Coordinated Disclosure Timeline
- 2020-11-30: Issue reported to maintainers.
- 2020-02-03: Partial fix is made.
- 2020-02-23: Additional details reported.
- 2021-02-23: Issue fixed.
Summary
The preview.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.
Product
Tested Version
The latest changeset 12a7001 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]
...
- uses: actions/checkout@v2
with:
ref: refs/pull/${{ github.event.pull_request.number }}/merge
- uses: afc163/surge-preview@v1
with:
surge_token: ${{ secrets.SURGE_TOKEN }}
github_token: ${{ secrets.GITHUB_TOKEN }}
build: |
yarn
yarn build
yarn doc:build
dist: dist
...
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-2020-268 in any communication regarding this issue.