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March 25, 2021

GHSL-2020-331: Unauthorized repository modification or secrets exfiltration in a GitHub workflow of appsmith

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

The client.yml GitHub workflow is vulnerable to unauthorized modification of the base repository and secret exfiltration.

Product

appsmithorg/appsmith GitHub repository

Tested Version

The latest changeset 75b3f18 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 access to secrets. In order to prevent untrusted code from execution, it runs in the context of the base repository.

By explicitly checking out and running the build script from a fork, the untrusted code will be able to push to the base repository and access its secrets.

on:
...
  pull_request_target:
    branches: [release, master]
    paths:
      - 'app/client/**'
      - '!app/client/cypress/manual_TestSuite/**'
...
      # Checkout the code
      - name: Checkout the merged commit from PR and base branch
        if: ${{ github.event_name == 'pull_request_target' }}
        uses: actions/checkout@v2
        with:
          ref: refs/pull/${{ github.event.pull_request.number }}/merge
...
      # Install all the dependencies
      - name: Install dependencies
        run: yarn install
...
      - name: Run the jest tests
        run: REACT_APP_ENVIRONMENT=${{steps.vars.outputs.REACT_APP_ENVIRONMENT}} yarn run test:unit
...

Impact

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

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-2020-331 in any communication regarding this issue.