Summary

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

Product

stm32-rs/stm32-rs GitHub repository

Tested Version

The latest changeset d04725f 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: ... - name: Checkout master uses: actions/checkout@v2 with: path: master - name: Checkout pull request uses: actions/checkout@v2 with: ref: refs/pull/${{ github.event.number }}/head path: stm32-rs ... - name: Build and publish run: | cd stm32-rs COMMIT=$(git rev-parse --short HEAD) BRANCH=pr-${{ github.event.number }}-$COMMIT echo "BRANCH=$BRANCH" >> $GITHUB_ENV cp ../master/Makefile . make -j2 mmaps ...

The build script overwrites the makefile with a file from the base repository. However it still calls other scripts from the PR, for example the extract.sh:

... # Each yaml file also corresponds to a mmap in mmaps/ MMAPS := $(patsubst devices/%.yaml, mmaps/%.mmap, $(YAMLS)) ... # Turn a devices/device.yaml and svd/device.svd into svd/device.svd.patched svd/%.svd.patched: devices/%.yaml svd/%.svd .deps/%.d svd patch $< ... # Generate mmap from patched SVD mmaps/%.mmap: svd/%.svd.patched @mkdir -p mmaps svd mmap $< > $@ ... svd/%.svd: svd/.extracted ; svd/.extracted: cd svd && ./extract.sh && touch .extracted ... mmaps: $(MMAPS)

Impact

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

Coordinated Disclosure Timeline

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-2020-278 in any communication regarding this issue.