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December 16, 2020

GHSL-2020-281: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of tskit-dev/msprime

Jaroslav Lobacevski

Summary

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

Product

tskit-dev/msprime GitHub repository

Tested Version

The latest changeset e2a01e8 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:
...
env:
...
  MAKE_TARGET: cmodule
...
      - uses: actions/checkout@v2
        # As we are using pull-request-target which uses the workflow from the base
        # of the PR, we need to be specific
        with:
            ref: ${{ github.event.pull_request.head.ref }}
            repository: ${{ github.event.pull_request.head.repo.full_name }}
            submodules: true
...
      - name: Build virtualenv
        if: steps.cache.outputs.cache-hit != 'true'
        run: python -m venv venv

      - name: Install deps
        run: venv/bin/activate && pip install -r requirements/CI-docs/requirements.txt

      - name: Install GSL
        if: env.BUILD_GSL == 'true'
        run: sudo apt-get install -y libgsl0-dev

      - name: Build C module
        if: env.MAKE_TARGET
        run: venv/bin/activate && make $MAKE_TARGET

      - name: Build Docs
        run: venv/bin/activate && make -C docs
...

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-2020-281 in any communication regarding this issue.