feat(#120): add GitHub Actions support with VCS host detection and security hardening

- Detect VCS host type from github.api_url (present on GitHub/GHES, absent on Gitea)
- Add action-repo input: specifies repo hosting review-bot releases, separate from
  the reviewed repo. Defaults to github.action_repository, then rodin/review-bot.
- Add action-repo-token input: auth for release downloads (defaults to github.token
  on GitHub, reviewer-token on Gitea).
- GitHub/GHES path: use github.api_url for version resolution and REST API asset
  download endpoint (required for private repos; web URLs redirect to S3 and don't
  support Authorization headers reliably).
- Gitea path: use validated SERVER_URL with direct download (no -L; prevents
  Authorization forwarding on potential CDN redirects).
- Security hardening:
  - inputs.vcs-url is IGNORED on GitHub/GHES to prevent token exfiltration
  - SERVER_URL validated for https scheme and no whitespace on Gitea path
  - action-repo validated against owner/repo pattern (prevent path traversal)
  - VERSION validated for no slashes/whitespace
  - TOKEN validated for no control characters (header injection defense)
  - ACTION_TOKEN passed via ::add-mask:: + GITHUB_ENV (not step output, which
    can leak in debug logs)
  - set -euo pipefail in both script steps
- Multi-arch support: OS/arch detection via uname (linux/darwin, amd64/arm64)
  for cache key and binary name — incorporates changes from #124
- Run review step: passes VCS_URL from step output (server_url) instead of
  direct input expression

