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You don’t need to make changes to workflows to run arbitrary code. Projects usually run tests, and you can just put whatever you want to execute in the code being tested, or exploit other holes in the existing workflow. TFA does not make any changes to existing workflows, for instance.

Also, you can disable workflow changes in GitHub Actions with the pull_request_target event, in which case it uses the workflow definition from the target branch.

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The pull_request_target event however allows for read/write access to the base repo and access to all of its secrets. It's not the event type you'd want to use whenever dealing with code that's incoming in a PR.

See the big warning at https://docs.github.com/en/actions/reference/events-that-tri....

That’s not true at all. Unless you expose the token or secrets in steps where you run untrusted code, the token and secrets are safe. This is explicitly designed for untrusted pull requests on public repos, solving the problem of, say, adding labels with the write token, or deploying a preview build to Netlify, while keeping the secrets safe from arbitrary code execution.

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