[BUG] v47: recommend `mapfile` for preventing unwanted code execution in CI?

[lkraav]

Is there an existing issue for this?

• I have searched the existing issues

Does this issue exist in the latest version?

• I'm using the latest release

Describe the bug?

Potential README update? Issue might in the consumer shell step, not in tj-actions/changed-files itself, but I don't see any mapfile or similar use recommendation in README.

Claimed vulnerable usage pattern ⬇️

run: npx prettier --check ${{ steps.changed-files.outputs.all_changed_files }}

If a PR includes a file whose name itself looks like a CLI flag, GitHub expands that output directly into the shell command line.

Example:

• attacker adds a file named --config=pwn.cjs
• attacker also adds pwn.cjs

Generated shell command becomes:

npx prettier --check --config=pwn.cjs pwn.cjs

At that point Prettier does not see --config=pwn.cjs as a filename — it sees it as an option. Since Prettier can load JS config/plugin files, that can turn attacker-controlled repo content into code execution in CI.

Recommendation is to make boundaries explicit:

• read the changed files as JSON
• parse them into a bash array
• invoke Prettier as: `npx prettier --check -- "${files[@]}"

Proposed solution template:

...
jobs:
  prettier:
    runs-on: ubuntu-slim
    steps:
      - uses: actions/checkout@v6
      - uses: ./.github/actions/restore-node-modules-cache
      - id: changed-files
        uses: tj-actions/changed-files@v47
        with:
          json: true
          escape_json: false
      - if: steps.changed-files.outputs.any_changed == 'true'
        env:
          CHANGED_FILES_JSON: ${{ steps.changed-files.outputs.all_changed_files }}
        run: |
          mapfile -t files < <(printf '%s' "$CHANGED_FILES_JSON" | jq -r '.[]')
          npx prettier --check -- "${files[@]}"

Is this a real problem? Should https://github.com/tj-actions/changed-files/blob/v47/README.md be upgraded?

To Reproduce

Follow https://github.com/tj-actions/changed-files/blob/v47/README.md as-is.

What OS are you seeing the problem on?

all

Expected behavior?

Perhaps README.md should harden its recommendations?

Relevant log output

N/A

Has all relevant logs been included?

• I've included all relevant logs

Anything else?

No response

Code of Conduct

• I agree to follow this project's Code of Conduct