Security in SingularityCE

Security Policy

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

Sylabs discloses vulnerabilities found in SingularityCE through public CVE reports, as well as 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 typically need to access the same shared resource. User accounts, groups, and standard file permissions limit their access to data and 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, and users can request that the daemon start 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 carry out themselves.

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.

Security Implications of Unprivileged User Namespaces


If you rely on the ECL or other container execution limits, you must disable unprivileged user namespace creation on your systems.

When unprivileged user namespace creation is allowed on a system, a user can supply and use their own unprivileged installation of Singularity or another container runtime. They may also be able to use standard system tools such as unshare, nsenter, and FUSE mounts to access / execute arbitrary containers without installing any runtime. Both of these approaches will allow users to bypass any restrictions that have been set in a system-wide installation of SingularityCE. These include:

  • The allow container and limit container directives in singularity.conf.

  • The Execution Control List, which restricts execution of SIF container images via signature checks.

Note also that SingularityCE’s –oci mode is an unprivileged runtime that requires unprivileged user namespace creation. It does not implement the container restrictions that cannot be effectively enforced when unprivileged user namespaces are available.

If your primary security concern is that of restricting the containers which users can execute, you should use singularity in setuid mode, and ensure unprivileged user namespace creation is disabled on the host.

Configuration and Limitations of User Namespace Mode

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 to this approach:

  • SIF and other single-file container images cannot be mounted using kernel mounts. SingularityCE will attempt to mount them in user space, using FUSE. If this is not possible, the container image must be extracted to a directory on disk to run. This impacts the speed of execution. As a result, workloads accessing large numbers of small files (as is the case with python application startup, for example) do not benefit from the reduced metadata load on the filesystem an image file provides. To force extraction to disk, instead of FUSE mount, use the --tmp-sandbox flag. To ensure containers are not extracted to disk, even when FUSE mounts fail, use the --no-tmp-sandbox flag.

  • The effectiveness of signing and verifying container images is reduced. With both FUSE mounts, and sandbox directories, the content of the container can easily be modified at runtime and verification against the image’s original signature cannot be performed.

  • Encryption is not supported. SingularityCE leverages kernel LUKS2 mounts to run encrypted containers without writing a decrypted version of their content to disk.

  • Some hold the opinion that vulnerabilities in kernel user namespace code could have greater impact than vulnerabilities confined to a single piece of setuid software, and are therefore reluctant to enable unprivileged user namespace creation.

  • Limitations on container execution by location, valid signatures, user/group cannot be enforced.

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 tackle security concerns by sandboxing containers executing as the root user so that they cannot affect the host system, SingularityCE has adopted a different 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, as well.

  • Favor integration over isolation: a user is allowed to easily use host resources such as GPUs, network filesystems, and high speed interconnects. 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 nosuid 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 other means.

If you do require the additional isolation of the network, devices, PIDs, etc., which other runtimes provide, 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 that involve many small files.

Holding the container image in a single file also enables unique security features. The container filesystem is immutable, and can be signed. The signature travels as part of 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 corresponding public keys to verify the container. Verification of signed containers happens automatically in singularity pull commands against the Singularity Container Services (SCS) Library. The SCS keystore 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. In setuid mode, SingularityCE can be configured with an execution control list (ECL). The ECL requires the presence of one or more valid signatures, to limit execution to approved containers on systems that have unprivileged user namespace creation disabled.

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 then remains 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, the encrypted container is run without ever decrypting its contents to disk.

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, authorize containers, etc.

For example, the ecl.toml configuration file allows blacklisting and whitelisting of containers.

Documentation for administrators about configuration files and their parameters is available here.

When running a container as root, SingularityCE can apply hardening rules using cgroups, seccomp, and apparmor. See here for details on these options.