Summary
The docs.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.
Product
tskit-dev/msprime GitHub repository
Tested Version
The latest changeset e2a01e8 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:
...
env:
...
MAKE_TARGET: cmodule
...
- uses: actions/checkout@v2
# As we are using pull-request-target which uses the workflow from the base
# of the PR, we need to be specific
with:
ref: ${{ github.event.pull_request.head.ref }}
repository: ${{ github.event.pull_request.head.repo.full_name }}
submodules: true
...
- name: Build virtualenv
if: steps.cache.outputs.cache-hit != 'true'
run: python -m venv venv
- name: Install deps
run: venv/bin/activate && pip install -r requirements/CI-docs/requirements.txt
- name: Install GSL
if: env.BUILD_GSL == 'true'
run: sudo apt-get install -y libgsl0-dev
- name: Build C module
if: env.MAKE_TARGET
run: venv/bin/activate && make $MAKE_TARGET
- name: Build Docs
run: venv/bin/activate && make -C docs
...
Impact
The vulnerability allows for unauthorized modification of the base repository and secrets exfiltration.
Coordinated Disclosure Timeline
- 2020-11-30: Report sent to maintainers
- 2020-11-30: Issue resolved
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-2020-281 in any communication regarding this issue.