Security Options

Apptainer implements security related options in the container runtime. This document describes the methods users have for specifying the security scope and context when running Apptainer containers.

Linux Capabilities

Note

It is extremely important to recognize that granting users Linux capabilities with the capability command group is usually identical to granting those users root level access on the host system. Most if not all capabilities will allow users to “break out” of the container and become root on the host. This feature is targeted toward special use cases (like cloud-native architectures) where an admin/developer might want to limit the attack surface within a container that normally runs as root. This is not a good option in multi-tenant HPC environments where an admin wants to grant a user special privileges within a container. For that and similar use cases, the fakeroot feature is a better option.

Apptainer provides full support for granting and revoking Linux capabilities on a user or group basis. For example, let us suppose that an admin has decided to grant a user (named pinger) capabilities to open raw sockets so that they can use ping in a container where the binary is controlled via capabilities. For information about how to manage capabilities as an admin please refer to the capability admin docs.

To take advantage of this granted capability as a user, pinger must also request the capability when executing a container with the --add-caps flag like so:

$ apptainer exec --add-caps CAP_NET_RAW library://sylabs/tests/ubuntu_ping:v1.0 ping -c 1 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=52 time=73.1 ms

--- 8.8.8.8 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 73.178/73.178/73.178/0.000 ms

If the admin decides that it is no longer necessary to allow the user pinger to open raw sockets within Apptainer containers, they can revoke the appropriate Linux capability and pinger will not be able to add that capability to their containers anymore:

$ apptainer exec --add-caps CAP_NET_RAW library://sylabs/tests/ubuntu_ping:v1.0 ping -c 1 8.8.8.8
WARNING: not authorized to add capability: CAP_NET_RAW
ping: socket: Operation not permitted

Another scenario which is atypical of shared resource environments, but useful in cloud-native architectures is dropping capabilities when spawning containers as the root user to help minimize attack surfaces. With a default installation of Apptainer, containers created by the root user will maintain all capabilities. This behavior is configurable if desired. Check out the capability configuration and root default capabilities sections of the admin docs for more information.

Assuming the root user will execute containers with the CAP_NET_RAW capability by default, executing the same container pinger executed above works without the need to grant capabilities:

# apptainer exec library://sylabs/tests/ubuntu_ping:v1.0 ping -c 1 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=52 time=59.6 ms

--- 8.8.8.8 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 59.673/59.673/59.673/0.000 ms

Now we can manually drop the CAP_NET_RAW capability like so:

# apptainer exec --drop-caps CAP_NET_RAW library://sylabs/tests/ubuntu_ping:v1.0 ping -c 1 8.8.8.8
ping: socket: Operation not permitted

And now the container will not have the ability to create new sockets, causing the ping command to fail.

The --add-caps and --drop-caps options will accept the all keyword. Of course appropriate caution should be exercised when using this keyword.

Building encrypted containers

With Apptainer it is possible to build and run encrypted containers. The containers are decrypted at runtime entirely in kernel space, meaning that no intermediate decrypted data is ever present on disk. See encrypted containers for more details.