CVE-2023-51664

### 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/)