Merge pull request 'feat: add dual-side diagnostic overview and fail-safe gating' (#10) from feat-diagnostic-overview-and-gating into main

Reviewed-on: #10
Reviewed-by: LLMReview <27+autoreview@noreplay.horstenkamp.eu>

README.md

@@ -27,6 +27,7 @@ A self-contained Gitea and GitHub Actions composite action that extracts multi-p
 | `output-delta` | Output path for the CVE delta Markdown report | No | `cve-delta.md` |
 | `output-summary` | Output path for the full vulnerability Markdown summary | No | `cve-summary.md` |
 | `write-summary` | Whether to append the generated reports to the job summary (`$GITHUB_STEP_SUMMARY`) | No | `true` |
+| `use-inbuild-sbom` | Whether to inspect and use built-in image SBOM attestations. If false, generates SBOMs directly with Trivy | No | `false` |
 
 ---
 
@@ -39,6 +40,31 @@ A self-contained Gitea and GitHub Actions composite action that extracts multi-p
 
 ---
 
+## 🔍 Built-in SBOM Attestations vs. Trivy Direct Scanning (Syft vs. Trivy)
+
+This action supports two modes for obtaining SBOMs:
+
+### 1. Direct Image Scanning with Trivy (`use-inbuild-sbom: false` — Default & Recommended)
+By default, the action uses `trivy image --format spdx-json` to generate the SBOM directly from the container images:
+- **Full Filesystem Fidelity**: Inspects the entire flattened root filesystem, reading `/etc/os-release` and native package databases (e.g. `/var/lib/dpkg/status`, RPM databases, apk).
+- **Accurate OS & Language Package Detection**: Successfully catalogs both OS packages (Debian, Alpine, RHEL, etc.) and application dependencies (Python, Node.js, Go, etc.).
+- **Symmetric, Apples-to-Apples Comparisons**: Both `head` and `target` images are analyzed using the identical Trivy engine and vulnerability database, preventing skewed diffs.
+
+### 2. Built-in Attestation Extraction (`use-inbuild-sbom: true`)
+When enabled, the action extracts SBOM attestations attached to the container image using `docker buildx imagetools inspect`:
+- **Speed**: Extracts the SBOM JSON metadata directly from the registry manifest index in seconds without pulling image layer blobs.
+- **Underlying Generator (Syft)**: Docker Buildx (`sbom: true`) uses BuildKit's embedded Syft scanner (`moby/buildkit-syft-scanner`).
+- **Known Limitations with Trivy**:
+  - **Testing/Rolling Distributions**: Embedded Syft versions may not recognize newer or unreleased distributions (such as Debian 13 "Trixie"), recording the OS as `family="none"` and skipping OS package vulnerability matching.
+  - **Multi-Stage Builds**: Analyzing layer diffs rather than the flattened container rootfs can cause package database paths to be omitted or detached from the OS manifest.
+  - **Schema/PURL Discrepancies**: Syft and Trivy express certain package qualifiers and namespaces differently in SPDX. When Trivy scans a Syft SPDX file that lacks explicit distro qualifiers, Trivy will silently skip OS vulnerability checks.
+  - **Differential Skew**: If a `head` image uses an incomplete Syft SBOM (detecting only language packages) while `target` is scanned directly with Trivy (detecting 400+ OS packages), the differential report will erroneously show hundreds of "resolved" CVEs.
+
+> [!TIP]
+> If your builds run on stable base images where BuildKit's Syft scanner accurately captures all OS packages, you can set `use-inbuild-sbom: 'true'` to speed up CI runs. Otherwise, leave it as `false` to ensure consistent and comprehensive scans.
