupt service, or possibly intercept private information. Your hosting infrastructure. The operating system or cloud environment that hosts your container environment needs to be secure—otherwise, it can become the front door for an attack against your environment. Storage systems. To protect the sensitive data hosted on your container cluster, you need to keep the storage system you use free of vulnerabilities. The container daemon. If the Docker daemon is compromised, attackers can shut down containers or gain unauthorized access to the ones you’re running. Application code inside your containers. Last but not least, you need to make sure the code that runs inside your containers is free of vulnerabilities that could allow attackers to disrupt or control your application when it is running.

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Securing the Stack

There are two main considerations to bear in mind when securing your image registry. First, you need to make sure to lock down access control. Your approach to doing this will vary depending on which registry you use. Some registries offer finer-tuned access control features than others, but all of the mainstream registries provide some security controls. (For an overview of different registry options, including a comparison of the security features built into them, check out Container Registries You May Have Missed) The second challenge is detecting security vulnerabilities inside container images themselves. For this task, two tools are available: Clair from CoreOS and Docker Security Scanning from Docker. Both of these image scanners will check an image for known malware signatures. They’re designed mainly to be integrated into CoreOS’s and Docker’s registries, but they can also work in standalones mode by manually scanning an image.


Securing your orchestrator requires more work than simply turning on some access control features or running a scanner. Orchestrators are complex tools, and their inner workings vary from one orchestrator to another. Explaining all of the details of configuring an orchestrator for maximum security is beyond the scope of this article. In general, however, key principles to follow include:

  • Making sure you install your orchestrator from an official source. Be wary of third-party package repositories.
  • Keep your orchestrator up-to-date.
  • When configuring your orchestrator and the cluster it manages, limit public-facing network connections to the minimum necessary to run your application.
  • Configure your orchestrator for automatic failover and high availability in order to mitigate the impact of potential DDoS or similar attacks.

If you use Kubernetes, you may also find this article on security best practices to be helpful. A similar guide for Mesos is available here.

Hosting Infrastructure

The infrastructure you use to host your container environment could be on-premises, in the cloud, or in some cases, a mix of both. Whatever it looks like, you should be sure to secure the infrastructure as much as possible. If you manage your host servers yourself, make sure they are locked down with kernel hardening tools like SELinux or AppArmor. For cloud-based deployments, take advantage of access control features (such as IAM roles on AWS) to configure access to your environment. Security auditing and monitoring tools will help to keep your infrastructure secure, too.


When it comes to containers and storage, however, one important point to keep in mind is that, in many cases, many containers might share access to the same storage directories. This happens if you map directories inside containers to a shared location on the host. Under these conditions, it’s especially important to make sure that any container with access to a shared storage location is secure. You should also limit storage access to read-only in cases where a container does not need write permissions. And it’s always a good idea to have rollback features built into your storage system so that you can undo changes to data if necessary.

Container Daemon

Running SELinux or AppArmor on the container host can help to defend the Docker daemon against attack, but that is only one security challenge to keep in mind when it comes to the daemon. You should also make sure that daemon socket connections are securely authenticated and encrypted. Of course, it’s also essential to keep your Docker installation up-to-date to avoid security vulnerabilities in the daemon.

Application Code

You should secure the code running inside your containers just as you would secure any type of application code—by obtaining the code from a trusted source and auditing it with security tools designed to catch vulnerabilities. Clair and Docker Security Scanning can help with the latter, but they are not designed to be all-purpose static application security testing solutions. For that reason, you may benefit from deploying a tool like Veracode or OWASP to scan your code for vulnerabilities.


Keeping a container environment secure is a big task because there are so many moving parts. The details of your security strategy will vary depending on exactly which types of registries, orchestrators, hosting infrastructure and so on that you choose to include in your stack. But whatever your environment looks like, the key to keeping it secure is to remember that there is no one-stop shopping. You have to keep all of the different layers in mind, and develop a security plan that addresses each one. Chris Riley (@HoardingInfo) is a technologist who has spent 12 years helping organizations transition from traditional development practices to a modern set of culture, processes and tooling. In addition to being a research analyst, he is an O’Reilly author, regular speaker, and subject matter expert in the areas of DevOps strategy and culture. Chris believes the biggest challenges faced in the tech market are not tools, but rather people and planning.

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