Closes #120

.gitea/actions/review/action.yml

@@ -1,17 +1,37 @@
-# This composite action is designed for Gitea Actions runners.
-# Gitea Actions supports GitHub Actions syntax including $GITHUB_OUTPUT,
-# actions/cache, and actions/checkout.
+# This composite action supports both Gitea Actions and GitHub Actions runners.
+# It detects the VCS host type by checking whether github.api_url is set
+# (present on GitHub.com and GHES runners, absent on Gitea runners) and uses
+# the appropriate releases API for version resolution and binary download
+# (REST API on GitHub, direct URLs on Gitea).
+#
+# Security notes:
+# - On GitHub/GHES (VCS_TYPE=github), inputs.vcs-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.vcs-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:
   vcs-url:
-    description: 'VCS server URL (defaults to server_url)'
+    description: 'VCS server URL (only used on Gitea runners; ignored on GitHub/GHES). Defaults to server_url.'
     required: false
     default: ''
   repo:
-    description: 'Repository (owner/name, defaults to current)'
+    description: 'Repository to review (owner/name, defaults to current)'
+    required: false
+    default: ''
+  action-repo:
+    description: 'Repository hosting review-bot releases (owner/name). Defaults to github.action_repository or rodin/review-bot.'
+    required: false
+    default: ''
+  action-repo-token:
+    description: 'Token for downloading release assets from action-repo (defaults to github.token on GitHub, reviewer-token on Gitea). Required for private repos.'
     required: false
     default: ''
   pr-number:
@@ -19,7 +39,7 @@ inputs:
     required: false
     default: ''
   reviewer-token:
-    description: 'Gitea token for posting the review'
+    description: 'Token for posting the review'
     required: true
   reviewer-name:
     description: 'Display name for the reviewer'
@@ -112,19 +132,120 @@ runs:
       id: version
       shell: bash
       run: |
-        BASE_URL="${{ inputs.vcs-url || github.server_url }}"
-        REPO="${{ inputs.repo || 'rodin/review-bot' }}"
+        set -euo pipefail
+
+        # --- Input Validation ---
+
+        # Determine the repo hosting review-bot releases (not the repo being reviewed)
+        ACTION_REPO="${{ inputs.action-repo }}"
+        if [ -z "$ACTION_REPO" ]; then
+          # github.action_repository is the repo containing the running action
+          ACTION_REPO="${{ github.action_repository }}"
+        fi
+        if [ -z "$ACTION_REPO" ]; then
+          # Final fallback for Gitea (which may not set action_repository)
+          ACTION_REPO="rodin/review-bot"
+          echo "::notice::action-repo not specified and github.action_repository is empty; falling back to rodin/review-bot"
+        fi
+
+        # Validate ACTION_REPO matches owner/repo pattern (prevent path traversal)
+        if ! printf '%s' "$ACTION_REPO" | grep -qE '^[a-zA-Z0-9._-]+/[a-zA-Z0-9._-]+$'; then
+          echo "Error: action-repo '${ACTION_REPO}' does not match expected owner/repo format" >&2
+          exit 1
+        fi
+
+        # Detect VCS host type using github.api_url context.
+        # github.api_url is set on GitHub.com (https://api.github.com) and GHES
+        # (https://<host>/api/v3). It is empty/unset on Gitea Actions runners.
+        GITHUB_API_URL="${{ github.api_url }}"
+        if [ -n "$GITHUB_API_URL" ]; then
+          VCS_TYPE="github"
+        else
+          VCS_TYPE="gitea"
+        fi
+
+        # Determine SERVER_URL based on VCS type.
+        # SECURITY: On GitHub/GHES, ALWAYS use github.server_url — never trust
+        # inputs.vcs-url to prevent token exfiltration to attacker-controlled hosts.
+        if [ "$VCS_TYPE" = "github" ]; then
+          SERVER_URL="${{ github.server_url }}"
+          if [ -n "${{ inputs.vcs-url }}" ]; then
+            echo "::warning::inputs.vcs-url is ignored on GitHub/GHES runners (VCS_TYPE=github). Using github.server_url instead."
+          fi
+        else
+          SERVER_URL="${{ inputs.vcs-url || github.server_url }}"
+        fi
+        # Strip trailing slash if present
+        SERVER_URL="${SERVER_URL%/}"
+
+        # Validate SERVER_URL for Gitea path: must be https, no whitespace/newlines.
+        # The [^[:space:]] class already rejects newlines, so no separate newline check needed.
+        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
+        fi
+
+        # Determine auth token for release API requests
+        ACTION_TOKEN="${{ inputs.action-repo-token }}"
+        if [ -z "$ACTION_TOKEN" ]; then
+          if [ "$VCS_TYPE" = "github" ]; then
+            ACTION_TOKEN="${{ github.token }}"
+          else
+            ACTION_TOKEN="${{ inputs.reviewer-token }}"
+          fi
+        fi
+
+        # Validate token contains no control characters (defense-in-depth against header injection)
+        if [ -n "$ACTION_TOKEN" ]; then
+          if printf '%s' "$ACTION_TOKEN" | LC_ALL=C grep -q '[^[:print:]]'; then
+            echo "Error: ACTION_TOKEN contains control characters" >&2
+            exit 1
+          fi
+        fi
+
         if [ "${{ inputs.version }}" = "latest" ]; then
-          VERSION=$(curl -sSf "${BASE_URL}/api/v1/repos/${REPO}/releases?limit=1" \
-            | python3 -c "import sys, json; releases = json.load(sys.stdin); print(releases[0]['tag_name'] if releases else '')")
+          if [ "$VCS_TYPE" = "github" ]; then
+            # 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 — SERVER_URL was validated above
+            API_URL="${SERVER_URL}/api/v1/repos/${ACTION_REPO}/releases?limit=1"
+          fi
+
+          # Fetch latest version with inline auth header (no intermediate variable)
+          if [ -n "$ACTION_TOKEN" ]; then
+            if [ "$VCS_TYPE" = "github" ]; then
+              VERSION=$(curl -sSf --connect-timeout 10 --max-time 30 \
+                -H "Authorization: Bearer ${ACTION_TOKEN}" "$API_URL" \
+                | python3 -c "import sys, json; releases = json.load(sys.stdin); print(releases[0]['tag_name'] if releases else '')")
+            else
+              VERSION=$(curl -sSf --connect-timeout 10 --max-time 30 \
+                -H "Authorization: token ${ACTION_TOKEN}" "$API_URL" \
+                | python3 -c "import sys, json; releases = json.load(sys.stdin); print(releases[0]['tag_name'] if releases else '')")
+            fi
+          else
+            VERSION=$(curl -sSf --connect-timeout 10 --max-time 30 "$API_URL" \
+              | python3 -c "import sys, json; releases = json.load(sys.stdin); print(releases[0]['tag_name'] if releases else '')")
+          fi
+
           if [ -z "$VERSION" ]; then
-            echo "Failed to determine latest version" >&2
+            echo "Failed to determine latest version from ${API_URL}" >&2
             exit 1
           fi
         else
           VERSION="${{ inputs.version }}"
         fi
 
