GitHub Security Lab (GHSL) Vulnerability Report: GHSL-2020-313

[JarLob]

We have been unsuccessfully trying to contact the repository owners since 2020-11-30.
The issue affects master, Janglee123-patch-2, Janglee123-patch-1 and revert-83-master branches.

Summary

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

Tested Version

The latest changeset a43fb3c 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:
    paths:
    - 'public/directory/**.json'
...
      uses: actions/checkout@v2
      with:
        ref: ${{github.event.pull_request.head.ref}}
        repository: ${{github.event.pull_request.head.repo.full_name}}
    
    - name: Yarn install
      uses: actions/setup-node@v1
    - run: yarn install

Impact

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

Remediation

Use pull_request trigger that doesn't have read/write repository token and no access to secrets. If at some point a write access is needed split the workflow in two and use workflow_run.

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-2020-313 in any communication regarding this issue.

Disclosure Policy

This report is subject to our coordinated disclosure policy.

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[Janglee123]

Disabled the workflow for now.