fix(action): address security review - prevent token exfiltration and add input validation

Security fixes:
- On GitHub/GHES (VCS_TYPE=github), inputs.gitea-url is now completely
  ignored. API calls use github.api_url; downloads use github.server_url.
  Tokens are never sent to user-supplied URLs.
- Replace action_token step output with masked GITHUB_ENV variable to
  prevent token leakage in debug logs.
- Validate action-repo against owner/repo pattern to prevent path traversal.
- Validate SERVER_URL in Gitea path: require https:// scheme, reject
  whitespace and newlines.
- Strengthen VERSION validation: block slashes and whitespace in addition
  to newlines.
- Add integrity check in Install step: verify SERVER_URL matches
  github.server_url on GitHub runners.

Addresses findings from security-review-bot on PR #121.
Deferred: full IP-level SSRF defense (see #123).

.gitea/actions/review/action.yml

@@ -2,13 +2,23 @@
 # It detects the VCS host type using the github.api_url context (set only on
 # GitHub/GHES runners) and uses the appropriate releases API for version
 # resolution and binary download.
+#
+# Security notes:
+# - On GitHub/GHES (VCS_TYPE=github), inputs.gitea-url is IGNORED to prevent
+#   token exfiltration. API calls use github.api_url; downloads use
+#   github.server_url. Tokens are never sent to user-supplied URLs.
+# - On Gitea (VCS_TYPE=gitea), inputs.gitea-url is validated (https scheme,
+#   no whitespace/newlines) before use.
+# - action-repo is validated against owner/repo pattern.
+# - Tokens are passed via masked environment variables, not step outputs.
+#
 # Requirements: python3, sha256sum, curl (all present on ubuntu-* runners).
 name: 'AI Code Review'
 description: 'Run AI-powered code review on a pull request using review-bot'
 
 inputs:
   gitea-url:
-    description: 'VCS instance URL (defaults to server_url). Works for both Gitea and GitHub.'
+    description: 'Gitea instance URL (only used on Gitea runners; ignored on GitHub/GHES). Defaults to server_url.'
     required: false
     default: ''
   repo:
@@ -121,9 +131,7 @@ runs:
       id: version
       shell: bash
       run: |
-        SERVER_URL="${{ inputs.gitea-url || github.server_url }}"
-        # Strip trailing slash if present
-        SERVER_URL="${SERVER_URL%/}"
+        # --- Input Validation ---
 
         # Determine the repo hosting review-bot releases (not the repo being reviewed)
         ACTION_REPO="${{ inputs.action-repo }}"
@@ -136,6 +144,12 @@ runs:
           ACTION_REPO="rodin/review-bot"
         fi
 
+        # Validate ACTION_REPO matches owner/repo pattern (防止 path traversal)
+        if ! printf '%s' "$ACTION_REPO" | grep -qE '^[a-zA-Z0-9._-]+/[a-zA-Z0-9._-]+$'; then
+          echo "Error: action-repo '${ACTION_REPO}' does not match expected owner/repo format" >&2
+          exit 1
+        fi
+
         # Detect VCS host type using github.api_url context.
         # github.api_url is set on GitHub.com (https://api.github.com) and GHES
         # (https://<host>/api/v3). It is empty/unset on Gitea Actions runners.
@@ -146,6 +160,32 @@ runs:
           VCS_TYPE="gitea"
         fi
 
+        # Determine SERVER_URL based on VCS type.
+        # SECURITY: On GitHub/GHES, ALWAYS use github.server_url — never trust
+        # inputs.gitea-url to prevent token exfiltration to attacker-controlled hosts.
+        if [ "$VCS_TYPE" = "github" ]; then
+          SERVER_URL="${{ github.server_url }}"
+          if [ -n "${{ inputs.gitea-url }}" ]; then
+            echo "::warning::inputs.gitea-url is ignored on GitHub/GHES runners (VCS_TYPE=github). Using github.server_url instead."
+          fi
+        else
+          SERVER_URL="${{ inputs.gitea-url || github.server_url }}"
+        fi
+        # Strip trailing slash if present
+        SERVER_URL="${SERVER_URL%/}"
+
+        # Validate SERVER_URL for Gitea path: must be https, no whitespace/newlines
+        if [ "$VCS_TYPE" = "gitea" ]; then
+          if ! printf '%s' "$SERVER_URL" | grep -qE '^https://[^[:space:]]+$'; then
+            echo "Error: SERVER_URL '${SERVER_URL}' must be an https:// URL with no whitespace" >&2
+            exit 1
+          fi
+          if printf '%s' "$SERVER_URL" | grep -q $'\n'; then
+            echo "Error: SERVER_URL contains unexpected newline" >&2
+            exit 1
+          fi
+        fi
+
         # Determine auth token for release API requests
         ACTION_TOKEN="${{ inputs.action-repo-token }}"
         if [ -z "$ACTION_TOKEN" ]; then
@@ -158,11 +198,11 @@ runs:
 