+        # Validate VERSION: no slashes or whitespace (prevent path traversal).
+        # [:space:] includes newlines and carriage returns in POSIX.
+        if printf '%s' "$VERSION" | grep -qE '[/[:space:]]'; then
+          echo "Error: VERSION '${VERSION}' contains invalid characters (newline, slash, or whitespace)" >&2
+          exit 1
+        fi
+
         # Detect OS and architecture for platform-specific binary download
         OS_RAW=$(uname -s | tr '[:upper:]' '[:lower:]')
         case "$OS_RAW" in
@@ -149,6 +270,16 @@ runs:
         echo "version=${VERSION}" >> "$GITHUB_OUTPUT"
         echo "os=${OS}" >> "$GITHUB_OUTPUT"
         echo "arch=${ARCH}" >> "$GITHUB_OUTPUT"
+        echo "action_repo=${ACTION_REPO}" >> "$GITHUB_OUTPUT"
+        echo "server_url=${SERVER_URL}" >> "$GITHUB_OUTPUT"
+        echo "vcs_type=${VCS_TYPE}" >> "$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
@@ -161,21 +292,101 @@ runs:
       if: steps.cache.outputs.cache-hit != 'true'
       shell: bash
       run: |
-        BASE_URL="${{ inputs.vcs-url || github.server_url }}"
-        REPO="${{ inputs.repo || 'rodin/review-bot' }}"
-        VERSION="${{ steps.version.outputs.version }}"
-        BINARY="review-bot-${{ steps.version.outputs.os }}-${{ steps.version.outputs.arch }}"
+        set -euo pipefail
 
-        curl -sSfL "${BASE_URL}/${REPO}/releases/download/${VERSION}/${BINARY}" \
-          -o "${{ runner.temp }}/review-bot"
-        curl -sSfL "${BASE_URL}/${REPO}/releases/download/${VERSION}/checksums.txt" \
-          -o "${{ runner.temp }}/checksums.txt"
+        SERVER_URL="${{ steps.version.outputs.server_url }}"
+        ACTION_REPO="${{ steps.version.outputs.action_repo }}"
+        VERSION="${{ steps.version.outputs.version }}"
+        VCS_TYPE="${{ steps.version.outputs.vcs_type }}"
+        OS="${{ steps.version.outputs.os }}"
+        ARCH="${{ steps.version.outputs.arch }}"
+        # Read token from masked environment variable (set in Determine version step)
+        # Falls back to empty if not set (public repos don't need auth)
+        ACTION_TOKEN="${ACTION_TOKEN:-}"
+        BINARY="review-bot-${OS}-${ARCH}"
+
+        if [ "$VCS_TYPE" = "github" ]; then
+          # GitHub/GHES: Use REST API for release asset downloads.
+          # Web release URLs ({server}/.../releases/download/{tag}/{asset}) redirect
+          # to S3 and don't reliably support Authorization headers for private repos.
+          # The REST API endpoint with Accept: application/octet-stream is required.
+          # GITHUB_API_URL: trusted platform value, same as detected in "Determine version" step.
+          GITHUB_API_URL="${{ github.api_url }}"
+
+          if [ -n "$ACTION_TOKEN" ]; then
+            RELEASE_JSON=$(curl -sSf --connect-timeout 10 --max-time 30 \
+              -H "Authorization: Bearer ${ACTION_TOKEN}" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/tags/${VERSION}")
+          else
+            RELEASE_JSON=$(curl -sSf --connect-timeout 10 --max-time 30 \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/tags/${VERSION}")
+          fi
+
+          # Extract asset IDs for binary and checksums
+          BINARY_ASSET_ID=$(printf '%s' "$RELEASE_JSON" | python3 -c "import sys, json; assets = json.load(sys.stdin).get('assets', []); matches = [a['id'] for a in assets if a['name'] == '${BINARY}']; print(matches[0] if matches else '')")
+          if [ -z "$BINARY_ASSET_ID" ]; then
+            echo "Error: could not find asset '${BINARY}' in release ${VERSION}" >&2
+            exit 1
+          fi
+
+          CHECKSUMS_ASSET_ID=$(printf '%s' "$RELEASE_JSON" | python3 -c "import sys, json; assets = json.load(sys.stdin).get('assets', []); matches = [a['id'] for a in assets if a['name'] == 'checksums.txt']; print(matches[0] if matches else '')")
+          if [ -z "$CHECKSUMS_ASSET_ID" ]; then
+            echo "Error: could not find asset 'checksums.txt' in release ${VERSION}" >&2
+            exit 1
+          fi
+
+          # Download assets via REST API with Accept: application/octet-stream
+          if [ -n "$ACTION_TOKEN" ]; then
+            curl -sSfL --connect-timeout 10 --max-time 120 \
+              -H "Authorization: Bearer ${ACTION_TOKEN}" \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${BINARY_ASSET_ID}" \
+              -o "${{ runner.temp }}/review-bot"
+            curl -sSfL --connect-timeout 10 --max-time 30 \
+              -H "Authorization: Bearer ${ACTION_TOKEN}" \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${CHECKSUMS_ASSET_ID}" \
+              -o "${{ runner.temp }}/checksums.txt"
+          else
+            curl -sSfL --connect-timeout 10 --max-time 120 \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${BINARY_ASSET_ID}" \
+              -o "${{ runner.temp }}/review-bot"
+            curl -sSfL --connect-timeout 10 --max-time 30 \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${CHECKSUMS_ASSET_ID}" \
+              -o "${{ runner.temp }}/checksums.txt"
+          fi
+        else
+          # Gitea: Direct download via web release URLs (Gitea serves assets
+          # directly without redirects — no -L needed).
+          # SECURITY: Omitting -L prevents forwarding Authorization header to
+          # unexpected hosts if Gitea ever introduces CDN redirects.
+          DOWNLOAD_URL="${SERVER_URL}/${ACTION_REPO}/releases/download/${VERSION}"
+
+          if [ -n "$ACTION_TOKEN" ]; then
+            curl -sSf --connect-timeout 10 --max-time 120 \
+              -H "Authorization: token ${ACTION_TOKEN}" \
+              "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
+            curl -sSf --connect-timeout 10 --max-time 30 \
+              -H "Authorization: token ${ACTION_TOKEN}" \
+              "${DOWNLOAD_URL}/checksums.txt" -o "${{ runner.temp }}/checksums.txt"
+          else
+            curl -sSf --connect-timeout 10 --max-time 120 \
+              "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
+            curl -sSf --connect-timeout 10 --max-time 30 \
+              "${DOWNLOAD_URL}/checksums.txt" -o "${{ runner.temp }}/checksums.txt"
+          fi
+        fi
 
