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February 3, 2021

GHSL-2020-333: Arbitrary code execution in osohq/oso workflow

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

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

Product

osohq/oso GitHub repository

Tested Version

The latest changeset 316cabc 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:
    branches:
...
      - uses: actions/checkout@v2
        with:
          # This prevents the Action from persisting the credentials it uses to
          # perform the fetch/checkout to the Runner's local Git config. On
          # `pull_request_target` events, the GITHUB_TOKEN provided to the
          # Runner has Write permissions to the base repository. We do **not**
          # want to allow untrusted code from forks to execute arbitrary Git
          # commands with those elevated permissions.
          #
          # More info:
          # https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/#improvements-for-public-repository-forks
          persist-credentials: false
          # Explicitly setting the `repository` and `ref` inputs ensures that
          # `pull_request_target` events trigger CI runs against the code from
          # the HEAD branch. By default, this Action checks out code from the
          # BASE branch. On `push` events, the `github.event.pull_request` path
          # will yield a null value, and passing nulls to these inputs causes
          # them to fall back to the defaults of `osohq/oso` and
          # `refs/heads/main`, respectively.
          repository: ${{ github.event.pull_request.head.repo.full_name }}
          ref: ${{ github.event.pull_request.head.sha }}
...
      - name: Run benchmark
        run: cargo bench -- --output-format bencher | tee output.txt
...

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-2020-333 in any communication regarding this issue.