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• always make sure to configure minimal permissions and branch restrictions.
• if using the 'checkout' action, always enable persist-credentials: false 

Dangerous workflows

There are a number of really dangerous workflows in GitHub actions that you should only consider when it's absolutely necessary - and you should be extremely careful when deciding to use them: "pull_request_target" and "workflow_run" are particularly dangerous, also "issue_comment" should be used with care. There are multiple ways malicious users can use capabilities that those workflows give them, and new ways are being continuously discovered (abusing environment creation, using malicious PR titles and descriptions, poisoning the cache etc.). Best if you do not use those workflows at all. 

There are several reasons why you might want to use the workflows - but most of them revolve around allowing "write" permissions in "secure way" for PRs coming from "forks". PRs from forks - by definition - cannot have any write permissions, and the separate workflows are supposed to bypass this limitation. One of the common problems is to be able to create and use "huge" cache between the runs, usually cache created in previous PR runs of the same un, or cache created by "target branch" runs. There is the https://github.com/apache/infrastructure-actions/tree/main/stash. action created by Unknown User (assignuser)  that allows to use artifacts to store even huge cache (and store/restore the cache quickly and efficiently share the cache between branches). For example it allowed Apache Airflow to get rid of the "pull_request_target" workflow where they needed to share 2GB images between PR runs - providing a much simpler and easier to maintain workflow. See https://github.com/apache/airflow/pull/45266 for PR implementing this change.

If you think you cannot avoid dangerous workflows - it's best if you reach out to #builds slack channel on ASF slack - there is a group of peple there who discuss various ways you can design your Github Actions in the way to avoid dangerous workflows.

It's highly recommended to use https://woodruffw.github.io/zizmor/ static analysis tool in your CI / pipelines to detect and fix potential security issues in your workflows.

Builds triggered with pull_request_target 

Builds triggered by the pull_request_target event (as opposed to pull_request) by default check out the 'target' branch of the PR, and run with a 'permissive' GITHUB_TOKEN. It is common for such workflows to switch to the PR that triggered the workflow. This is dangerous, as any code that is loaded from the repo and run after the switch (including actions, scripts, build tools and test code) may be untrusted, and will have access to the GITHUB_TOKEN.

Builds

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triggered with

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workflow_run

A common technique for building untrusted code but also using privileges to act on the build result is to split the build into two parts: a low-privilege one triggered by pull_request that runs the untrusted code, stores the result in an artifact, and triggers a second, high-privilege build with workflow_run  that acts on that result.

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• Always extract into a directory separate from the trusted code in a step before checking out said  trusted code. This stops files extracted from the archive from clobbering trusted code.
• A separate directory can also prevent files in the artifact impersonating often used python modules like pip  as running python -m pip ...  would execute a pip.py file in the cwd
• Validate any content you retrieve from an artifact before you use it to avoid command injection, especially in steps using bash  and Github Actions macros.
  • This includes using cat  on such files as well as putting their content into environment variables (a popular exploit is to modify LD_PRELOAD, some examples ), step outputs to use via ${{ steps.id.outputs.sus_content }} to be used in e.g. if  in bash.

Builds triggered with issue_comment 

Similarly, builds triggered with issue_comment  run with a 'permissive' GITHUB_TOKEN. Again, no code may be loaded after switching to the PR branch. You use this technique to switch to the branch.

3rd-party actions

The Apache Infrastructure GitHub Actions Policy states actions outside of apache/*, github/* and actions/* must be pinned to the specific git hash (SHA1) of the action that has been reviewed for use by the project. For instance, you MUST pin foobar/baz-action@8843d7f92416211de9ebb963ff4ce28125932878.

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