This is why I avoid using non-official actions where possible and always set a version for the action.

We had a contractor that used some random action to ssh files to the server and referenced master as the version to boot. First, ssh isn't that difficult to upload files and run commands but the action owner could easily add code to save private keys and information to another server.

I am a bit confused on the "bypass" though. Wouldn't the adversary need push access to the repository to edit the workflow file? So, the portion that needs hardening is ensuring the wrong people do not have access to push files to the repository?

On public repositories I could see this being an issue if they do it in a section of the workflow that is run when a PR is created. Private repositories, you should take care with who you give access.