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CircleCI server security features

3 months ago6 min read
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This document outlines security features built into CircleCI and related integrations.


Security is our top priority at CircleCI. We are proactive and we act on security issues immediately. Report security issues to with an encrypted message using our security team’s GPG key (ID: 0x4013DDA7, fingerprint: 3CD2 A48F 2071 61C0 B9B7 1AE2 6170 15B8 4013 DDA7).


CircleCI uses HTTPS or SSH for all networking in and out of our service, including from the browser to our services application, from the services application to your builder fleet, from our builder fleet to your source control system, and all other points of communication. None of your code or data travels to or from CircleCI without being encrypted, unless you have code in your builds that does so at your discretion. Operators may also choose to bypass our SSL configuration or not use TLS for communicating with underlying systems.

The nature of CircleCI is that our software has access to your code and whatever data that code interacts with. All jobs on CircleCI run in a sandbox (specifically, a Docker container or an ephemeral VM) that stands alone from all other builds and is not accessible from the Internet or from your own network. The build agent pulls code via git over SSH. Your particular test suite or job configurations may call out to external services or integration points within your network, and the response from such calls will be pulled into your jobs and used by your code at your discretion. After a job is complete, the container that ran the job is destroyed and rebuilt. All environment variables are encrypted using HashiCorp Vault. Environment variables are encrypted using AES256-GCM96 and are unavailable to CircleCI employees.


With CircleCI, you control the resources allocated to run the builds of your code. This will be done through instances of our builder boxes that set up the containers in which your builds will run. By their nature, build containers will pull down source code and run whatever test and deployment scripts are part of the codebase or your configuration. The containers are sandboxed, each created and destroyed for one build only (or one slice of a parallel build), and they are not available from outside themselves. The CircleCI service provides the ability to SSH directly to a particular build container. When accessing a container this way, a user will have complete access to any files or processes being run inside that build container. Only provide CircleCI access to those also trusted with your source code.


A few different external services and technology integration points touch CircleCI. The following list explains those integration points.


CircleCI uses Pusher client libraries for WebSocket communication between the server and the browser. However, for installs CircleCI uses an internal server called Slanger, so Pusher servers have no access to your instance of CircleCI, nor your source control system. This is how CircleCI, for instance, updates the builds list dynamically, or show the output of a build line-by-line as it occurs. CircleCI sends build status and lines of your build output through the web socket server (which unless you have configured your installation to run without SSL is done using the same certs over SSL), so it is encrypted in transit.

Source control systems

To use CircleCI you will set up a direct connection with your instance of GitHub Enterprise or When you set up CircleCI, you authorize the system to check out your private repositories. You may revoke this permission at any time through your GitHub application settings page and by removing Circle’s Deploy Keys and Service Hooks from your repositories' Admin pages. While CircleCI allows you to selectively build your projects, GitHub’s permissions model is "all or nothing" — CircleCI gets permission to access all of a user’s repositories or none of them. Your instance of CircleCI will have access to anything hosted in those git repositories and will create webhooks for a variety of events (for example, when code is pushed, when a user is added, etc.) that will call back to CircleCI, triggering one or more git commands that will pull down code to your build fleet.

Dependency and source caches

Most CircleCI customers use S3 or equivalent cloud-based storage inside their private cloud infrastructure (Amazon VPC, etc) to store their dependency and source caches. These storage servers are subject to the normal security parameters of anything stored on such services, meaning in most cases our customers prevent any outside access.


It is common to use S3 or similar hosted storage for artifacts. Assuming these resources are secured per your normal policies, they are as safe from any outside intrusion as any other data you store there.

Audit logs

The audit log feature is available for CircleCI installed on your servers or private cloud.

CircleCI logs important events in the system for audit and forensic analysis purposes. Audit logs are separate from system logs that track performance and network metrics.

Complete audit logs may be downloaded as a CSV file from the audit log page within the admin section of the application. Audit log fields with nested data contain JSON blobs.

Audit log events

The following are the system events that are logged. See action in the Field section below for the definition and format.

  • context.create

  • context.delete

  • context.env_var.delete


  • context.secrets.accessed

  • project.env_var.create

  • project.env_var.delete

  • project.settings.update

  • user.create

  • user.logged_in

  • user.logged_out

  • workflow.job.approve

  • workflow.job.finish

  • workflow.job.scheduled

  • workflow.job.start

Audit log fields



The action taken that created the event. The format is ASCII lowercase words separated by dots, with the entity acted upon first and the action taken last. In some cases entities are nested, for example, workflow.job.start.


The actor who performed this event. In most cases, this will be a CircleCI user. This data is a JSON blob that will always contain id and type and will likely contain name.


The entity instance acted upon for this event, for example, a project, an org, an account, or a build. This data is a JSON blob that will always contain id and type and will likely contain name.


A JSON blob of action-specific information. The schema of the payload is expected to be consistent for all events with the same action and version.


When the event occurred in UTC expressed in ISO-8601 format with up to nine digits of fractional precision, for example '2017-12-21T13:50:54.474Z'.


A set of key/value pairs that can be attached to any event. All keys and values are strings. This can be used to add additional information to certain types of events.


A UUID that uniquely identifies this event. This is intended to allow consumers of events to identify duplicate deliveries.


Version of the event schema. Currently the value will always be 1. Later versions may have different values to accommodate schema changes.


If the target is owned by an account in the CircleCI domain model, the account field should be filled in with the account name and ID. This data is a JSON blob that will always contain id and type and will likely contain name.


A flag to indicate if the action was successful.


If this event was triggered by an external request, this data will be populated and may be used to connect events that originate from the same external request. The format is a JSON blob containing id (the unique ID assigned to this request by CircleCI).

Checklist to using CircleCI securely as a customer

If you are getting started with CircleCI, there are some points you can ask your team to consider for security best practices as users of CircleCI:

  • Minimize the number of secrets (private keys / environment variables) your build needs and rotate secrets regularly.

    • It is important to rotate secrets regularly in your organization, especially as team members come and go.

    • Rotating secrets regularly means your secrets are only active for a certain amount of time, helping to reduce possible risks if keys are compromised.

    • Ensure the secrets you do use are of limited scope, with only enough permissions for the purposes of your build. Consider carefully adjudicating the role and permission systems of other platforms you use outside of CircleCI; for example, when using something such as IAM permissions on AWS, or GitHub’s Machine User feature.

  • Sometimes user misuse of certain tools might accidentally print secrets to stdout which will appear in your logs. Be aware of the following:

    • Running env or printenv which will print all your environment variables to stdout.

    • Literally printing secrets in your codebase or in your shell with echo.

    • Programs or debugging tools that print secrets on error.

  • Consult your VCS provider’s permissions for your organization (if you are in an organization) and try to follow the Principle of Least Privilege.

  • Use Restricted Contexts with teams to share environment variables with a select security group. Read through the contexts document to learn more.

  • Ensure you regularly audit who has access to SSH keys in your organization.

  • Ensure that your team is using Two-Factor Authentication (2FA) with your VCS ( GitHub 2FA, Bitbucket). If a user’s GitHub or Bitbucket account is compromised, a nefarious actor could push code or potentially steal secrets.

  • If your project is open source and public, make note of whether you want to share your environment variables. On CircleCI, you can change a project’s settings to control whether your environment variables can pass on to forked versions of your repository. This is not enabled by default. You can read more about these settings and open source security in our Open Source Projects Document.

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