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March 3, 2021

GHSL-2020-287: Unauthorized repository modification or secrets exfiltration in a GitHub workflow of jdf2e/nutui

Jaroslav Lobačevski

Coordinated Disclosure Timeline


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


jdf2e/nutui GitHub repository

Tested Version

The latest changeset 9d59193 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: refs/pull/${{ github.event.pull_request.number }}/merge
      - uses: afc163/surge-preview@v1
          surge_token: ${{ secrets.SURGE_TOKEN }}
          github_token: ${{ secrets.GITHUB_TOKEN }}
          build: |
            cnpm install
            npm run build:site
            mv dist/default.html dist/index.html
          dist: site/public


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


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-2020-287 in any communication regarding this issue.