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December 16, 2020

GHSL-2020-279: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of is-a-dev/register

Jaroslav Lobacevski


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


is-a-dev/register GitHub repository

Tested Version

The latest changeset 7a651a1 to the date.


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
          ref: ${{ github.event.pull_request.head.sha }}
          fetch-depth: 0
      - name: Installing dependencies
        uses: borales/actions-yarn@v2.0.0
          cmd: install --ignore-engines
      - name: Running tests
        uses: borales/actions-yarn@v2.0.0
          cmd: test


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

Coordinated Disclosure Timeline


This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).


You can contact the GHSL team at, please include a reference to GHSL-2020-279 in any communication regarding this issue.