December 16, 2020

GHSL-2020-284: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of dazuma/toys

Jaroslav Lobačevski

Summary

The release-hook-on-open.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

dazuma/toys GitHub repository

Tested Version

The latest changeset 5a73898 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:
    types: [opened, edited, synchronize, reopened]
...
    if: ${{ github.repository == 'dazuma/toys' }}
...
      - name: Checkout repo
        uses: actions/checkout@v2
        with:
          ref: refs/pull/${{ github.event.pull_request.number }}/merge
      - name: Install Toys
        run: "gem install --no-document toys"
      - name: Check commit messages
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        run: |
          toys release _onopen --verbose \
            "--event-path=${{ github.event_path }}" \
            < /dev/null

The workflow above runs the _onopen.rb ruby script from a Pull Request. There is a repository name check, but even in a case of a Pull Request from an external fork the name is still dazuma/toys.

Impact

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

Coordinated Disclosure Timeline

  • 2020-11-30: Report sent to maintainers
  • 2020-11-30: Got a confirmation that the issue was just resolved before the report was received

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-2020-284 in any communication regarding this issue.