+
+---
+
 ## 🚀 Usage Example
 
 ```yaml
@@ -52,7 +78,6 @@ jobs:
         with:
           context: .
           push: true
-          sbom: true
           tags: registry.example.com/my-app:git-${{ github.sha }}
 
       - name: Compare Vulnerabilities (Head vs Target)

action.yaml

@@ -24,6 +24,10 @@ inputs:
     description: Whether to append the generated reports to the job summary ($GITHUB_STEP_SUMMARY)
     required: false
     default: 'true'
+  use-inbuild-sbom:
+    description: Whether to inspect and use built-in image SBOM attestations. If false, generates SBOM directly from images with Trivy.
+    required: false
+    default: 'false'
 
 outputs:
   delta-report:
@@ -58,14 +62,15 @@ runs:
   - name: Install and Verify Trivy
     shell: bash
     run: |
-      mkdir -p "$HOME/.cache/trivy-bin" "$HOME/.local/bin"
-      echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+      set -euo pipefail
 
-      ARCH=$(uname -m)
-      case "$ARCH" in
+      mkdir -p "$HOME/.local/bin" "$HOME/.cache/trivy-bin"
+      echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+      ARCH="$(uname -m)"
+      case "${ARCH}" in
         x86_64) T_ARCH="64bit" ;;
         aarch64|arm64) T_ARCH="ARM64" ;;
-        *) echo "Unsupported architecture: $ARCH"; exit 1 ;;
+        *) echo "::error::Unsupported architecture: ${ARCH}"; exit 1 ;;
       esac
 
       checksums_file="${{ github.action_path }}/checksums.txt"
@@ -86,60 +91,121 @@ runs:
     shell: bash
     run: |
       export PATH="$HOME/.local/bin:$PATH"
+      export TRIVY_SKIP_VERSION_CHECK="true"
       mkdir -p sboms/head sboms/target
 
       cleanup_file() {
         local f="$1"
-        if [ ! -s "$f" ] || [ "$(cat "$f" 2>/dev/null)" = "null" ]; then
-          rm -f "$f"
+        if [ -f "$f" ]; then
+          if [ ! -s "$f" ] || [ "$(cat "$f" 2>/dev/null)" = "null" ]; then
+            rm -f "$f"
+          fi
         fi
       }
 
-      # Extract platform SBOMs for head image
-      echo "Extracting platform SBOMs for head image: ${{ inputs.head-image }}..."
-      docker buildx imagetools inspect "${{ inputs.head-image }}" \
-        --format '{{ json (index .SBOM "linux/amd64").SPDX }}' > sboms/head/sbom-amd64.spdx.json 2>/dev/null || true
-      cleanup_file sboms/head/sbom-amd64.spdx.json
+      head_sbom_err=""
+      target_sbom_err=""
+      use_inbuild="${{ inputs.use-inbuild-sbom }}"
 
-      docker buildx imagetools inspect "${{ inputs.head-image }}" \
-        --format '{{ json (index .SBOM "linux/arm64").SPDX }}' > sboms/head/sbom-arm64.spdx.json 2>/dev/null || true
-      cleanup_file sboms/head/sbom-arm64.spdx.json
-
-      # If single-arch, extract root .SPDX
-      if [ ! -f sboms/head/sbom-amd64.spdx.json ] && [ ! -f sboms/head/sbom-arm64.spdx.json ]; then
+      # --- Head Image SBOM Extraction ---
+      if [ "$use_inbuild" = "true" ]; then
+        echo "Attempting extraction of built-in SBOM attestations for head image: ${{ inputs.head-image }}..."
