Writing GitHub Actions securely is notoriously difficult. As increasingly we're exposing secrets to our GitHub Actions builds, it's worth carefully auditing that those secrets cannot by abused by third parties.

• 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.
• 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.
• 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.