December 16, 2020

GHSL-2020-249: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of aws/amazon-chime-sdk-js

Jaroslav Lobačevski

Summary

The 'continuous-integration.yml' GitHub workflow is vulnerable to arbitrary code execution.

Product

aws/amazon-chime-sdk-js GitHub repository

Tested Version

The latest changeset bfc4c60 to the date.

Details

Issue: Untrusted code is explicitly checked out and run on a Pull Request from a fork

pull_request_target was introduced to allow triggered workflows to comment on PRs, label them, assign people, etc.. In order to make it possible the triggered action runner has read/write token for the base repository and the access to secrets. In order to prevent untrusted code from execution it runs in a context of the base repository.

By explicitly checking out and running build script from a fork the untrusted code is running in an environment that is able to push to the base repository and to access secrets.

on:
  pull_request_target:
    branches:
      - master
      - release-1.x
...
    steps:
    - name: Checkout Package
      uses: actions/checkout@v2
      with:
        fetch-depth: 0
        ref: ${{github.event.pull_request.head.ref}}
        repository: ${{github.event.pull_request.head.repo.full_name}}
    - name: Using Node.js 12.x
      uses: actions/setup-node@v1
      with:
        node-version: 12.x
    - name: Clean Install
      run: npm ci
    - name: Build
      run: npm run build:release
...

Impact

The vulnerability allows for unauthorized modification of the base repository and secrets exfiltration.

Coordinated Disclosure Timeline

  • 2020-11-26: Report sent to maintainer
  • 2020-11-26: Maintainer acknowledges
  • 2020-11-26: Issue resolved
  • 2020-11-26: The internal investigation concluded that the vulnerability has not been exploited

Credit

This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2020-249 in any communication regarding this issue.