         docker buildx imagetools inspect "${{ inputs.head-image }}" \
-          --format '{{ json .SBOM.SPDX }}' > sboms/head/sbom-amd64.spdx.json 2>/dev/null || true
+          --format '{{ json (index .SBOM "linux/amd64").SPDX }}' > sboms/head/sbom-amd64.spdx.json 2>/dev/null || true
         cleanup_file sboms/head/sbom-amd64.spdx.json
+
+        docker buildx imagetools inspect "${{ inputs.head-image }}" \
+          --format '{{ json (index .SBOM "linux/arm64").SPDX }}' > sboms/head/sbom-arm64.spdx.json 2>/dev/null || true
+        cleanup_file sboms/head/sbom-arm64.spdx.json
+
+        # If single-arch, extract root .SPDX
+        if [ ! -f sboms/head/sbom-amd64.spdx.json ] && [ ! -f sboms/head/sbom-arm64.spdx.json ]; then
+          docker buildx imagetools inspect "${{ inputs.head-image }}" \
+            --format '{{ json .SBOM.SPDX }}' > sboms/head/sbom-amd64.spdx.json 2>/dev/null || true
+          cleanup_file sboms/head/sbom-amd64.spdx.json
+        fi
       fi
 
-      # Fallback to generating head SBOM with Trivy if attestation missing
+      # Generate head SBOM with Trivy if attestation not used or missing
       if [ ! -f sboms/head/sbom-amd64.spdx.json ] && [ ! -f sboms/head/sbom-arm64.spdx.json ]; then
-        echo "Head image lacks SBOM attestation; generating with Trivy..."
-        trivy image --format spdx-json --output sboms/head/sbom-amd64.spdx.json "${{ inputs.head-image }}" || true
+        echo "Generating SBOM for head image directly with Trivy: ${{ inputs.head-image }}..."
+        if ! trivy image --skip-version-check --format spdx-json --output sboms/head/sbom-amd64.spdx.json "${{ inputs.head-image }}"; then
+          head_sbom_err="Failed to generate SBOM with Trivy for head image"
+        fi
         cleanup_file sboms/head/sbom-amd64.spdx.json
       fi
 
-      # Extract platform SBOMs for target image
-      echo "Extracting platform SBOMs for target image: ${{ inputs.target-image }}..."
-      docker buildx imagetools inspect "${{ inputs.target-image }}" \
-        --format '{{ json (index .SBOM "linux/amd64").SPDX }}' > sboms/target/sbom-amd64.spdx.json 2>/dev/null || true
-      cleanup_file sboms/target/sbom-amd64.spdx.json
+      if [ ! -f sboms/head/sbom-amd64.spdx.json ] && [ ! -f sboms/head/sbom-arm64.spdx.json ] && [ -z "$head_sbom_err" ]; then
+        head_sbom_err="No SBOM generated and head image could not be inspected"
+      fi
 
-      docker buildx imagetools inspect "${{ inputs.target-image }}" \
-        --format '{{ json (index .SBOM "linux/arm64").SPDX }}' > sboms/target/sbom-arm64.spdx.json 2>/dev/null || true
-      cleanup_file sboms/target/sbom-arm64.spdx.json
-
-      if [ ! -f sboms/target/sbom-amd64.spdx.json ] && [ ! -f sboms/target/sbom-arm64.spdx.json ]; then
+      # --- Target Image SBOM Extraction ---
+      if [ "$use_inbuild" = "true" ]; then
+        echo "Attempting extraction of built-in SBOM attestations for target image: ${{ inputs.target-image }}..."
         docker buildx imagetools inspect "${{ inputs.target-image }}" \
-          --format '{{ json .SBOM.SPDX }}' > sboms/target/sbom-amd64.spdx.json 2>/dev/null || true
+          --format '{{ json (index .SBOM "linux/amd64").SPDX }}' > sboms/target/sbom-amd64.spdx.json 2>/dev/null || true
+        cleanup_file sboms/target/sbom-amd64.spdx.json
+
+        docker buildx imagetools inspect "${{ inputs.target-image }}" \
+          --format '{{ json (index .SBOM "linux/arm64").SPDX }}' > sboms/target/sbom-arm64.spdx.json 2>/dev/null || true
+        cleanup_file sboms/target/sbom-arm64.spdx.json
+
+        if [ ! -f sboms/target/sbom-amd64.spdx.json ] && [ ! -f sboms/target/sbom-arm64.spdx.json ]; then
+          docker buildx imagetools inspect "${{ inputs.target-image }}" \
+            --format '{{ json .SBOM.SPDX }}' > sboms/target/sbom-amd64.spdx.json 2>/dev/null || true
+          cleanup_file sboms/target/sbom-amd64.spdx.json
+        fi
+      fi
+
+      # Generate target SBOM with Trivy if attestation not used or missing
+      if [ ! -f sboms/target/sbom-amd64.spdx.json ] && [ ! -f sboms/target/sbom-arm64.spdx.json ]; then
+        echo "Generating SBOM for target image directly with Trivy: ${{ inputs.target-image }}..."
