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Source code, the pipeline's YAML file, and necessary scripts & tools are all stored in a version control repository. Permissions and branch policies must be employed to ensure changes to the code and pipeline are safe. You can also add pipeline permissions and checks to repositories.
Also, you should review default access control for repositories.
Because of Git's design, protection at a branch level will only carry you so far. Users with push access to a repo can usually create new branches. If you use GitHub open-source projects, anyone with a GitHub account can fork your repository and propose contributions back. Since pipelines are associated with a repository and not with specific branches, you must assume the code and YAML files are untrusted.
If you build public repositories from GitHub, you must consider your stance on fork builds. Forks are especially dangerous since they come from outside your organization. To protect your products from contributed code, consider the following recommendations.
The following recommendations apply primarily to building public repos from GitHub.
Don't provide secrets to fork builds
By default, your pipelines are configured to build forks, but secrets and protected resources aren't made available to the jobs in those pipelines by default. Don't turn off this latter protection.
When you enable fork builds to access secrets, Azure Pipelines by default restricts the access token used for fork builds. It has more limited access to open resources than a normal access token. To give fork builds the same permissions as regular builds, enable the Make fork builds have the same permissions as regular builds setting.
Even if you enable fork builds to access secrets, Azure Pipelines restricts the access token used for fork builds. It has more limited access to open resources than a normal access token. You cannot disable this protection.
Consider manually triggering fork builds
You can turn off automatic fork builds and instead use pull request comments as a way to manually building these contributions. This setting will give you an opportunity to review the code before triggering a build.
Use Microsoft-hosted agents for fork builds
Don't run builds from forks on self-hosted agents. By doing so, you're effectively providing a path to external organizations to run outside code on machines inside your corporate network. Use Microsoft-hosted agents whenever possible. For your self-hosted agent, use some form of network isolation and ensure agents don't persist their state between jobs.
Review code changes
Before you run your pipeline on a forked pull-request, carefully review the proposed changes, and make sure you're comfortable running it.
The version of the YAML pipeline you'll run is the one from the pull request. Thus, pay special attention to changes to the YAML code and to the code that runs when the pipeline runs, such as command line scripts or unit tests.
GitHub token scope limitation
When you build a GitHub forked pull request, Azure Pipelines ensures the pipeline can't change any GitHub repository content. This restriction applies only if you use the Azure Pipelines GitHub app to integrate with GitHub. If you use other forms of GitHub integration, for example, the OAuth app, the restriction isn't enforced.
Users in your organization with the right permissions can create new branches containing new or updated code. That code can run through the same pipeline as your protected branches. Further, if the YAML file in the new branch is changed, then the updated YAML will be used to run the pipeline. While this design allows for great flexibility and self-service, not all changes are safe (whether made maliciously or not).
If your pipeline consumes source code or is defined in Azure Repos, you must fully understand the Azure Repos permissions model. In particular, a user with Create Branch permission at the repository level can introduce code to the repo even if that user lacks Contribute permission.
Next, learn about the more protection offered by checks on protected resources.