diff --git a/README.md b/README.md
index bccb3d9..7560878 100644
--- a/README.md
+++ b/README.md
@@ -37,10 +37,14 @@ jobs:
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
 
       - name: Pre-fetch base and head refs for the PR
+        env:
+          PR_BASE_REF: ${{ github.event.pull_request.base.ref }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
         run: |
+          # Pass GitHub expressions through env and quote shell expansions.
           git fetch --no-tags origin \
-            ${{ github.event.pull_request.base.ref }} \
-            +refs/pull/${{ github.event.pull_request.number }}/head
+            "$PR_BASE_REF" \
+            "+refs/pull/$PR_NUMBER/head"
 
       # If you want Codex to build and run code, install any dependencies that
       # need to be downloaded before the "Run Codex" step because Codex's
diff --git a/action.yml b/action.yml
index d83acc2..c1cb84f 100644
--- a/action.yml
+++ b/action.yml
@@ -107,8 +107,10 @@ runs:
     - name: Validate Windows safety strategy
       if: ${{ runner.os == 'Windows' }}
       shell: bash
+      env:
+        SAFETY_STRATEGY: ${{ inputs['safety-strategy'] }}
       run: |
-        if [ "${{ inputs['safety-strategy'] }}" != "unsafe" ]; then
+        if [ "$SAFETY_STRATEGY" != "unsafe" ]; then
           echo "On Windows, inputs['safety-strategy'] must be 'unsafe'" >&2
           echo "because no viable sandboxing options are available at this time." >&2
           exit 1
@@ -123,46 +125,63 @@ runs:
 
     - name: Check repository write access
       env:
+        ACTION_PATH: ${{ github.action_path }}
         GITHUB_TOKEN: ${{ github.token }}
+        ALLOW_BOTS: ${{ inputs['allow-bots'] }}
+        ALLOW_USERS: ${{ inputs['allow-users'] }}
       shell: bash
       run: |
-        node "${{ github.action_path }}/dist/main.js" check-write-access \
-          --allow-bots "${{ inputs['allow-bots'] }}" \
-          --allow-users "${{ inputs['allow-users'] }}"
+        node "$ACTION_PATH/dist/main.js" check-write-access \
+          --allow-bots "$ALLOW_BOTS" \
+          --allow-users "$ALLOW_USERS"
 
     - name: Install Codex CLI
       shell: bash
-      run: npm install -g "@openai/codex@${{ inputs['codex-version'] }}"
+      env:
+        CODEX_VERSION: ${{ inputs['codex-version'] }}
+      run: npm install -g "@openai/codex@${CODEX_VERSION}"
 
     - name: Install Codex Responses API proxy
       shell: bash
-      run: npm install -g "@openai/codex-responses-api-proxy@${{ inputs['codex-version'] }}"
+      env:
+        CODEX_VERSION: ${{ inputs['codex-version'] }}
+      run: npm install -g "@openai/codex-responses-api-proxy@${CODEX_VERSION}"
 
     - name: Resolve Codex home
       id: resolve_home
       shell: bash
+      env:
+        ACTION_PATH: ${{ github.action_path }}
+        CODEX_HOME_OVERRIDE: ${{ inputs['codex-home'] }}
+        SAFETY_STRATEGY: ${{ inputs['safety-strategy'] }}
+        CODEX_USER: ${{ inputs['codex-user'] }}
+        CODEX_RUN_ID: ${{ github.run_id }}
       run: |
-        node "${{ github.action_path }}/dist/main.js" resolve-codex-home \
-          --codex-home-override "${{ inputs['codex-home'] }}" \
-          --safety-strategy "${{ inputs['safety-strategy'] }}" \
-          --codex-user "${{ inputs['codex-user'] }}" \
-          --github-run-id "${{ github.run_id }}"
+        node "$ACTION_PATH/dist/main.js" resolve-codex-home \
+          --codex-home-override "$CODEX_HOME_OVERRIDE" \
+          --safety-strategy "$SAFETY_STRATEGY" \
+          --codex-user "$CODEX_USER" \
+          --github-run-id "$CODEX_RUN_ID"
 
