Flatcar Container Linux hardening guide

    This guide covers the basics of securing a Flatcar Container Linux instance. Flatcar Container Linux has a very slim network profile and the only service that listens by default on Flatcar Container Linux is sshd on port 22 on all interfaces. There are also some defaults for local users and services that should be considered.

    Remote listening services

    Disabling sshd

    To disable sshd from listening you can stop the socket:

    systemctl mask sshd.socket --now
    

    If you wish to make further customizations see our customize sshd guide .

    Remote non-listening services

    etcd and Locksmith

    etcd and Locksmith should be secured and authenticated using TLS if you are using these services. Please see the relevant guides for details.

    Local services

    Local users

    Flatcar Container Linux has a single default user account called “core”. Generally this user is the one that gets ssh keys added to it via a Container Linux Config for administrators to login. The core user, by default, has access to the wheel group which grants sudo access. The group can’t be easily changed and thus the solution to restrict access is to either require a password for sudo but not setting one, or disable login for the core user.

    A sudo drop-in can be created under /etc/sudoers.d/core-passwd with the contents core ALL=(ALL) ALL and as long as the core user has no password set it can’t use sudo. Here is a CLC snippet:

    storage:
      files:
        - path: /etc/sudoers.d/core-passwd
          mode: 0644
          contents:
            inline: |
              core	ALL=(ALL) 	ALL
    

    You can disable the core user by setting the login shell to /sbin/nologin, here a CLC snippet:

    passwd:
      users:
        - name: core
          shell: /sbin/nologin
    

    Docker daemon

    The docker daemon is accessible via a unix domain socket at /run/docker.sock. Users in the “docker” group have access to this service and access to the docker socket grants similar capabilities to sudo. The core user, by default, has access to the docker group. The group can’t be easily changed and thus the solution to restrict access is to disable login for the core user or restrict the Docker socket permissions.

    You can restrict the Docker socket to root by creating a unit drop-in for docker.socket in /etc/systemd/system/docker.socket.d/10-restrict.conf, here a CLC snippet:

    systemd:
      units:
        - name: docker.socket
          dropins:
            - name: 10-restrict.conf
              contents: |
                [Socket]
                SocketGroup=root
    

    Additional hardening

    Disabling Simultaneous Multi-Threading

    Recent Intel CPU vulnerabilities cannot be fully mitigated in software without disabling Simultaneous Multi-Threading. This can have a substantial performance impact and is only necessary for certain workloads, so for compatibility reasons, SMT is enabled by default.

    The SMT on Container Linux guide provides guidance and instructions for disabling SMT.

    Disable USB

    If you don’t expect to ever use USB, you can disable the kernel module, here a CLC snippet:

    storage:
      files:
        - path: /etc/modprobe.d/blacklist.conf
          mode: 0644
          contents:
            inline: |
              blacklist usb-storage
    

    SELinux

    SELinux is a fine-grained access control mechanism integrated into Flatcar Container Linux. Each container runs in its own independent SELinux context, increasing isolation between containers and providing another layer of protection should a container be compromised.

    Flatcar Container Linux implements SELinux, but currently does not enforce SELinux protections by default. The SELinux on Flatcar Container Linux guide covers the process of checking containers for SELinux policy compatibility and switching SELinux into enforcing mode.