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December 16, 2020

GHSL-2020-278: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of stm32-rs/stm32-rs

Jaroslav Lobacevski

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.