Writing GitHub Actions securely is notoriously difficult. While the default behavior is relatively secure, certain features may have nonobvious security implications. As we're increasingly exposing secrets to our GitHub Actions builds, it's worth carefully auditing that those secrets cannot be abused by third parties.

Default GITHUB_TOKEN permissions

Each build will have access to a GITHUB_TOKEN to perform GitHub API calls. The permissions associated with this token depend on the trigger that started the build. You can find an overview here.

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 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.

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.

(TODO follow up): There are two critical security vulnerability reports opened by Unknown User (potiuk) 30 December 2020 with GitHub Actions - both of them triaged and awaiting for actions on the GitHub side. GitHub Security Lab who in December encouraged users to post their experiences is engaged as well.  Those issues can be all mitigated (Apache Airflow implemented all mitigations) but they are not what most projects do. 

Mitigations

If you decide to use GitHub Actions,  those are recommendations (there are varying opinions on sub-modules use, though):

• ALWAYS limit your GitHub write token to as little scope as possible. You can specify scopes for the permissions of the token you automatically get during your build. https://github.blog/changelog/2021-04-20-github-actions-control-permissions-for-github_token/ . This could help preventing sophisticated supply-chain attacks like the codecov attack.
• (TODO: verify this advice is still current. using external actions but pinning them and having them reviewed by Infra might supersede this advice) NEVER use 3rd-party actions directly in your workflows - use the "submodule" pattern. Example PR Tobiasz Kędzierski  opened in SuperSet showing how this could be done. Also ASF INFRA allow-listed some of the popular Actions out there, including my "cancel workflow" action, but I there is no public list of those available. The nice things about submodules is that they do not bring action code to your repo. They link to commit hashes of the Actions, and that integrates well with the GitHub review process so that committers have better chance to review the changes before they are merged. By using submodules, you are automatically following the GitHub recommendations for hardening of security for 3rd-party actions.
• (TODO: verify this advice is still current. checkout actions should have read-only access unless using 'pull_request_target', and for 'pull_request_target' you (also?) must lock down the GITHUB_TOKEN) ALWAYS add "persist-credentials: false" to all your checkout actions. This is not done by default and is a huge security risk because it leaves your repository (and hundreds of thousands of others) open to 3rd-party dependencies to modify your repository (!) if you have any kind of "master" builds enabled. This is a "hidden" feature of the checkout action that is not at all obvious, but it leaves write access to your repository widely open to any code that you install during the build process. This is a very dangerous default.
• NEVER directly run code that might come with "forked" PRs in your workflows. There are certain exotic (but useful) workflows that are dangerous. For example, with "workflow_run" you might need to cancel duplicate workflows. Those workflows by default run with "master" code, but sometimes you might need to check out the incoming PR code for those. The host environment can have access (in various ways) to the "WRITE" GITHUB_TOKEN that has permission to modify your repository WITHOUT RESTRICTION OR NOTIFICATION. NEVER run the code that is checked out from the PR in your host environment. If you need to, run it in Docker Container to provide isolation from the host environment to avoid the "write" access leaking to users who prepare such a PR from their fork.
• NEVER install and run 3rd-party dependencies in the host of your build workflow code. Again there are ways those dependencies can obtain the "WRITE" GITHUB_TOKEN and change anything in your repository without your knowledge.  There are very common "schedule" and  "push" workflows that are especially prone to such abuse. Those run with "WRITE" access, and again there are ways to obtain the GitHub Token by these Actions and code that runs in your workflow. If you execute any 3rd-party code, run it in Docker containers to keep isolation from your "build" host environment to avoid leaking "write" access to those 3rd parties.