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.

Security related action options

Apptainer has many security related flags that can be passed to the action commands; shell, exec, and run allowing fine grained control of security.

--add-caps

As explained above, --add-caps will “activate” Linux capabilities when a container is initiated, providing those capabilities have been granted to the user by an administrator using the capability add command. This option will also accept the case insensitive keyword all to add every capability granted by the administrator.

--allow-setuid

The SetUID bit allows a program to be executed as the user that owns the binary. The most well-known SetUID binaries are owned by root and allow a user to execute a command with elevated privileges. But other SetUID binaries may allow a user to execute a command as a service account.

By default SetUID is disallowed within Apptainer containers as a security precaution. But the root user can override this precaution and allow SetUID binaries to behave as expected within an Apptainer container with the --allow-setuid option like so:

$ sudo apptainer shell --allow-setuid some_container.sif

--keep-privs

It is possible for an admin to set a different set of default capabilities or to reduce the default capabilities to zero for the root user by setting the root default capabilities parameter in the apptainer.conf file to file or no respectively. If this change is in effect, the root user can override the apptainer.conf file and enter the container with full capabilities using the --keep-privs option.

$ sudo apptainer exec --keep-privs library://centos 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=128 time=18.8 ms

--- 8.8.8.8 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 18.838/18.838/18.838/0.000 ms

--drop-caps

By default, the root user has a full set of capabilities when they enter the container. You may choose to drop specific capabilities when you initiate a container as root to enhance security.

For instance, to drop the ability for the root user to open a raw socket inside the container:

$ sudo apptainer exec --drop-caps CAP_NET_RAW library://centos ping -c 1 8.8.8.8
ping: socket: Operation not permitted

The drop-caps option will also accept the case insensitive keyword all as an option to drop all capabilities when entering the container.

--security

The --security flag allows the root user to leverage security modules such as SELinux, AppArmor, and seccomp within your Apptainer container. You can also change the UID and GID of the user within the container at runtime.

For instance:

$ sudo whoami
root

$ sudo apptainer exec --security uid:1000 my_container.sif whoami
david

To use seccomp to blacklist a command follow this procedure. (It is actually preferable from a security standpoint to whitelist commands but this will suffice for a simple example.) Note that this example was run on Ubuntu and that Apptainer was installed with the libseccomp-dev and pkg-config packages as dependencies.

First write a configuration file. An example configuration file is installed with Apptainer, normally at /usr/local/etc/apptainer/seccomp-profiles/default.json. For this example, we will use a much simpler configuration file to blacklist the mkdir command.

{
    "defaultAction": "SCMP_ACT_ALLOW",
    "archMap": [
        {
            "architecture": "SCMP_ARCH_X86_64",
            "subArchitectures": [
                "SCMP_ARCH_X86",
                "SCMP_ARCH_X32"
            ]
        }
    ],
    "syscalls": [
        {
            "names": [
                "mkdir"
            ],
            "action": "SCMP_ACT_KILL",
            "args": [],
            "comment": "",
            "includes": {},
            "excludes": {}
        }
    ]
}

We’ll save the file at /home/david/no_mkdir.json. Then we can invoke the container like so:

$ sudo apptainer shell --security seccomp:/home/david/no_mkdir.json my_container.sif

Apptainer> mkdir /tmp/foo
Bad system call (core dumped)

Note that attempting to use the blacklisted mkdir command resulted in a core dump.

The full list of arguments accepted by the --security option are as follows:

--security="seccomp:/usr/local/etc/apptainer/seccomp-profiles/default.json"
--security="apparmor:/usr/bin/man"
--security="selinux:context"
--security="uid:1000"
--security="gid:1000"
--security="gid:1000:1:0" (multiple gids, first is always the primary group)