STIGs for Chainguard Images

The practice of using Security Technical Implementation Guides, or “STIGs,” to secure various technologies originated with the United States Department of Defense (DoD). If an organization uses a certain kind of software, say MySQL 8.0, they must ensure that their implementation of it meets the requirements of the associated Security Requirements Guides (SRG) in order to qualify as a vendor for the DoD. More recently, other compliance frameworks have begun acknowledging the value of STIGS, with some going so far as to require the use of STIGs in their guidelines.

Chainguard announced the release of a STIG for the General Purpose Operating System (GPOS) SRG — an SRG that specifies security requirements for general purpose operating systems running in a network. The goal for this new STIG is that it will help customers confidently and securely integrate Chainguard Images into their workflows. This conceptual article aims to give a brief overview of what STIGs are and how they can be valuable in the context of container images. It also includes instructions on how to get started with Chainguard’s STIG for the GPOS SRG.

Getting Started

The recommended way to get started with Chainguard’s STIG for the GPOS SRG is to use the Chainguard openscap Image. This includes the openscap tool itself, the oscap-docker libraries, and the Chainguard GPOS STIG profile. This image is built with the same capabilities and low-to-zero CVEs as every other Chainguard Image, and makes the openscap tool — which can be difficult to set up — more portable.

The following instructions assume that you have docker installed and running on your system, and are intended to be performed on a non-production system, similar to the process outlined in DISA’s Container Hardening Whitepaper.

For ease of use, we’ll use the datastream file sourced from the Chainguard STIGs repository, and available within Chainguard’s openscap image. This file serves as a sort of checklist, outlining each of the requirements that must be met in order to conform with the STIG.

If you’d like, you can download this datastream file — named ssg-chainguard-gpos-ds.xml — with a command like the following:

curl -fsSLO https://raw.githubusercontent.com/chainguard-dev/stigs/main/gpos/xml/scap/ssg/content/ssg-chainguard-gpos-ds.xml

The -O option in this example will redirect the file’s contents into a local file also named ssg-chainguard-gpos-ds.xml in your working directory. You can then view the checklist locally.

We’ll refer to this as the scan image, and the target image we’ll be scanning will be: cgr.dev/chainguard/wolfi-base:latest.

First, start the target image:

docker run --name target -d cgr.dev/chainguard/wolfi-base:latest tail -f /dev/null

Next, run the scan image against the target image.

docker run -i --rm -u 0:0 --pid=host \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v $(pwd)/out:/out \
  --entrypoint sh \
  cgr.dev/chainguard/openscap:latest-dev <<_END_DOCKER_RUN
oscap-docker container target xccdf eval \
  --profile "xccdf_basic_profile_.check" \
  --report /out/report.html \
  --results /out/results.xml \
  /usr/share/xml/scap/ssg/content/ssg-chainguard-gpos-ds.xml
_END_DOCKER_RUN

Note that this is a highly privileged container since we’re scanning a container being run by the host’s Docker daemon.

The results of the scan will be written to a new subdirectory named out/ within the current working directory. The report.html file will contain a human-readable report of the scan results, and the results.xml file will contain the raw results of the scan.

What are STIGs?

As mentioned previously, “STIG” is an acronym that stands for Security Technical Implementation Guide. A STIG is akin to the implementation of a Security Requirements Guide (SRG) that a security administrator can go through to ensure that a given piece of software has been hardened against cybersecurity threats.

A STIG is typically written by the developer or vendor of the given piece of software against a published DOD Security Requirements Guide (SRG). STIGs are presented in the XCCDF (Extensible Configuration Checklist Description Format), allowing them to be ingested into a SCAP-validated tool to validate that a given target is in compliance with them.

After drafting the STIG, the vendor will submit it to the Defense Information Systems Agency (DISA), an agency within the DoD. One of DISA’s responsibilities is publishing and maintaining STIGs on the DoD Cyber Exchange website, and the process from a STIG being submitting to it being published by DISA can take years. As of this writing, DISA has published over 450 STIGs for a wide variety of software applications.

How STIGs can be used to harden images

STIGs are typically published for hardware, firmware, and specific applications. However, in recent years containerization has grown in popularity and is now the modern way to deploy applications. This has resulted in there being a gap in terms of clear instructions on how to securely deploy containerized applications.

Chainguard produces hardened, minimal container images containing few to zero CVEs. Because of this, they adhere to many compliance standards where there’s a control for vulnerability or risk management, including FedRAMP and PCI DSS v4.0.

For many risk management frameworks, the system categorization will result in varying levels of hardening guidelines. Using FedRAMP as an example, FIPS-199 defines the security categorization which will result in a baseline set of controls (defined in NIST 800-53) that must be implemented.

Because the NIST 800-53 controls are technology-neutral, the STIGs published by DISA provide technology-specific configurations on how to satisfy the applicable NIST 800-53 control set. As stated in the CM-6 (a) Requirement 1 of the FedRAMP System Security Plan:

“The service provider shall use the DoD STIGs to establish configuration settings; Center for Internet Security up to Level 2 (CIS Level 2) guidelines shall be used if STIGs are not available; Custom baselines shall be used if CIS is not available.”

