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December 16, 2020

GHSL-2020-320: Unauthorized repository modification or secrets exfiltration in illright/attractions repository

Jaroslav Lobacevski

Summary

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

Product

illright/attractions GitHub repository

Tested Version

The latest changeset a22cae7 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
...
    - uses: actions/checkout@v2
      name: Checkout PR
      with:
        persist-credentials: false
        ref: ${{ github.event.pull_request.head.sha }}

    - uses: actions/setup-node@v1
    - name: Install and Build
      run: |
        yarn install
        yarn build
...

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-2020-320 in any communication regarding this issue.