+        if ! trivy image --skip-version-check --format spdx-json --output sboms/target/sbom-amd64.spdx.json "${{ inputs.target-image }}"; then
+          target_sbom_err="Failed to generate SBOM with Trivy for target image"
+        fi
         cleanup_file sboms/target/sbom-amd64.spdx.json
       fi
 
-      # Fallback to generating target SBOM with Trivy if attestation missing
-      if [ ! -f sboms/target/sbom-amd64.spdx.json ] && [ ! -f sboms/target/sbom-arm64.spdx.json ]; then
-        echo "Target image lacks SBOM attestation; attempting to generate with Trivy..."
-        trivy image --format spdx-json --output sboms/target/sbom-amd64.spdx.json "${{ inputs.target-image }}" || true
-        cleanup_file sboms/target/sbom-amd64.spdx.json
+      if [ ! -f sboms/target/sbom-amd64.spdx.json ] && [ ! -f sboms/target/sbom-arm64.spdx.json ] && [ -z "$target_sbom_err" ]; then
+        target_sbom_err="No SBOM generated and target baseline image could not be inspected"
+      fi
+
+      # Report on both sides
+      echo ""
+      echo "================================================================================"
+      echo "                      SBOM EXTRACTION DIAGNOSTIC OVERVIEW                       "
+      echo "================================================================================"
+      if [ -n "$head_sbom_err" ]; then
+        echo "HEAD Image (${{ inputs.head-image }}): ❌ FAILED - $head_sbom_err"
+      else
+        echo "HEAD Image (${{ inputs.head-image }}): ✅ SUCCESS"
+        [ -f sboms/head/sbom-amd64.spdx.json ] && echo "  - sbom-amd64.spdx.json ($(wc -c < sboms/head/sbom-amd64.spdx.json) bytes)"
+        [ -f sboms/head/sbom-arm64.spdx.json ] && echo "  - sbom-arm64.spdx.json ($(wc -c < sboms/head/sbom-arm64.spdx.json) bytes)"
+      fi
+
+      if [ -n "$target_sbom_err" ]; then
+        echo "TARGET Baseline Image (${{ inputs.target-image }}): ❌ FAILED - $target_sbom_err"
+      else
+        echo "TARGET Baseline Image (${{ inputs.target-image }}): ✅ SUCCESS"
+        [ -f sboms/target/sbom-amd64.spdx.json ] && echo "  - sbom-amd64.spdx.json ($(wc -c < sboms/target/sbom-amd64.spdx.json) bytes)"
+        [ -f sboms/target/sbom-arm64.spdx.json ] && echo "  - sbom-arm64.spdx.json ($(wc -c < sboms/target/sbom-arm64.spdx.json) bytes)"
+      fi
+      echo "================================================================================"
+      echo ""
+
+      has_sbom_err=false
+      if [ -n "$head_sbom_err" ]; then
+        echo "::error::Head SBOM extraction failed: $head_sbom_err"
+        has_sbom_err=true
+      fi
+      if [ -n "$target_sbom_err" ]; then
+        echo "::error::Target baseline SBOM extraction failed: $target_sbom_err"
+        has_sbom_err=true
+      fi
+
+      if [ "$has_sbom_err" = "true" ]; then
+        echo "::error::Both head and target images must have valid SBOMs to proceed with differential scanning."
