:title: Docker Security
:description: Review of the Docker Daemon attack surface
:keywords: Docker, Docker documentation, security

.. _dockersecurity:

Docker Security
===============

  *Adapted from* `Containers & Docker: How Secure are They? <blogsecurity>`_

There are three major areas to consider when reviewing Docker security:

* the intrinsic security of containers, as implemented by kernel
  namespaces and cgroups;
* the attack surface of the Docker daemon itself;
* the "hardening" security features of the kernel and how they
  interact with containers.

Kernel Namespaces
-----------------

Docker containers are essentially LXC containers, and they come with
the same security features. When you start a container with ``docker
run``, behind the scenes Docker uses ``lxc-start`` to execute the
Docker container. This creates a set of namespaces and control groups
for the container. Those namespaces and control groups are not created
by Docker itself, but by ``lxc-start``. This means that as the LXC
userland tools evolve (and provide additional namespaces and isolation
features), Docker will automatically make use of them.

**Namespaces provide the first and most straightforward form of
isolation**: processes running within a container cannot see, and even
less affect, processes running in another container, or in the host
system.

**Each container also gets its own network stack**, meaning that a
container doesn’t get a privileged access to the sockets or interfaces
of another container. Of course, if the host system is setup
accordingly, containers can interact with each other through their
respective network interfaces — just like they can interact with
external hosts. When you specify public ports for your containers or
use :ref:`links <working_with_links_names>` then IP traffic is allowed
between containers. They can ping each other, send/receive UDP
packets, and establish TCP connections, but that can be restricted if
necessary. From a network architecture point of view, all containers
on a given Docker host are sitting on bridge interfaces. This means
that they are just like physical machines connected through a common
Ethernet switch; no more, no less.

How mature is the code providing kernel namespaces and private
networking?  Kernel namespaces were introduced `between kernel version
2.6.15 and 2.6.26
<http://lxc.sourceforge.net/index.php/about/kernel-namespaces/>`_. This
means that since July 2008 (date of the 2.6.26 release, now 5 years
ago), namespace code has been exercised and scrutinized on a large
number of production systems. And there is more: the design and
inspiration for the namespaces code are even older. Namespaces are
actually an effort to reimplement the features of `OpenVZ
<http://en.wikipedia.org/wiki/OpenVZ>`_ in such a way that they could
be merged within the mainstream kernel. And OpenVZ was initially
released in 2005, so both the design and the implementation are
pretty mature.

Control Groups
--------------

Control Groups are the other key component of Linux Containers. They
implement resource accounting and limiting. They provide a lot of very
useful metrics, but they also help to ensure that each container gets
its fair share of memory, CPU, disk I/O; and, more importantly, that a
single container cannot bring the system down by exhausting one of
those resources.

So while they do not play a role in preventing one container from
accessing or affecting the data and processes of another container,
they are essential to fend off some denial-of-service attacks. They
are particularly important on multi-tenant platforms, like public and
private PaaS, to guarantee a consistent uptime (and performance) even
when some applications start to misbehave.

Control Groups have been around for a while as well: the code was
started in 2006, and initially merged in kernel 2.6.24.

Docker Daemon Attack Surface
----------------------------

Running containers (and applications) with Docker implies running the
Docker daemon. This daemon currently requires root privileges, and you
should therefore be aware of some important details.

First of all, **only trusted users should be allowed to control your
Docker daemon**. This is a direct consequence of some powerful Docker
features. Specifically, Docker allows you to share a directory between
the Docker host and a guest container; and it allows you to do so
without limiting the access rights of the container. This means that
you can start a container where the ``/host`` directory will be the
``/`` directory on your host; and the container will be able to alter
your host filesystem without any restriction. This sounds crazy? Well,
you have to know that **all virtualization systems allowing filesystem
resource sharing behave the same way**. Nothing prevents you from
sharing your root filesystem (or even your root block device) with a
virtual machine.

This has a strong security implication: if you instrument Docker from
e.g. a web server to provision containers through an API, you should
be even more careful than usual with parameter checking, to make sure
that a malicious user cannot pass crafted parameters causing Docker to
create arbitrary containers.

For this reason, the REST API endpoint (used by the Docker CLI to
communicate with the Docker daemon) changed in Docker 0.5.2, and now
uses a UNIX socket instead of a TCP socket bound on 127.0.0.1 (the
latter being prone to cross-site-scripting attacks if you happen to
run Docker directly on your local machine, outside of a VM). You can
then use traditional UNIX permission checks to limit access to the
control socket.

You can also expose the REST API over HTTP if you explicitly decide
so. However, if you do that, being aware of the abovementioned
security implication, you should ensure that it will be reachable
only from a trusted network or VPN; or protected with e.g. ``stunnel``
and client SSL certificates.

Recent improvements in Linux namespaces will soon allow to run
full-featured containers without root privileges, thanks to the new
user namespace. This is covered in detail `here
<http://s3hh.wordpress.com/2013/07/19/creating-and-using-containers-without-privilege/>`_. Moreover,
this will solve the problem caused by sharing filesystems between host
and guest, since the user namespace allows users within containers
(including the root user) to be mapped to other users in the host
system.

