Security in Singularity

Containers are popular for many good reasons. They are light weight, easy to spin-up and require reduced IT management resources as compared to hardware VM environments. More importantly, container technology facilitates advanced research computing by granting the ability to package software in highly portable and reproducible environments encapsulating all dependencies, including the operating system. But there are still some challenges to container security.

Singularity addresses some core missions of containers : Mobility of Compute, Reproducibility, HPC support, and Security. This section gives an overview of security features supported by Singularity, especially where they differ from other container runtimes.

Security Policy

Security is not a check box that one can tick and forget. Ensuring security is a ongoing process that begins with software architecture, and continues all the way through to ongoing security practices. In addition to ensuring that containers are run without elevated privileges where appropriate, and that containers are produced by trusted sources, users must monitor their containers for newly discovered vulnerabilities and update when necessary just as they would with any other software. Sylabs is constantly probing to find and patch vulnerabilities within Singularity, and will continue to do so.

If you suspect you have found a vulnerability in Singularity, please follow the steps in our published Security Policy.

so that it can be disclosed, investigated, and fixed in an appropriate manner.

Singularity PRO - Long Term Support & Security Patches

Security patches for Singularity are applied to the latest open-source version, so it is important to follow new releases and upgrade when neccessary.

SingularityPRO is a professionally curated and licensed version of Singularity 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 Singularity PRO, so that they can be deployed long-term where required. PRO users receive security fixes (without specific notification or detail) prior to public disclosure, as detailed in the Sylabs Security Policy.

Singularity Runtime & User Privilege

The Singularity Runtime enforces a unique security model that makes it appropriate for untrusted users to run untrusted containers safely on multi-tenant resources. When you run a container, the processes in the container will run as your user account. Singularity dynamically writes UID and GID information to the appropriate files within the container, and the user remains the same inside and outside the container, i.e., if you’re an unprivileged user while entering the container you’ll remain an unprivileged user inside the container.

Additional blocks are in place to prevent users from escalating privileges once they are inside of a container. 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 priveleges by other means. This approach provides a secure way for users to run containers and greatly simplifies things like reading and writing data to the host system with appropriate ownership.

It is also important to note that the philosophy of Singularity is Integration over Isolation. Most container run times strive to isolate your container from the host system and other containers as much as possible. Singularity, on the other hand, assumes that the user’s primary goals are portability, reproducibility, and ease of use and that isolation is often a tertiary concern. Therefore, Singularity only isolates the mount namespace by default, and will bind mount several host directories such as $HOME and /tmp into the container at runtime. If needed, additional levels of isolation can be achieved by passing options causing Singularity to enter any or all of the other kernel namespaces and to prevent automatic bind mounting. These measures allow users to interact with the host system from within the container in sensible ways.

Singularity Image Format (SIF)

Ensuring container security as a continuous process. Singularity provides ways to ensure integrity throughout the lifecyle of a container, i.e. at rest, in transit and while running. The SIF Singularity Image Format has been designed to achieve these goals.

A SIF file is an immutable container image that packages the container environment into a single file. SIF supports security and integrity through the ability to cryptographically sign a container, creating a signature block within the SIF file which can guarantee immutability and provide accountability as to who signed it. Singularity follows the OpenPGP standard to create and manage these signatures, and the keys used to create them. After building an image with Singularity, a user can singularity sign the container and push it to the Library along with its public PGP key (stored in Keystore). The signature can be verified (singularity verify) while pulling or downloading the image. This feature makes it easy to to establish trust in collaborations within and between teams.

In Singularity 3.4 and above, the root file system 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. Other users on the system will be able to look inside your container files. The content of the container is private, even if the SIF file is shared in public.

Unlike other container platforms where execution requires a number of layers to be extracted to a rootfs directory on the host, Singularity executes containers in a single step, directly from the immutable .sif. This reduces the attack surface and allows the container to be easily verified at runtime, to ensure it has not been tampered with.

Admin Configurable Files

System administrators who manage Singularity can use configuration files, to set security restrictions, grant or revoke a user’s capabilities, manage resources and authorize containers etc.

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

Configuration files and their parameters are documented for administrators documented here.

cgroups support

Starting with v3.0, Singularity added native support for cgroups, allowing users to limit the resources their containers consume without the help of a separate program like a batch scheduling system. This feature can help to prevent DoS attacks where one container seizes control of all available system resources in order to stop other containers from operating properly. To use this feature, a user first creates a cgroups configuration file. An example configuration file is installed by default with Singularity as a guide. At runtime, the --apply-cgroups option is used to specify the location of the configuration file to apply to the container and cgroups are configured accordingly. More about cgroups support here.

--security options

Singularity supports a number of methods for further modifying the security scope and context when running Singularity containers. Flags can be passed to the action commands; shell, exec, and run allowing fine grained control of security. Details about them are documented here.

Security in the Sylabs Cloud

Sylabs Cloud consists of a Remote Builder, a Container Library, and a Keystore. Together, theses services provide an end-to-end solution for packaging and distributing applications in secure and trusted containers.

Remote Builder

As mentioned earlier, the Singularity runtime prevents executing code with root-level permissions on the host system. However, building a container requires elevated privileges that most shared environments do not grant their users. The Build Service aims to address this by allowing unprivileged users to build containers remotely, with root level permissions inside the secured service. System administrators can use the system to monitor which users are building containers, and the contents of those containers. The Singularity CLI has native integration with the Build Service from version 3.0 onwards. In addition, a browser interface to the Build Service also exists, which allows users to build containers using only a web browser.


Please also see the Fakeroot feature which is a secure option for admins in multi-tenant HPC environments and similar use cases where they might want to grant a user special privileges inside a container.

Fakeroot has some limitations, and requires unpriveleged user namespace support in the host kernel.

Container Library

The Container Library allows users to store and share Singularity container images in the Singularity Image Format (SIF). A web front-end allows users to create new projects within the Container Library, edit documentation associated with container images, and discover container images published by their peers.

Key Store

The Key Store is a key management system offered by Sylabs that uses an OpenPGP implementation to permit sharing and discovery of PGP public keys used to sign and verify Singularity container images. This service is based on the OpenPGP HTTP Keyserver Protocol (HKP), with several enhancements:

  • The Service requires connections to be secured with Transport Layer Security (TLS).

  • The Service implements token-based authentication, allowing only authenticated users to add or modify PGP keys.

  • A web front-end allows users to view and search for PGP keys using a web browser.

Authentication and encryption

  1. Communication between users, the authentication service other services is secured via TLS encryption.

  2. The services support authentication of users via signed and encrypted authentication tokens.

  3. There is no implicit trust relationship between each service. Each request between the services is authenticated using the authentication token supplied by the user in the associated request.