         # Verify SHA-256 checksum
+        # NOTE: This verifies integrity (download wasn't corrupted) but not
+        # authenticity — both binary and checksums come from the same server.
+        # For stronger guarantees, consider GPG signature verification.
         cd "${{ runner.temp }}"
-        EXPECTED=$(grep -E "^[[:xdigit:]]+[[:space:]]+\*?${BINARY}$" checksums.txt | awk '{print $1}')
+        EXPECTED=$(grep -E "^[0-9a-f]+[[:space:]]+\*?${BINARY}$" checksums.txt | awk '{print $1}')
         # sha256sum (GNU) is not available on macOS; use shasum -a 256 on darwin.
-        if [ "${{ steps.version.outputs.os }}" = "darwin" ]; then
+        if [ "${OS}" = "darwin" ]; then
           ACTUAL=$(shasum -a 256 review-bot | awk '{print $1}')
         else
           ACTUAL=$(sha256sum review-bot | awk '{print $1}')
@@ -193,12 +404,12 @@ runs:
         fi
 
         chmod +x "${{ runner.temp }}/review-bot"
-        echo "Installed review-bot-${{ steps.version.outputs.os }}-${{ steps.version.outputs.arch }} ${VERSION} (checksum verified)"
+        echo "Installed review-bot-${OS}-${ARCH} ${VERSION} (checksum verified)"
 
     - name: Run review
       shell: bash
       env:
-        VCS_URL: ${{ inputs.vcs-url || github.server_url }}
+        VCS_URL: ${{ steps.version.outputs.server_url }}
         GITEA_REPO: ${{ inputs.repo || github.repository }}
         PR_NUMBER: ${{ inputs.pr-number || github.event.pull_request.number }}
         REVIEWER_TOKEN: ${{ inputs.reviewer-token }}