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December 16, 2020

GHSL-2020-248: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of rism-ch/verovio

Jaroslav Lobacevski

Summary

The ‘tests_build.yml’ GitHub workflow is vulnerable to arbitrary code execution.

Product

rism-ch/verovio GitHub repository

Tested Version

The latest changeset 4923b8f 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: develop
...
      - name: Checkout the PR
        uses: actions/checkout@v2
        with:
          ref: ${{ github.event.pull_request.head.sha }}
          path: ${{ env.PR_DIR }}/

      - name: Build Python toolkit and run the tests for the PR
        working-directory: ${{ github.workspace }}/${{ env.PR_DIR }}/bindings
        run: |
          cmake ../cmake -DBUILD_AS_PYTHON=ON -B python
...

Impact

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

Coordinated Disclosure Timeline

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-2020-248 in any communication regarding this issue.