GHSA-mcph-m25j-8j63

### Summary
The `tj-actions/changed-files` workflow allows for command injection in changed filenames, allowing an attacker to execute arbitrary code and potentially leak secrets.

### Details
The [`changed-files`](https://github.com/tj-actions/changed-files) action returns a list of files changed in a commit or pull request which provides an `escape_json` input [enabled by default](https://github.com/tj-actions/changed-files/blob/94549999469dbfa032becf298d95c87a14c34394/action.yml#L136), only escapes `"` for JSON values.

This could potentially allow filenames that contain special characters such as `;` and \` (backtick) which can be used by an attacker to take over the [GitHub Runner](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners) if the output value is used in a raw fashion (thus being directly replaced before execution) inside a `run` block. By running custom commands an attacker may be able to steal **secrets** such as `GITHUB_TOKEN` if triggered on other events than `pull_request`. For example on `push`.

#### Proof of Concept

1. Submit a pull request to a repository with a new file injecting a command. For example `$(whoami).txt` which is a valid filename.
2. Upon approval of the workflow (triggered by the pull request), the action will get executed and the malicious pull request filename will flow into the `List all changed files` step below.

```yaml
- name: List all changed files
run: |
for file in ${{ steps.changed-files.outputs.all_changed_files }}; do
echo "$file was changed"
done
```

Example output:

```yaml
##[group]Run for file in $(whoami).txt; do
for file in $(whoami).txt; do
echo "$file was changed"
done
shell: /usr/bin/bash -e {0}
##[endgroup]
runner.txt was changed
```

### Impact

This issue may lead to arbitrary command execution in the GitHub Runner.

### Resolution
- A new `safe_output` input would be enabled by default and return filename paths escaping special characters like ;, ` (backtick), $, (), etc for bash environments.

- A safe recommendation of using environment variables to store unsafe outputs.

```yaml
- name: List all changed files
env:
ALL_CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
run: |
for file in "$ALL_CHANGED_FILES"; do
echo "$file was changed"
done
```

### Resources

* [Keeping your GitHub Actions and workflows secure Part 2: Untrusted input](https://securitylab.github.com/research/github-actions-untrusted-input/)
* [Keeping your GitHub Actions and workflows secure Part 1: Preventing pwn requests](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/)