However, the requirements for how a STIG applies to a container image are rather unclear. For example, some controls apply to the host operating system instead of the image. Similarly, other controls apply to the container runtime instead of the container itself. Knowing what controls are relevant for containers and how to check for them in a STIG are key to achieving and maintaining FedRAMP compliance.

DISA understands that containers have different requirements than traditional operating systems. In an effort to highlight these differences, the DOD DevSecOps Initiative released the Container Hardening Process Guide which describes the Initiative’s approach to hardening images and how other agencies should handle applying STIGs to containers.

Appendix C of the Hardening Process Guide highlights an important point about STIG compliance for containers:

“With a properly locked down hosting environment, containers inherit most of the security controls and benefits from infrastructure to host OS-level remediation requirements.”

Deploying containers on a STIG hardened host provides many of the security features that are difficult or sometimes impossible to implement inside a container. What’s left then is the application-level security configuration — in particular vulnerability remediation — which Chainguard provides through our guaranteed vulnerability remediation SLAs.

False positives and the General Purpose OS STIG

Here we’ve assembled several explanations for requirements from Chainguard’s General Purpose Operating System STIG that are likely to cause false positives when scanning containers, as well as the rationale for those requirements. By disambiguating these false positives, the following sections should be helpful to any administrators deploying containers in environments where STIG hardening is necessary both as a means to understand where to expend effort performing hardening and for discussions with assessors and compliance personnel.

Auditing

Auditing capabilities for Linux containers are implemented by the underlying host’s audit configuration. In Linux, the auditing program auditd leverages multiple functions in the running kernel through the Linux Auditing System to capture runtime information such as starting and stopping processes, opening sockets, and accessing files.

Linux containers use their host’s kernel, making it impossible to install and operate a separate auditing package inside the container itself. Instead, auditing information must be configured and collected through the auditing configuration of the host.

Once configured, logs of container actions are written to the host’s audit log files and are readable only by the host superuser account. These logs must be collected from the host for incident response and reporting. Storage capacity limits, audit process monitoring, remote upload of logs, and associated alerts are the responsibility of the host where the containers are running.

Isolation

Containers provide process isolation by executing their applications in a constrained environment using the Linux Namespace and cgroup subsystems. Inside the namespace, container processes are only permitted to access a limited set of system resources defined when the container is launched. Processes are further restricted by limits imposed through the cgroup for access to system resources such as system memory or CPU use.

Together, namespaces and cgroups isolate security functions of the host operating system from non-security functions of applications running inside the container. This separation makes it possible for container failures to not directly impact the operation of the host and its security functions when caused through processing of invalid inputs or other runtime errors. In the event of a container failure during initialization or shutdown, for example, the host operating system’s security capabilities will continue to function as configured.

Minimal images

Chainguard Images contain only the minimal software needed for the container to perform its intended function. For Nonessential capabilities such as package managers, shell environments, executables, and process launching functions have been removed from many Chainguard Images and may not be installed once the container is running.

This limited implementation means that only the necessary software to operate can run on the container and restricts the installation of additional software on the image during operation. Be sure to have fixed permissions on libraries and executable files in place so that any software installed can’t be modified.

The host’s container execution environment further reduces the risk of unauthorized modification of software through Linux container isolation capabilities including namespaces and cgroups. These restrictions prevent unauthorized modification of the host operating system environment.

Address Space Layout Randomization (ASLR)

ASLR configuration is the responsibility of the host operating system on which containers run. Applications running within a container on a host that has ASLR enabled will automatically be protected by the configuration. No additional action is needed to ensure that container-based applications are protected.

Host firewall

Linux containers inherit the firewall configuration of their host operating system which dictates which ports on the container can be accessed from the network. Selection of which ports to make accessible on the applications running on the container is the responsibility of the host firewall configuration — an additional application-level firewall inside the container is not necessary.

Host filesystem

Linux containers use the host’s filesystem for storage of their files and configuration. To protect data at rest inside containers from unauthorized access or modification, you must modify the host operating system’s configuration. As an example, one might set up encrypted virtual filesystems. The host filesystem is also responsible for the size, utilization, and capacity of the physical disks that are used by containers running on that host.

Vulnerability scanning

The team deploying the container is also responsible for scanning it for vulnerabilities. This scanning can be executed from the host operating system or against the container image when it is stored in a registry. Continuous scanning can be used to detect vulnerabilities that have been identified / announced since the previous scan and determine when updated images should be built and deployed in the environment.

Time

Linux containers inherit the system time from the underlying host; likewise, containers don’t operate their own separate time services. The host owner is responsible for configuring the host’s time service to generate the timestamp used by its auditing system, perform periodic synchronization, and ensure that only authorized time servers are used as the authoritative source. Time synchronization of the host clock is automatically reflected in the time used by the container.

STIGs

These containers can be validated against the General Purpose Operating System STIG and other applicable STIGs using the OpenSCAP toolset. OpenSCAP can validate configuration of container images by reviewing the configuration of the image filesystem and can perform interactive checks by executing commands against running containers.

Learn more

Chainguard’s STIG hardened FIPS Images are now generally available. You can check out our STIG repo or contact us for more information. If you’d like to learn more about how Chainguard Images can help you meet FedRAMP compliance, we encourage you to refer to our overview of Chainguard’s FIPS-ready Images.