     - name: Determine server info path
       id: derive_server_info
       shell: bash
+      env:
+        CODEX_HOME: ${{ steps.resolve_home.outputs.codex-home }}
+        CODEX_RUN_ID: ${{ github.run_id }}
       run: |
-        server_info_file="${{ steps.resolve_home.outputs.codex-home }}/${{ github.run_id }}.json"
+        server_info_file="$CODEX_HOME/$CODEX_RUN_ID.json"
         echo "server_info_file=$server_info_file" >> "$GITHUB_OUTPUT"
 
     - name: Check Responses API proxy status
       id: start_proxy
       if: ${{ inputs['openai-api-key'] != '' }}
       shell: bash
+      env:
+        SERVER_INFO_FILE: ${{ steps.derive_server_info.outputs.server_info_file }}
       run: |
-        server_info_file="${{ steps.derive_server_info.outputs.server_info_file }}"
-        if [ -s "$server_info_file" ]; then
-          echo "Responses API proxy already appears to be running (found $server_info_file)."
+        if [ -s "$SERVER_INFO_FILE" ]; then
+          echo "Responses API proxy already appears to be running (found $SERVER_INFO_FILE)."
           echo "server_info_file_exists=true" >> "$GITHUB_OUTPUT"
         else
           echo "server_info_file_exists=false" >> "$GITHUB_OUTPUT"
@@ -175,19 +194,19 @@ runs:
     - name: Start Responses API proxy
       if: ${{ inputs['openai-api-key'] != '' && steps.start_proxy.outputs.server_info_file_exists == 'false' }}
       env:
+        SERVER_INFO_FILE: ${{ steps.derive_server_info.outputs.server_info_file }}
         PROXY_API_KEY: ${{ inputs['openai-api-key'] }}
+        UPSTREAM_URL: ${{ inputs['responses-api-endpoint'] }}
       shell: bash
       run: |
-        upstream_url="${{ inputs['responses-api-endpoint'] }}"
-
         args=(
           codex-responses-api-proxy
           --http-shutdown
-          --server-info "${{ steps.derive_server_info.outputs.server_info_file }}"
+          --server-info "$SERVER_INFO_FILE"
         )
 
-        if [ -n "$upstream_url" ]; then
-          args+=(--upstream-url "$upstream_url")
+        if [ -n "$UPSTREAM_URL" ]; then
+          args+=(--upstream-url "$UPSTREAM_URL")
         fi
 
         (
@@ -197,22 +216,23 @@ runs:
     - name: Wait for Responses API proxy
       if: ${{ inputs['openai-api-key'] != '' && steps.start_proxy.outputs.server_info_file_exists == 'false' }}
       shell: bash
+      env:
+        SERVER_INFO_FILE: ${{ steps.derive_server_info.outputs.server_info_file }}
       run: |
-        server_info_file="${{ steps.derive_server_info.outputs.server_info_file }}"
         for _ in {1..10}; do
-          if [ -s "$server_info_file" ]; then
+          if [ -s "$SERVER_INFO_FILE" ]; then
             break
           fi
           sleep 1
         done
-        if [ ! -s "$server_info_file" ]; then
+        if [ ! -s "$SERVER_INFO_FILE" ]; then
           echo "responses-api-proxy did not write server info" >&2
           exit 1
         fi
 
         if [ "${RUNNER_OS}" != "Windows" ]; then
-          sudo chmod 444 "$server_info_file"
-          sudo chown root "$server_info_file"
+          sudo chmod 444 "$SERVER_INFO_FILE"
+          sudo chown root "$SERVER_INFO_FILE"
         fi
 
     # This step has an output named `port`.
@@ -220,27 +240,37 @@ runs:
       id: read_server_info
       if: ${{ inputs['openai-api-key'] != '' || inputs.prompt != '' || inputs['prompt-file'] != '' }}
       shell: bash
-      run: node "${{ github.action_path }}/dist/main.js" read-server-info "${{ steps.derive_server_info.outputs.server_info_file }}"
+      env:
+        ACTION_PATH: ${{ github.action_path }}
+        SERVER_INFO_FILE: ${{ steps.derive_server_info.outputs.server_info_file }}
+      run: node "$ACTION_PATH/dist/main.js" read-server-info "$SERVER_INFO_FILE"
 
