Security in SingularityCE
If you suspect you have found a vulnerability in SingularityCE we want to work with you so that it can be investigated, fixed, and disclosed in a responsible manner. Please follow the steps in our published Security Policy, which begins with contacting us privately via firstname.lastname@example.org
Sylabs discloses vulnerabilities found in SingularityCE through public CVE reports, and notifications on our community channels. We encourage all users to monitor new releases of SingularityCE for security information. Security patches are applied to the latest open-source release.
SingularityPRO is a professionally curated and licensed version of SingularityCE that provides added security, stability, and support beyond that offered by the open source project. Security and bug-fix patches are backported to select versions of SingularityPRO, so that they can be deployed long-term where required. PRO users receive security fixes as detailed in the Sylabs Security Policy.
SingularityCE grew out of the need to implement a container platform that was suitable for use on shared systems, such as HPC clusters. In these environments multiple people access a shared resource. User accounts, groups, and standard file permissions limit their access to data, devices, and prevent them from disrupting or accessing others’ work.
To provide security in these environments a container needs to run as the user who starts it on the system. Before the widespread adoption of the Linux user namespace, only a privileged user could perform the operations which are needed to run a container. A default Docker installation uses a root-owned daemon to start containers. Users can request that the daemon starts a container on their behalf. However, coordinating a daemon with other schedulers is difficult and, since the daemon is privileged, users can ask it to carry out actions that they wouldn’t normally have permission to do.
When a user runs a container with SingularityCE, it is started as a normal process running under the user’s account. Standard file permissions and other security controls based on user accounts, groups, and processes apply. In a default installation SingularityCE uses a setuid starter binary to perform only the specific tasks needed to setup the container.
Setuid & User Namespaces
Using a setuid binary to run container setup operations is essential to support containers on older Linux distributions, such as CentOS 6, that were previously common in HPC and enterprise. Newer distributions have support for ‘unprivileged user namespace creation’. This means a normal user can create a user namespace, in which most setup operations needed to run a container can be run, unprivileged.
SingularityCE supports running containers without setuid, using user
namespaces. It can be compiled with the
--without-setuid option, or
allow setuid = no can be set in
singularity.conf to enable this.
In this mode all operations run as the user who starts the
singularity program. However, there are some disadvantages:
SIF and other single file container images cannot be mounted directly. The container image must be extracted to a directory on disk to run. This impact the speed of execution. Workloads accessing large numbers of small files (such as python application startup) do not benefit from the reduced metadata load on the filesystem an image file provides.
Replacing direct kernel mounts with a FUSE approach is likely to cause a significant reduction in performance.
The effectiveness of signing and verifying container images is reduced as, when extracted to a directory, modification is possible and verification of the image’s original signature cannot be performed.
Encryption is not supported. SingularityCE leverages kernel LUKS2 mounts to run encrypted containers without decrypting their content to disk.
Some sites hold the opinion that vulnerabilities in kernel user namespace code could have greater impact than vulnerabilities confined to a single piece of setuid software. Therefore they are reluctant to enable unprivileged user namespace creation.
Because of the points above, the default mode of operation of SingularityCE uses a setuid binary. Sylabs aims to reduce the circumstances that require this as new functionality is developed and reaches commonly deployed Linux distributions.
Runtime & User Privilege Model
While other runtimes have aimed to safely sandbox containers executing
root user, so that they cannot affect the host system,
SingularityCE has adopted an alternative security model:
Containers should be run as an unprivileged user.
The user should never be able to elevate their privileges inside the container to gain control over the host.
All permission restrictions on the user outside of a container should apply inside the container.
Favor integration over isolation. Allow a user to use host resources such as GPUs, network file systems, high speed interconnects easily. The process ID space, network etc. are not isolated in separate namespaces by default.
To accomplish this, SingularityCE uses a number of Linux kernel
features. The container file system is mounted using the
option, and processes are started with the
PR_NO_NEW_PRIVS flag set.
This means that even if you run
sudo inside your container, you
won’t be able to change to another user, or gain root privileges by
If you do require the additional isolation of the network, devices, PIDs etc. provided by other runtimes, SingularityCE can make use of additional namespaces and functionality such as seccomp and cgroups.
Singularity Image Format (SIF)
SingularityCE uses SIF as its default container format. A SIF container is a single file, which makes it easy to manage and distribute. Inside the SIF file, the container filesystem is held in a SquashFS object. By default, we mount the container filesystem directly using SquashFS. On a network filesystem this means that reads from the container are data-only. Metadata operations happen locally, speeding up workloads with many small files.
Holding the container image in a single file also enable unique security features. The container filesystem is immutable, and can be signed. The signature travels in the SIF image itself so that it is always possible to verify that the image has not been tampered with or corrupted.
We use private PGP keys to create a container signature, and the public
key in order to verify the container. Verification of signed containers
happens automatically in
singularity pull commands against the
Sylabs Cloud Container Library. A Keystore in the Sylabs Cloud makes it
easier to share and obtain public keys for container verification.
A container may be signed once, by a trusted individual who approves its use. It could also be signed with multiple keys to signify it has passed each step in a CI/CD QA & Security process. SingularityCE can be configured with an execution control list (ECL), which requires the presence of one or more valid signatures, to limit execution to approved containers.
In SingularityCE 3.4 and above, the root filesystem of a container (stored in the squashFS partition of SIF) can be encrypted. As a result, everything inside the container becomes inaccessible without the correct key or passphrase. The content of the container is private, even if the SIF file is shared in public.
Encryption and decryption are performed using the Linux kernel’s LUKS2 feature. This is the same technology routinely used for full disk encryption. The encrypted container is mounted directly through the kernel. Unlike other container formats, an encrypted container is not decrypted to disk in order to run it.
Configuration & Runtime Options
System administrators who manage SingularityCE can use configuration files to set security restrictions, grant or revoke a user’s capabilities, manage resources and authorize containers etc.
For example, the Execution Control List file allows restricting usage of SIF containers based on their signature and the key used to sign them.
Configuration files and their parameters are documented for administrators here.
When running a container as root, SingularityCE can apply hardening rules using seccomp and apparmor. See the ‘Security Options’ section of the user guide.
Limits on resource usage by containers can be enforced using cgroups. On systems that use cgroups v1, only the root user can set resource limits. On systems that use cgroups v2 and systemd, all users can apply resource limits as long as the system is configured for delegation.
By default, EL9, Ubuntu 22.04, Debian 11, Fedora 31 and newer use cgroups v2 and
are configured for delegation so that unprivileged users will be able to use the
--apply-cgroups and other resource limit flags of SingularityCE without
On EL8 and Ubuntu 20.04 it is possible to setup a compatible configuration by following the ‘Enabling cgroup v2’ and ‘Enabling CPU, CPUSET, and I/O delegation’ steps at the rootless containers website
See the ‘Limiting Container Resources’ section of the user guide for more details of how to apply cgroups limits to containers at runtime.