GHSL-2020-271: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of openzfs/zfs

Summary

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

Product

openzfs/zfs GitHub repository

Tested Version

The latest changeset d6f90c7 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:
  push:
  pull_request_target:
...
    steps:
    - uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
...
    - name: Prepare
      run: |
        sh ./autogen.sh
        ./configure
    - name: Checkstyle
      run: |
        make checkstyle
    - name: Lint
      run: |
        make lint
...

Impact

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

Coordinated Disclosure Timeline

• 2020-11-30: Report sent to maintainers
• 2020-12-01: Maintainers acknowledged
• 2020-12-01: Issue resolved

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-2020-271 in any communication regarding this issue.