ci: run CodeQL Actions security scan on stacked and backport PRs

.github/workflows/action-security-scan.yaml

@@ -0,0 +1,94 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Runs the same CodeQL `actions` security query suite as Alphabet's Enterprise
+# `Google GitHub Admin: Actions Workflow Security Scan` workflow on stacked PRs
+# and backport PRs (`base != default_branch`), which the Enterprise ruleset
+# does not trigger on unless the PR targets `main` (or belongs to a native
+# GitHub Stack targeting `main`).
+
+name: Actions Workflow Security Scan
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  scan-pr:
+    name: Scan workflow files (stacked & backport PRs)
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    if: github.base_ref != github.event.repository.default_branch
+    steps:
+    - name: Checkout .github directory
+      uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      with:
+        fetch-depth: 1
+        sparse-checkout: .github
+        persist-credentials: false
+
+    - name: Create CodeQL query suite
+      run: |
+        cat <<'EOF' > actions-suite.qls
+        - qlpack: codeql/actions-queries
+        - include:
+            id: actions/envvar-injection/critical
+        - include:
+            id: actions/envpath-injection/critical
+        - include:
+            id: actions/cache-poisoning/poisonable-step
+        - include:
+            id: actions/artifact-poisoning/critical
+        - include:
+            id: actions/untrusted-checkout/critical
+        - include:
+            id: actions/untrusted-checkout/high
+        EOF
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@9b3254085494b55a0f7c4c2b3d2d52cc712ce370 # v4.39.1
+      with:
+        languages: actions
+        config: |
+          name: "Custom Action Scan"
+          disable-default-queries: true
+          queries:
+            - uses: ./actions-suite.qls
+
+    - name: Perform CodeQL analysis
+      uses: github/codeql-action/analyze@9b3254085494b55a0f7c4c2b3d2d52cc712ce370 # v4.39.1
+      with:
+        output: ../results
+        upload: never
+
+    - name: Check for vulnerabilities
+      run: |
+        SARIF_FILE="${RUNNER_WORKSPACE}/results/actions.sarif"
+        if [[ ! -f "$SARIF_FILE" ]]; then
+          echo "::error::SARIF file not found at ${SARIF_FILE}"
+          exit 1
+        fi
+        RESULT_COUNT=$(jq '.runs[0].results | length' "$SARIF_FILE")
+        if [[ "$RESULT_COUNT" -gt 0 ]]; then
+          echo "::error::CodeQL found ${RESULT_COUNT} potential workflow security vulnerabilities."
+          echo "---"
+          jq -r '.runs[0].results[] | ("Rule ID: " + .ruleId + "\nMessage: " + .message.text + "\nFile: " + .locations[0].physicalLocation.artifactLocation.uri + "\nLine: " + (.locations[0].physicalLocation.region.startLine | tostring) + "\n---")' "$SARIF_FILE"
+          exit 1
+        fi
+        echo "No vulnerabilities found. Check passed."