I can't believe the default is to persist credentials and expose them to other jobs :( this is a major security issue.

Just as a heads up for anyone stumbling upon this issue:

1. persist-credentials: false is only relevant when you use ssh authentication, because
2. GITHUB_TOKEN is always exposed to all jobs, and by default has write access to your repo.

So if you want to harden security, apart from setting persist-credentials: false for ssh auth, make sure that GITHUB_TOKEN auth has no write permission to your repo.

See https://github.blog/changelog/2021-04-20-github-actions-control-permissions-for-github_token/ for reference.

+1

Agreed this seems a severe security issue, because it means any workflow using actions/checkout basically leaks the token to any process/action in that workflow which can just read it from .git/config.

@haampie IIUC it is a problem also with no ssh authentication (the default). The GitHub token is given only to this action and maybe a few other actions/* actions (default: ${{ github.token }} only work for those AFAIK), but is otherwise given to no other action unless done explicitly (like with: token: ${{ github.token }}/${{ secrets.GITHUB_TOKEN }}).
The token is not in the environment.

In other words, actions/checkout leaks the token to .git/config, making it very easy to read for anything running inside the workflow.
If the token was not written to .git/config, then I think stealing the GitHub token would require (one of):

• passing the token explicitly to some action in the workflow and that action gets compromised
• actions/checkout gets compromised
• compromise the workflow definition to e.g. print the token

So, depending on whether the token is explicitly passed to some action:

• If yes, then it seems the only safety net is to set token permissions
• If no, it would be safe by default with this change or with persist-credentials: false, regardless of the token permissions. A workaround is to set token permissions

I guess GitHub sees setting token permissions as the more general solution.
If so, fine, but then the default should be secure and so the default workflow permissions should be just contents: read.

https://securitylab.github.com/research/github-actions-preventing-pwn-requests/ also has a mention related to this, search for persist-credentials:

If the workflow uses actions/checkout and does not pass the optional parameter persist-credentials as false, it makes it even worse. The default for the parameter is true. It means that in any subsequent steps any running code can simply read the stored repository token from the disk.

+1