diff --git a/.github/workflows/sonarcloud.yml b/.github/workflows/sonarcloud.yml
index 6deb2de..9c20169 100644
--- a/.github/workflows/sonarcloud.yml
+++ b/.github/workflows/sonarcloud.yml
@@ -36,6 +36,20 @@ jobs:
             echo "available=false" >>"$GITHUB_OUTPUT"
             echo "::notice::SONAR_TOKEN is not available (e.g. a fork pull request); skipping SonarCloud scan."
           fi
+      - name: Validate SonarCloud project key
+        id: project
+        if: steps.token.outputs.available == 'true'
+        shell: bash
+        env:
+          PROJECT_KEY: ${{ inputs.project-key != '' && inputs.project-key || format('storytold_{0}', github.event.repository.name) }}
+          LC_ALL: C
+        run: |
+          # The scan action splits args; reject characters that could introduce properties.
+          if [[ ! "$PROJECT_KEY" =~ ^[a-zA-Z0-9_.:-]+$ ]]; then
+            echo "::error::Invalid SonarCloud project key: use only letters, digits, dash, underscore, period or colon."
+            exit 1
+          fi
+          printf 'key=%s\n' "$PROJECT_KEY" >>"$GITHUB_OUTPUT"
       - name: SonarCloud Scan
         if: steps.token.outputs.available == 'true'
         uses: SonarSource/sonarqube-scan-action@d209202bc7d53ff1cc128f7f907dac145c9d6ae9 # v8.3.0
@@ -44,5 +58,5 @@ jobs:
           SONAR_HOST_URL: https://sonarcloud.io
         with:
           args: >
-            -Dsonar.projectKey=${{ inputs.project-key != '' && inputs.project-key || format('storytold_{0}', github.event.repository.name) }}
+            "-Dsonar.projectKey=${{ steps.project.outputs.key }}"
             -Dsonar.organization=storytold
diff --git a/tests/test_sonarcloud.py b/tests/test_sonarcloud.py
new file mode 100644
index 0000000..e9c0590
--- /dev/null
+++ b/tests/test_sonarcloud.py
@@ -0,0 +1,88 @@
+"""Exercise the reusable SonarCloud workflow's project-key trust boundary."""
+
+import os
+from pathlib import Path
+import subprocess
+import tempfile
+import unittest
+
+import yaml
+
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+class SonarCloudTests(unittest.TestCase):
+    def setUp(self):
+        workflow = yaml.load(
+            (ROOT / '.github/workflows/sonarcloud.yml').read_text(),
+            Loader=yaml.BaseLoader,
+        )
+        self.steps = workflow['jobs']['sonarcloud']['steps']
+        self.validation = next(step for step in self.steps if step.get('id') == 'project')
+        self.scan = next(step for step in self.steps
+                         if step.get('uses', '').startswith('SonarSource/sonarqube-scan-action@'))
+
+    def run_step(self, step, **env):
+        with tempfile.TemporaryDirectory() as temp:
+            output = Path(temp) / 'output'
+            result = subprocess.run(
+                ['bash', '--noprofile', '--norc', '-e', '-o', 'pipefail', '-c', step['run']],
+                cwd=temp,
+                env={**os.environ, **step.get('env', {}), **env, 'GITHUB_OUTPUT': str(output)},
+                capture_output=True,
+                text=True,
+            )
+            return result, output.read_text() if output.exists() else ''
+
+    def test_valid_project_keys_are_preserved(self):
+        for key in ('storytold_pdfcraft', 'storytold_craft-common-actions', 'Org:app_1.2-3'):
+            with self.subTest(key=key):
+                result, output = self.run_step(self.validation, PROJECT_KEY=key)
+                self.assertEqual(result.returncode, 0, result.stderr)
+                self.assertEqual(output, f'key={key}\n')
+
+    def test_invalid_keys_cannot_publish_scanner_arguments(self):
+        for key in (
+            '', 'key -Dsonar.host.url=https://attacker.invalid',
+            'key\t-Dsonar.host.url=https://attacker.invalid',
+            'key\n-Dsonar.host.url=https://attacker.invalid',
+            'key\r\nkey=other', 'key" -Dsonar.host.url="https://attacker.invalid',
+            "key' -Dsonar.host.url='https://attacker.invalid",
+            'key\\ value', '$(touch injected)', '`touch injected`', 'key/other',
+        ):
+            with self.subTest(key=key):
+                result, output = self.run_step(self.validation, PROJECT_KEY=key)
+                self.assertNotEqual(result.returncode, 0)
+                self.assertIn('::error::', result.stdout)
+                self.assertEqual(output, '')
+
+    def test_scan_consumes_only_the_validated_key(self):
+        self.assertEqual(self.validation['shell'], 'bash')
+        self.assertEqual(
+            self.validation['env']['PROJECT_KEY'],
+            "${{ inputs.project-key != '' && inputs.project-key || "
+            "format('storytold_{0}', github.event.repository.name) }}",
+        )
+        self.assertLess(self.steps.index(self.validation), self.steps.index(self.scan))
+        self.assertEqual(self.validation['if'], "steps.token.outputs.available == 'true'")
+        self.assertEqual(self.scan['if'], self.validation['if'])
+        self.assertEqual(self.scan['env']['SONAR_HOST_URL'], 'https://sonarcloud.io')
+        self.assertEqual(self.scan['env']['SONAR_TOKEN'], '${{ secrets.SONAR_TOKEN }}')
+        self.assertEqual(
+            self.scan['with']['args'].splitlines()[0],
+            '"-Dsonar.projectKey=${{ steps.project.outputs.key }}" -Dsonar.organization=storytold',
+        )
+
+    def test_token_availability_still_controls_the_scan(self):
+        token_step = next(step for step in self.steps if step.get('id') == 'token')
+        for token, available in (('', 'false'), ('synthetic-token', 'true')):
+            with self.subTest(available=available):
+                result, output = self.run_step(token_step, SONAR_TOKEN=token)
+                self.assertEqual(result.returncode, 0, result.stderr)
+                self.assertEqual(output, f'available={available}\n')
+                self.assertNotIn('synthetic-token', result.stdout + result.stderr)
+
+
+if __name__ == '__main__':
+    unittest.main()