check-spelling workflow vulnerable to token leakage via symlink attack

Impact

For a repository with the check-spelling action enabled that triggers on pull_request_target (or schedule), an attacker can send a crafted Pull Request that causes a GITHUB_TOKEN to be exposed.

With the GITHUB_TOKEN, it’s possible to push commits to the repository bypassing standard approval processes. Commits to the repository could then steal any/all secrets available to the repository.

Workarounds

You can either:

• Disable the workflow until you’ve fixed all branches.

or

• Set repository to Allow specific actions. You can check:
  • [x] Allow actions created by GitHub
  • [x] Allow Marketplace actions by verified creators

check-spelling isn’t a verified creator and it certainly won’t be anytime soon. You could then explicitly add other actions that your repository uses.

or

• Set repository Workflow permissions to Read repository contents permission.

Solution

Workflows using check-spelling/check-spelling@main were fixed automatically with the release of v0.0.19.

Workflows using a pinned sha or tagged version will need to change the affected workflows for all repository branches to the latest version.

The simple case

In the simple case, you have few enough open branches that you can do the following on all branches.

• Edit the workflow to use check-spelling/check-spelling@main, or
• Edit the workflow to use check-spelling/[email protected], or
• Delete the workflow file, or
• Change the workflow to only use on: push
  • this will result in PRs losing status checks (commits will still have statuses)

The complex case

If you have too many open branches to feasibly fix all of them as per the above, you can instead do the following:

1. Perform the above solution on all open branches for which you need check-spelling to be active.
2. On all open branches on which you need check-spelling to be active, rename the workflow file (e.g. to spelling2.yml)
3. On the default branch, create a dummy workflow file with the old name (this is usually spelling.yml).
4. Use the GitHub Actions UI to disable the workflow with the old name (this is usually spelling.yml).

This should prevent the vulnerable workflow from executing on any branches that you have not applied the proper solution to.

The reason for creating the dummy file (Step 3) before disabling the workflow (Step 4) is that, in our testing, GitHub may un-disable a workflow if it does not exist on your default branch.

Example dummy workflow file (For step 3):

# spelling.yml is disabled per https://github.com/check-spelling/check-spelling/security/advisories/GHSA-g86g-chm8-7r2p
name: Workflow should not run!
on:
  push:
    branches: ''

jobs:
  placeholder:
    name: Should be disabled
    runs-on: ubuntu-latest
    if: false
    steps:
    - name: Task
      run: |
        echo 'Running this task would be bad'
        exit 1

You should also include a comment in the new workflow to remind people not to resurrect the old name, for example:

# spelling.yml is disabled per https://github.com/check-spelling/check-spelling/security/advisories/GHSA-g86g-chm8-7r2p

Finally, you should consider sending a Pull Request to an open branch in which you have not performed the proper solution to verify that the old version of check-spelling does not execute.

How to upgrade

Perform this change to your impacted workflow file (typically .github/workflows/spelling.yml):

@@ -24 +24 @@
-    - uses: check-spelling/[email protected]
+    - uses: check-spelling/[email protected]

As noted above, if you have many branches, you should additionally rename the workflow and include a comment to remind people not to use the old workflow file name:

# spelling.yml is blocked per https://github.com/check-spelling/check-spelling/security/advisories/GHSA-g86g-chm8-7r2p

Reviewing workflow runs

Users can verify who and which Pull Requests have been running the action by looking up the spelling.yml action in the Actions tab of their repositories, e.g., https://github.com/check-spelling/check-spelling/actions/workflows/spelling.yml - you can filter PRs by adding ?query=event%3Apull_request_target, e.g., https://github.com/check-spelling/check-spelling/actions/workflows/spelling.yml?query=event%3Apull_request_target.

References

• For more information on pull_request_target attacks, see GitHub Security Lab: Keeping your GitHub Actions and workflows secure: Preventing pwn requests
• For information on workflow hardening techniques, see GitHub: Security hardening for GitHub Actions

Credit

Thanks to @justinsteven for reporting as well as in helping validate the fix.

For more information

For questions or comments about this advisory:

• Email us at [email protected]