Summary
The scala3doc.yaml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.
Product
lampepfl/dotty GitHub repository
Tested Version
The latest changeset 3f4b101 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:
jobs:
build:
runs-on: ubuntu-latest
...
- name: Checkout to PR code
env:
PR_NUMBER: ${{ github.event.pull_request.number }}
run : |
if [[ -z "${PR_NUMBER}" ]]; then
echo Not a pull request do not need to checkout
else
REF=refs/pull/$PR_NUMBER/merge
echo checking $REF
git fetch origin $REF:$REF
git checkout $REF
fi
...
- name: Compile and test
run: ./project/scripts/sbt scala3doc/test
...
Impact
The vulnerability allows for unauthorized modification of the base repository and secrets exfiltration.
Coordinated Disclosure Timeline
- 2020-11-30: Report sent to maintainers
- 2020-12-01: 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-2020-272 in any communication regarding this issue.