+        exit 1
       fi
 
       # Expose extracted head SBOMs to workspace root for workflow artifact uploads
@@ -150,31 +216,134 @@ runs:
     shell: bash
     run: |
       export PATH="$HOME/.local/bin:$PATH"
+      export TRIVY_SKIP_VERSION_CHECK="true"
       mkdir -p cve-reports/head cve-reports/target
 
-      # Scan each available head SBOM
-      for f in sboms/head/sbom-*.spdx.json; do
-        [ -f "$f" ] || continue
-        arch=$(basename "$f" .spdx.json | sed 's/sbom-//')
-        echo "Scanning head SBOM for architecture: $arch"
-        trivy sbom --format json --output "cve-reports/head/cve-${arch}.json" "$f" || true
-      done
+      shopt -s nullglob
+      head_sboms=(sboms/head/sbom-*.spdx.json)
+      target_sboms=(sboms/target/sbom-*.spdx.json)
 
-      # Scan each available target SBOM
-      for f in sboms/target/sbom-*.spdx.json; do
-        [ -f "$f" ] || continue
-        arch=$(basename "$f" .spdx.json | sed 's/sbom-//')
-        echo "Scanning target SBOM for architecture: $arch"
-        trivy sbom --format json --output "cve-reports/target/cve-${arch}.json" "$f" || true
-      done
+      head_ok=true
+      target_ok=true
+      head_report_count=0
+      target_report_count=0
 
-      # Run multi-arch set-based diff
-      node "${{ github.action_path }}/cve-diff.js" \
+      head_summary=()
+      target_summary=()
+
+      # 1. Scan Head SBOMs
+      if [ ${#head_sboms[@]} -eq 0 ]; then
+        head_ok=false
+        head_summary+=("  - ❌ No SBOM files found in sboms/head/")
+      else
+        for f in "${head_sboms[@]}"; do
+          arch=$(basename "$f" .spdx.json | sed 's/sbom-//')
+          out_json="cve-reports/head/cve-${arch}.json"
+          err_log="cve-reports/head/cve-${arch}.err"
+
+          echo "Scanning head SBOM for architecture: $arch"
+          if trivy sbom --skip-version-check --format json --output "$out_json" "$f" 2>"$err_log"; then
+            if [ -s "$out_json" ]; then
+              vuln_cnt=$(grep -o '"VulnerabilityID"' "$out_json" 2>/dev/null | wc -l || echo 0)
+              if [ "$vuln_cnt" -eq 0 ]; then
+                echo "::warning::Head image architecture '${arch}' has 0 CVE findings. Verify that OS/packages were recognized."
+                head_summary+=("  - ${arch}: ⚠️ WARNING (0 vulnerability findings detected)")
+              else
+                head_summary+=("  - ${arch}: ✅ OK (${vuln_cnt} vulnerability findings)")
+              fi
+              head_report_count=$((head_report_count + 1))
+            else
+              head_ok=false
+              head_summary+=("  - ${arch}: ❌ FAILED (Output report was empty / 0 bytes)")
+            fi
+          else
+            head_ok=false
+            err_msg=$(tail -n 2 "$err_log" | tr '\n' ' ' | sed 's/  */ /g')
+            [ -z "$err_msg" ] && err_msg="Trivy scan exited with non-zero status"
+            head_summary+=("  - ${arch}: ❌ FAILED (${err_msg})")
+          fi
+        done
+      fi
+
+      # 2. Scan Target SBOMs
+      if [ ${#target_sboms[@]} -eq 0 ]; then
+        target_ok=false
+        target_summary+=("  - ❌ No SBOM files found in sboms/target/")
+      else
+        for f in "${target_sboms[@]}"; do
+          arch=$(basename "$f" .spdx.json | sed 's/sbom-//')
+          out_json="cve-reports/target/cve-${arch}.json"
+          err_log="cve-reports/target/cve-${arch}.err"
+
+          echo "Scanning target SBOM for architecture: $arch"
+          if trivy sbom --skip-version-check --format json --output "$out_json" "$f" 2>"$err_log"; then
+            if [ -s "$out_json" ]; then
+              vuln_cnt=$(grep -o '"VulnerabilityID"' "$out_json" 2>/dev/null | wc -l || echo 0)
+              if [ "$vuln_cnt" -eq 0 ]; then
+                echo "::warning::Target baseline image architecture '${arch}' has 0 CVE findings. Verify that OS/packages were recognized."
