GHSL-2020-316: Arbitrary code execution in indico/newdle workflow

Coordinated Disclosure Timeline

• 2020-11-30: Report sent to maintainers.
• 2020-12-01: Maintainers acknowledged.
• 2020-12-01: Temporary fix is applied.
• 2021-01-18: Issue resolved.

Summary

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

Product

indico/newdle GitHub repository

Tested Version

The latest changeset 697bb0c 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]
...
      - uses: actions/checkout@v2
        with:
          ref: ${{ github.event.pull_request.head.ref }}
          repository: ${{ github.event.pull_request.head.repo.full_name }}
...
      - name: Setup venv and config file
        if: steps.migrations-changed.outputs.changed == 'true'
        run: |
          make config
          make env
...

Impact

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

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-2020-316 in any communication regarding this issue.