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December 16, 2020

GHSL-2020-276: Unauthorized repository modification or secrets exfiltration in nuxt repositories

Jaroslav Lobacevski

Summary

snapshot.yml and sync.yml GitHub workflows are vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

nuxt/create-nuxt-app GitHub repository
nuxt/modules GitHub repository

Tested Version

The latest changesets to the date 1aa53fd and 2ae2a0c.

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, pull_request_target]
...
    if: ${{ contains(github.actor, 'renovate') }}
...
      - name: Checkout
        uses: actions/checkout@v2
        with:
          ref: ${{ github.head_ref }}
...
      - name: Install dependencies
        if: steps.cache.outputs.cache-hit != 'true'
        run: yarn --frozen-lockfile --non-interactive

      - name: Test with update-snapshots
        run: AVA_FORCE_CI="not-ci" yarn ava --verbose --update-snapshots
...

There is a check supposed to trigger the workflow only for renovate actor. However it is bypassable with a user name like renovate2, notrenovate, etc.

on: [pull_request, pull_request_target]
...
      - name: Checkout
        uses: actions/checkout@v2
        with:
          ref: ${{ github.head_ref }}
...
      - name: Install dependencies
        if: steps.cache.outputs.cache-hit != 'true'
        run: yarn --frozen-lockfile --non-interactive

      - name: Sync
        run: yarn sync

Impact

Currently, because of a mistake, the script doesn’t checkout the Pull Request, but a branch from the base repository named as the branch from the PR. If there is no branch with the same name the script fails. However it is likely the bug to be fixed to ref: refs/pull/${{ github.event.pull_request.number }}/merge and the vulnerabilty to be made exploitable.

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-2020-276 in any communication regarding this issue.