ci(fix): Do not trust user controlled input (#2337)

The prepare workflow runs in an untrusted context already and thus should not have anything worthwhile to exploit.

However care should still be taken to avoid interpolating expressions into shell scripts directly that is data a user can control the value of. Especially to avoid any maintainer referencing an existing workflow from copying a risky snippet unaware of different security contexts for workflows.

In this case, as per Github Documentation and referenced issue comment, the PR title is user controllable data, which if directly interpolated into the shell script being run (as it previously was), allows for injecting commands to execute.

.github/workflows/docs-preview-prepare.yml

@@ -26,7 +26,7 @@ jobs:
       NETLIFY_SITE_PREFIX: pullrequest-${{ github.event.pull_request.number }}
       NETLIFY_SITE_NAME: dms-doc-previews
     steps:
-      - uses: actions/checkout@v2.3.4
+      - uses: actions/checkout@v2.4.0
 
       - name: 'Build with mkdocs-material via Docker'
         working-directory: docs
@@ -53,12 +53,17 @@ jobs:
       # Minimize risk of upload failure by bundling files to a single compressed archive (tar + zstd).
       # Bundles build dir and env file into a compressed archive, nested file paths will be preserved.
       - name: 'Prepare artifact for transfer'
+        env:
+          # As a precaution, reference this value by an interpolated ENV var;
+          # instead of interpolating user controllable input directly in the shell script..
+          # https://github.com/docker-mailserver/docker-mailserver/issues/2332#issuecomment-998326798
+          PR_TITLE: ${{ github.event.pull_request.title }}
         run: |
           # Save ENV for transfer
           {
             echo "PR_HEADSHA=${{ github.event.pull_request.head.sha }}"
             echo "PR_NUMBER=${{ github.event.pull_request.number }}"
-            echo "PR_TITLE=${{ github.event.pull_request.title }}"
+            echo "PR_TITLE=${PR_TITLE}"
             echo "NETLIFY_SITE_PREFIX=${{ env.NETLIFY_SITE_PREFIX }}"
             echo "BUILD_DIR=${{ env.BUILD_DIR }}"
           } >> pr.env

.github/workflows/docs-production-deploy.yml

@@ -26,7 +26,7 @@ jobs:
     name: 'Deploy Docs'
     runs-on: ubuntu-20.04
     steps:
-      - uses: actions/checkout@v2.3.4
+      - uses: actions/checkout@v2.4.0
 
       - name: 'Check if deploy is for a `v<major>.<minor>` tag version instead of `edge`'
         if: startsWith(github.ref, 'refs/tags/')
@@ -75,10 +75,10 @@ jobs:
     needs: deploy
     steps:
       - name: 'Checkout the tagged commit (shallow clone)'
-        uses: actions/checkout@v2.3.4
+        uses: actions/checkout@v2.4.0
 
       - name: 'Checkout the docs deployment branch to a subdirectory'
-        uses: actions/checkout@v2.3.4
+        uses: actions/checkout@v2.4.0
         with:
           ref: gh-pages
           path: gh-pages