         if [ "${{ inputs.version }}" = "latest" ]; then
           if [ "$VCS_TYPE" = "github" ]; then
-            # Use github.api_url which resolves correctly for both github.com
-            # (https://api.github.com) and GHES (https://<host>/api/v3)
+            # SECURITY: Use github.api_url which is a trusted platform-provided value.
+            # Never construct API URLs from user-supplied inputs on GitHub.
             API_URL="${GITHUB_API_URL}/repos/${ACTION_REPO}/releases?per_page=1"
           else
-            # Gitea API
+            # Gitea API — SERVER_URL was validated above
             API_URL="${SERVER_URL}/api/v1/repos/${ACTION_REPO}/releases?limit=1"
           fi
 
@@ -193,9 +233,9 @@ runs:
           VERSION="${{ inputs.version }}"
         fi
 
-        # Validate outputs don't contain newlines (defensive against injection)
-        if printf '%s' "$VERSION" | grep -q $'\n'; then
-          echo "Error: VERSION contains unexpected newline" >&2
+        # Validate VERSION: no newlines, no slashes, no whitespace (防止 path traversal)
+        if printf '%s' "$VERSION" | grep -qE '[\n\r/[:space:]]'; then
+          echo "Error: VERSION '${VERSION}' contains invalid characters (newline, slash, or whitespace)" >&2
           exit 1
         fi
 
@@ -203,7 +243,13 @@ runs:
         echo "action_repo=${ACTION_REPO}" >> "$GITHUB_OUTPUT"
         echo "server_url=${SERVER_URL}" >> "$GITHUB_OUTPUT"
         echo "vcs_type=${VCS_TYPE}" >> "$GITHUB_OUTPUT"
-        echo "action_token=${ACTION_TOKEN}" >> "$GITHUB_OUTPUT"
+
+        # SECURITY: Pass token via masked environment variable instead of step output.
+        # Step outputs can leak in debug logs; GITHUB_ENV with masking is safer.
+        if [ -n "$ACTION_TOKEN" ]; then
+          echo "::add-mask::${ACTION_TOKEN}"
+          echo "ACTION_TOKEN=${ACTION_TOKEN}" >> "$GITHUB_ENV"
+        fi
 
     - name: Cache review-bot binary
       id: cache
@@ -220,16 +266,21 @@ runs:
         ACTION_REPO="${{ steps.version.outputs.action_repo }}"
         VERSION="${{ steps.version.outputs.version }}"
         VCS_TYPE="${{ steps.version.outputs.vcs_type }}"
-        ACTION_TOKEN="${{ steps.version.outputs.action_token }}"
+        # Read token from masked environment variable (set in Determine version step)
+        # Falls back to empty if not set (public repos don't need auth)
+        ACTION_TOKEN="${ACTION_TOKEN:-}"
         BINARY="review-bot-linux-amd64"
 
-        # Build auth args if token is available (supports private repos)
-        AUTH_ARGS=""
-        if [ -n "$ACTION_TOKEN" ]; then
-          if [ "$VCS_TYPE" = "github" ]; then
-            AUTH_ARGS="-H \"Authorization: Bearer ${ACTION_TOKEN}\""
-          else
-            AUTH_ARGS="-H \"Authorization: token ${ACTION_TOKEN}\""
+        # SECURITY: On GitHub/GHES, use github.server_url for downloads.
+        # SERVER_URL is already set correctly per VCS_TYPE in Determine version step,
+        # but verify the download destination matches expectations.
+        if [ "$VCS_TYPE" = "github" ]; then
+          # Double-check: SERVER_URL must be github.server_url (platform-provided)
+          EXPECTED_SERVER="${{ github.server_url }}"
+          EXPECTED_SERVER="${EXPECTED_SERVER%/}"
+          if [ "$SERVER_URL" != "$EXPECTED_SERVER" ]; then
+            echo "Error: SERVER_URL mismatch on GitHub runner (possible tampering)" >&2
+            exit 1
           fi
         fi