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Container Security 2018: Securing Container Contents

By Adrian Lane

Testing the code and supplementary components which will execute within containers, and verifying that everything conforms to security and operational practices, is core to any container security effort. One of the major advances over the last year or so is the introduction of security features for the software supply chain, from container engine providers including Docker, Rocket, OpenShift and so on. We also see a number of third-party vendors helping to validate container content, both before and after deployment. Each solution focuses on slightly different threats to container construction – Docker, for example, offers tools to certify that a container has gone through your process without alteration, using digital signatures and container repositories. Third-party tools focus on security benefits outside what engine providers offer, such as examining libraries for known flaws. So while things like process controls, digital signing services to verify chain of custody, and creation of a bill of materials based on known trusted libraries are all important, you’ll need more than what is packaged with your base container management platform. You will want to consider third-party to help harden your container inputs, analyze resource usage, analyze static code, analyze library composition, and check for known malware signatures. In a nutshell, you need to look for risks which won’t be caught by your base platform.

Container Validation and Security Testing

  • Runtime User Credentials: We could go into great detail here about user IDs, namespace views, and resource allocation; but instead we’ll focus on the most important thing: don’t run container processes as root, because that would provide attackers too-easy access to the underlying kernel and a direct path to attack other containers and the Docker engine itself. We recommend using specific user ID mappings with restricted permissions for each class of container. We understand roles and permissions change over time, which requires ongoing work to keep kernel views up to date, but user segregation offers a failsafe to limit access to OS resources and virtualization features underlying the container engine.
  • Security Unit Tests: Unit tests are a great way to run focused test cases against specific modules of code – typically created as your development teams find security and other bugs – without needing to build the entire product every time. They cover things such as XSS and SQLi testing of known attacks against test systems. As the body of tests grows over time it provides an expanding regression testbed to ensure that vulnerabilities do not creep back in. During our research we were surprised to learn that many teams run unit security tests from Jenkins. Even though most are moving to microservices, fully supported by containers, they find it easier to run these tests earlier in the cycle. We recommend unit tests somewhere in the build process to help validate the code in containers is secure.
  • Code Analysis: A number of third-party products perform automated binary and white box testing, rejecting builds when critical issues are discovered. We also see several new tools available as plug-ins to common Integrated Development Environments (IDE), where code is checked for security issues prior to check-in. We recommend you implement some form of code scanning to verify the code you build into containers is secure. Many newer tools offer full RESTful API integration within the software delivery pipeline. These tests usually take a bit longer to run but still fit within a CI/CD deployment framework.
  • Composition Analysis: Another useful security technique is to check libraries and supporting code against the CVE (Common Vulnerabilities and Exposures) database to determine whether you are using vulnerable code. Docker and a number of third parties – including some open source distributions – provide tools for checking common libraries against the CVE database, and can be integrated into your build pipeline. Developers are not typically security experts, and new vulnerabilities are discovered in common tools weekly, so an independent checker to validate components of your container stack is both simple and essential.
  • Hardening: Over and above making sure what you use is free of known vulnerabilities, there are other tricks for securing containers before deployment. This type of hardening is similar to OS hardening, which will we discuss in the next section; removal of libraries and unneeded packages reduces attack surface. There are several ways to check for unused items in a container, and you can then work with the development team to verify and remove unneeded items. Another hardening technique is to check for hard-coded passwords, keys, and other sensitive items in the container – these breadcrumbs makes things easy for developers, but help attackers even more. Some firms use manual scanning for this, while others leverage tools to automate it.
  • Container Signing and Chain of Custody: How do you know where a container came from? Did it complete your build process? These techniques address “image to container drift”: addition of unwanted or unauthorized items. You want to ensure your entire process was followed, and that nowhere along the way did a well-intentioned developer subvert your process with untested code. You can accomplish this by creating a cryptographic digest of all image contents, and then track it though your container lifecycle to ensure that no unapproved images run in your environment. Digests and digital fingerprints help you detect code changes and identify where each container came from. Some conatiner management platfroms offer tools to digitially fingerprint code at each phase of the development process, alongside tools to validate the signature chain. But these capabilities are seldom used, and platforms such as Docker may only optionally produce signatures. While all code should be checked prior to being placed into a registry or container library, signing images and code modules happens during building. You will need to create specific keys for each phase of the build, sign code snippets on test completion but before code is sent on to the next step in the process, and (most important) keep these keys secured so attackers cannot create their own trusted code signatures. This offers some assurance that your vetting process proceeded as intended.
  • Bill of Materials: What’s in each container? What code is running in your production environment? How long ago did you build this container image? These are common questions when something goes awry. In case of container compromise a very practical question is: how many containers are currently running this software bundle? One recommendation – especially for teams which don’t perform much code validation during the build process – is to leverage scanning tools to check pre-built containers for common vulnerabilities, malware, root account usage, bad libraries, and so on. If you keep containers around for weeks or months, it is entirely possible that a new vulnerability has been discovered since one of them was approved and launched, so such containers are now suspect. Additionally, we recommend using bill of materials capabilities available offered by some scanning tools to catalog container contents. This helps to identify other potentially vulnerable containers, and scope remediation efforts.

Our next post will discuss how to protect containers in production.

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