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December 16, 2020

GHSL-2020-315: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of rux616/karabiner-windows-mode

Jaroslav Lobacevski

Summary

The acceptance-tests.yaml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

rux616/karabiner-windows-mode GitHub repository

Tested Version

The latest changeset 0125d82 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'
...
    - name: Checkout Repo
      # https://github.com/marketplace/actions/checkout
      uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
    - name: Install Jsonnet
      run: sudo apt-get install -y jsonnet
    - name: Test Syntactic Validity
      run: .github/render_json.sh

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-2020-315 in any communication regarding this issue.