• Security risks for different executors
  • Usage of Shell executor
  • Usage of Docker executor
  • Usage of private Docker images with if-not-present pull policy
• Systems with Docker installed
  • Usage of SSH executor
  • Usage of Parallels executor
• Cloning a runner
• Security risks when using GIT_STRATEGY: fetch on shared environments
• Security hardening options
  • Reduce the security risk of using privileged containers
  • Network segmentation
  • Secure the runner host

Security for self-managed runners

A GitLab CI/CD pipeline is a workflow automation engine used for simple or complex DevOps automation tasks. Because these pipelines enable a remote code execution service, you should implement the following process to reduce security risks:

• A systematic approach to configuring the security of the entire technology stack.
• Ongoing rigorous reviews of the configuration and use of the platform.

If you plan to run your GitLab CI/CD jobs on self-managed runners, then security risks exist for your compute infrastructure and network.

The runner executes code defined in the CI/CD job. Any user that has the Developer role for the project’s repository could compromise the security of the environment hosting the runner, whether intentional or not.

This risk is even more acute if your self-managed runners are non-ephemeral and used for multiple projects.

• A job from a repository embedded with malicious code can compromise the security of other repositories serviced by the non-ephemeral runner.
• Depending on the executor, a job can install malicious code on the virtual machine where the runner is hosted.
• Secret variables exposed to jobs running in a compromised environment can be stolen, including but not limited to the CI_JOB_TOKEN.
• Users with the Developer role have access to submodules associated with the project, even if they don’t have access to the upstream projects of the submodule.

Security risks for different executors

Depending on the executor you are using, you can face different security risks.

Usage of Shell executor

High-security risks exist to your runner host and network when running builds with the shell executor. The jobs are run with the permissions of the GitLab Runner’s user and can steal code from other projects that are run on this server. Use it only for running trusted builds.

Usage of Docker executor

Docker can be considered safe when running in non-privileged mode. To make such a configuration more secure, run jobs as a non-root user in Docker containers with disabled sudo or dropped SETUID and SETGID capabilities.

More granular permissions can be configured in non-privileged mode via the cap_add/cap_drop settings.

It is not advised to run containers in privileged mode.

When privileged mode is enabled, a user running a CI/CD job could gain full root access to the runner’s host system, permission to mount and unmount volumes, and run nested containers.

By enabling privileged mode, you are effectively disabling all the container’s security mechanisms and exposing your host to privilege escalation, which can lead to container breakout.

It is especially risky when runners are shared between several organizations. For example, an instance-wide runner in a service like GitLab.com, where multiple separate organizations can work concurrently.

If you use a Docker Machine executor, we also strongly recommend to use the MaxBuilds = 1 setting, which ensures that a single autoscaled VM (potentially compromised because of the security weakness introduced by the privileged mode) is used to handle one and only one job.

Usage of private Docker images with if-not-present pull policy

When using the private Docker images support described in advanced configuration: using a private container registry you should use always as the pull_policy value. Especially you should use always pull policy if you are hosting a public, shared Runner with the Docker or Kubernetes executors.

Let’s consider an example where the pull policy is set to if-not-present:

1. User A has a private image at registry.example.com/image/name.
2. User A starts a build on a shared runner: The build receives the registry credentials and pulls the image after authorization in registry.
3. The image is stored on a shared runner’s host.
4. User B doesn’t have access to the private image at registry.example.com/image/name.
5. User B starts a build that is using this image on the same shared runner as User A: Runner finds a local version of the image and uses it even if the image could not be pulled because of missing credentials.

Therefore, if you host a runner that can be used by different users and different projects (with mixed private, and public access levels) you should never use if-not-present as the pull policy value, but use:

• never - If you want to limit users to use the only image pre-downloaded by you.
• always - If you want to give users the possibility to download any image from any registry.

The if-not-present pull policy should be used only for specific runners used by trusted builds and users.

Read the pull policies documentation for more information.

Systems with Docker installed

When installing the GitLab Runner package on Linux systems with Docker installed, gitlab-runner creates a user that has permission to access the Docker daemon. This makes the jobs that run with the shell executor able to access docker with full permissions and potentially allows root access to the server.

Usage of SSH executor

SSH executors are susceptible to MITM attack (man-in-the-middle), because of missing StrictHostKeyChecking option. This will be fixed in one of the future releases.

Usage of Parallels executor

Parallels executor is the safest possible option because it uses full system virtualization and with VM machines that are configured to run in the isolated virtualization and VM machines that are configured to run in isolated mode. It blocks access to all peripherals and shared folders.

Cloning a runner

Runners use a token to identify to the GitLab Server. If you clone a runner then the cloned runner could be picking up the same jobs for that token. This is a possible attack vector to “steal” runner jobs.

Security risks when using GIT_STRATEGY: fetch on shared environments

When you set GIT_STRATEGY to fetch, the runner attempts to reuse the local working copy of the Git repository.

Using a local copy can improve the performance of CI/CD jobs. However, any user with access to that reusable copy can add code that executes in other users’ pipelines.

Git stores the contents of a submodule (a repository embedded inside another repository) in the parent repository’s Git reflog. As a result, after a project’s submodules have been initially cloned, subsequent jobs can access the contents of the submodules by running git submodule update in their script. This applies even if the submodules have been deleted and the user that initiated the job doesn’t have access to the submodule projects.

Use GIT_STRATEGY: fetch only when you trust all users who have access to the shared environment.

Security hardening options

Reduce the security risk of using privileged containers

If you must run CI/CD jobs that require the use of Docker’s --privileged flag, you can take these steps to reduce the security risk:

• Run Docker containers with the --privileged flag enabled only on isolated and ephemeral virtual machines.
• Configure dedicated runners that are meant to execute jobs that require the use of Docker’s --privileged flag. Then configure these runners to execute jobs only on protected branches.

Network segmentation

GitLab Runner is designed to run user-controlled scripts. To reduce the attack surface if a job is malicious, you can consider running them in their own network segment. This would provide network separation from other infrastructure and services.

All needs are unique, but for a cloud environment, this could include:

• Configuring runner virtual machines in their own network segment
• Blocking SSH access from the Internet to runner virtual machines
• Restricting traffic between runner virtual machines
• Filtering access to cloud provider metadata endpoints

Secure the runner host

If you are using a static host for a runner, whether bare-metal or virtual machine, you should implement security best practices for the host operating system.

Malicious code executed in the context of a CI job could compromise the host, so security protocols can help mitigate the impact. Other points to keep in mind include securing or removing files such as SSH keys from the host system that may enable an attacker to access other endpoints in the environment.