December 16, 2020

GHSL-2020-320: Unauthorized repository modification or secrets exfiltration in illright/attractions repository

Jaroslav Lobačevski

Summary

The pr_demo.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

illright/attractions GitHub repository

Tested Version

The latest changeset a22cae7 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
...
    - uses: actions/checkout@v2
      name: Checkout PR
      with:
        persist-credentials: false
        ref: ${{ github.event.pull_request.head.sha }}

    - uses: actions/setup-node@v1
    - name: Install and Build
      run: |
        yarn install
        yarn build
...

Impact

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

Coordinated Disclosure Timeline

  • 2020-11-30: Report sent to maintainers
  • 2020-12-01: Maintainers acknowledged
  • 2020-12-02: Issue fixed

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-2020-320 in any communication regarding this issue.