The end goal for Docker is therefore to implement two additional
security improvements:

* map the root user of a container to a non-root user of the Docker
  host, to mitigate the effects of a container-to-host privilege
  escalation;
* allow the Docker daemon to run without root privileges, and delegate
  operations requiring those privileges to well-audited sub-processes,
  each with its own (very limited) scope: virtual network setup,
  filesystem management, etc.

Finally, if you run Docker on a server, it is recommended to run
exclusively Docker in the server, and move all other services within
containers controlled by Docker. Of course, it is fine to keep your
favorite admin tools (probably at least an SSH server), as well as
existing monitoring/supervision processes (e.g. NRPE, collectd, etc).

Linux Kernel Capabilities
-------------------------

By default, Docker starts containers with a very restricted set of
capabilities. What does that mean?

Capabilities turn the binary "root/non-root" dichotomy into a
fine-grained access control system. Processes (like web servers) that
just need to bind on a port below 1024 do not have to run as root:
they can just be granted the ``net_bind_service`` capability
instead. And there are many other capabilities, for almost all the
specific areas where root privileges are usually needed.

This means a lot for container security; let’s see why!

Your average server (bare metal or virtual machine) needs to run a
bunch of processes as root. Those typically include SSH, cron,
syslogd; hardware management tools (to e.g. load modules), network
configuration tools (to handle e.g. DHCP, WPA, or VPNs), and much
more. A container is very different, because almost all of those tasks
are handled by the infrastructure around the container:

* SSH access will typically be managed by a single server running in
  the Docker host;
* ``cron``, when necessary, should run as a user process, dedicated
  and tailored for the app that needs its scheduling service, rather
  than as a platform-wide facility;
* log management will also typically be handed to Docker, or by
  third-party services like Loggly or Splunk;
* hardware management is irrelevant, meaning that you never need to
  run ``udevd`` or equivalent daemons within containers;
* network management happens outside of the containers, enforcing
  separation of concerns as much as possible, meaning that a container
  should never need to perform ``ifconfig``, ``route``, or ip commands
  (except when a container is specifically engineered to behave like a
  router or firewall, of course).

This means that in most cases, containers will not need "real" root
privileges *at all*. And therefore, containers can run with a reduced
capability set; meaning that "root" within a container has much less
privileges than the real "root". For instance, it is possible to:

* deny all "mount" operations;
* deny access to raw sockets (to prevent packet spoofing);
* deny access to some filesystem operations, like creating new device
  nodes, changing the owner of files, or altering attributes
  (including the immutable flag);
* deny module loading;
* and many others.

This means that even if an intruder manages to escalate to root within
a container, it will be much harder to do serious damage, or to
escalate to the host.

This won't affect regular web apps; but malicious users will find that
the arsenal at their disposal has shrunk considerably! You can see
`the list of dropped capabilities in the Docker code
<https://github.com/dotcloud/docker/blob/v0.5.0/lxc_template.go#L97>`_,
and a full list of available capabilities in `Linux manpages
<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_.

Of course, you can always enable extra capabilities if you really need
them (for instance, if you want to use a FUSE-based filesystem), but
by default, Docker containers will be locked down to ensure maximum
safety.

Other Kernel Security Features
------------------------------

Capabilities are just one of the many security features provided by
modern Linux kernels. It is also possible to leverage existing,
well-known systems like TOMOYO, AppArmor, SELinux, GRSEC, etc. with
Docker.

While Docker currently only enables capabilities, it doesn't interfere
with the other systems. This means that there are many different ways
to harden a Docker host. Here are a few examples.

* You can run a kernel with GRSEC and PAX. This will add many safety
  checks, both at compile-time and run-time; it will also defeat many
  exploits, thanks to techniques like address randomization. It
  doesn’t require Docker-specific configuration, since those security
  features apply system-wide, independently of containers.
* If your distribution comes with security model templates for LXC
  containers, you can use them out of the box. For instance, Ubuntu
  comes with AppArmor templates for LXC, and those templates provide
  an extra safety net (even though it overlaps greatly with
  capabilities).
* You can define your own policies using your favorite access control
  mechanism. Since Docker containers are standard LXC containers,
  there is nothing “magic” or specific to Docker.

Just like there are many third-party tools to augment Docker
containers with e.g. special network topologies or shared filesystems,
you can expect to see tools to harden existing Docker containers
without affecting Docker’s core.

Conclusions
-----------

Docker containers are, by default, quite secure; especially if you
take care of running your processes inside the containers as
non-privileged users (i.e. non root).

You can add an extra layer of safety by enabling Apparmor, SELinux,
GRSEC, or your favorite hardening solution.

Last but not least, if you see interesting security features in other
containerization systems, you will be able to implement them as well
with Docker, since everything is provided by the kernel anyway.

For more context and especially for comparisons with VMs and other
container systems, please also see the `original blog post
<blogsecurity>`_.

.. _blogsecurity: http://blog.docker.io/2013/08/containers-docker-how-secure-are-they/