     - name: Write Codex proxy config
       if: ${{ inputs['openai-api-key'] != '' }}
       shell: bash
+      env:
+        ACTION_PATH: ${{ github.action_path }}
+        CODEX_HOME: ${{ steps.resolve_home.outputs.codex-home }}
+        PROXY_PORT: ${{ steps.read_server_info.outputs.port }}
+        SAFETY_STRATEGY: ${{ inputs['safety-strategy'] }}
       run: |
-        node "${{ github.action_path }}/dist/main.js" write-proxy-config \
-          --codex-home "${{ steps.resolve_home.outputs.codex-home }}" \
-          --port "${{ steps.read_server_info.outputs.port }}" \
-          --safety-strategy "${{ inputs['safety-strategy'] }}"
+        node "$ACTION_PATH/dist/main.js" write-proxy-config \
+          --codex-home "$CODEX_HOME" \
+          --port "$PROXY_PORT" \
+          --safety-strategy "$SAFETY_STRATEGY"
 
     - name: Drop sudo privilege, if appropriate
       if: ${{ inputs['safety-strategy'] == 'drop-sudo' && inputs['openai-api-key'] != '' }}
       shell: bash
+      env:
+        ACTION_PATH: ${{ github.action_path }}
       run: |
         case "${RUNNER_OS}" in
           Linux)
-            node "${{ github.action_path }}/dist/main.js" drop-sudo --user runner --group sudo
+            node "$ACTION_PATH/dist/main.js" drop-sudo --user runner --group sudo
             ;;
           macOS)
-            node "${{ github.action_path }}/dist/main.js" drop-sudo --user runner --group admin
+            node "$ACTION_PATH/dist/main.js" drop-sudo --user runner --group admin
             ;;
           *)
             echo "Unsupported OS for drop-sudo: ${RUNNER_OS}" >&2
@@ -275,10 +305,11 @@ runs:
         CODEX_EFFORT: ${{ inputs.effort }}
         CODEX_SAFETY_STRATEGY: ${{ inputs['safety-strategy'] }}
         CODEX_USER: ${{ inputs['codex-user'] }}
+        ACTION_PATH: ${{ github.action_path }}
         FORCE_COLOR: 1
       shell: bash
       run: |
-        node "${{ github.action_path }}/dist/main.js" run-codex-exec \
+        node "$ACTION_PATH/dist/main.js" run-codex-exec \
             --prompt "${CODEX_PROMPT}" \
             --prompt-file "${CODEX_PROMPT_FILE}" \
             --output-file "$CODEX_OUTPUT_FILE" \
diff --git a/docs/security.md b/docs/security.md
index 1c86388..8bdec4b 100644
--- a/docs/security.md
+++ b/docs/security.md
@@ -16,6 +16,20 @@ There is a lot of valuable context that can be used to fuel your invocation of C
 - **Commit messages**: a pull request can be composed of many commits. The messages for individual commits often go unnoticed, but could read by Codex.
 - **Screenshots** screenshots and other media have been known to be used as vehicles for prompt injection.
 
+## Avoid shell injection in workflow steps
+
+GitHub Actions expands `${{ ... }}` expressions before the shell runs your `run:` script. If you splice untrusted values such as branch names, issue titles, comment bodies, or action inputs directly into the script, those values can break shell quoting and execute arbitrary commands.
+
+Instead, pass those values through `env:` and quote the shell variables that consume them:
+
+```yaml
+- name: Safe shell usage
+  env:
+    PR_BASE_REF: ${{ github.event.pull_request.base.ref }}
+  run: |
+    git fetch origin "$PR_BASE_REF"
+```
+
 <!-- TODO ## Protecting secrets -->
 
 ## Look out for API key abuse
diff --git a/examples/test-sandbox-protections.yml b/examples/test-sandbox-protections.yml
index 8347f26..5c2b80e 100644
--- a/examples/test-sandbox-protections.yml
+++ b/examples/test-sandbox-protections.yml
@@ -29,9 +29,11 @@ jobs:
           safety-strategy: ${{ matrix.safety-strategy }}
 
       - name: Try to dump the key from the codex-responses-api-proxy process
+        env:
+          CODEX_RUN_ID: ${{ github.run_id }}
         run: |
           # Find the PID for the codex-responses-api-proxy process.
-          SERVER_INFO_FILE="$HOME/.codex/${{ github.run_id }}.json"
+          SERVER_INFO_FILE="$HOME/.codex/$CODEX_RUN_ID.json"
           PID=$(jq .pid < "$SERVER_INFO_FILE")
 
           # Using standard filesystem read operations (albeit privileged ones),