+                target_summary+=("  - ${arch}: ⚠️ WARNING (0 vulnerability findings detected)")
+              else
+                target_summary+=("  - ${arch}: ✅ OK (${vuln_cnt} vulnerability findings)")
+              fi
+              target_report_count=$((target_report_count + 1))
+            else
+              target_ok=false
+              target_summary+=("  - ${arch}: ❌ FAILED (Output report was empty / 0 bytes)")
+            fi
+          else
+            target_ok=false
+            err_msg=$(tail -n 2 "$err_log" | tr '\n' ' ' | sed 's/  */ /g')
+            [ -z "$err_msg" ] && err_msg="Trivy scan exited with non-zero status"
+            target_summary+=("  - ${arch}: ❌ FAILED (${err_msg})")
+          fi
+        done
+      fi
+
+      # 3. Print Comprehensive Diagnostic Overview
+      echo ""
+      echo "================================================================================"
+      echo "                   CVE VULNERABILITY SCAN DIAGNOSTIC OVERVIEW                  "
+      echo "================================================================================"
+      echo "HEAD Image: ${{ inputs.head-image }}"
+      for line in "${head_summary[@]}"; do
+        echo "$line"
+      done
+      echo ""
+      echo "TARGET Baseline Image: ${{ inputs.target-image }} (branch: '${{ inputs.target-branch }}')"
+      for line in "${target_summary[@]}"; do
+        echo "$line"
+      done
+      echo "================================================================================"
+      echo ""
+
+      # 4. Enforce Gating: Both must have succeeded with valid reports
+      has_error=false
+      if [ "$head_ok" != "true" ] || [ "$head_report_count" -eq 0 ]; then
+        echo "::error::Head image CVE analysis failed or produced no reports."
+        has_error=true
+      fi
+
+      if [ "$target_ok" != "true" ] || [ "$target_report_count" -eq 0 ]; then
+        echo "::error::Target baseline CVE analysis failed or produced no reports."
+        has_error=true
+      fi
+
+      if [ "$has_error" = "true" ]; then
+        echo "::error::Aborting differential analysis: both head and target scans must succeed before diffing."
+        exit 1
+      fi
+
+      # 5. Run multi-arch set-based diff (both are guaranteed present and valid)
+      echo "Both head and target scans succeeded. Computing set-based vulnerability delta..."
+      if ! node "${{ github.action_path }}/cve-diff.js" \
         "cve-reports/target" \
         "cve-reports/head" \
-        "${{ inputs.target-branch }}" > "${{ inputs.output-delta }}" || true
+        "${{ inputs.target-branch }}" > "${{ inputs.output-delta }}"; then
+        echo "::error::Failed to generate CVE differential report"
+        exit 1
+      fi
 
-      # Generate full markdown summary report for primary platform
+      # 6. Generate full markdown summary report for primary platform
       primary_sbom=""
       if [ -f sboms/head/sbom-amd64.spdx.json ]; then
         primary_sbom="sboms/head/sbom-amd64.spdx.json"
@@ -183,11 +352,15 @@ runs:
       fi
 
       if [ -n "$primary_sbom" ]; then
-        trivy sbom \
+        if ! trivy sbom \
+          --skip-version-check \
           --format template \
           --template "@${{ github.action_path }}/markdown.tpl" \
           --output "${{ inputs.output-summary }}" \
-          "$primary_sbom" || true
+          "$primary_sbom"; then
+          echo "::error::Trivy summary generation failed for primary SBOM: $primary_sbom"
+          exit 1
+        fi
       fi
 
   - name: Publish Reports to Step Summary

cve-diff.js

@@ -41,14 +41,24 @@ function loadReportsForArg(arg) {
     }
   }
 
+  if (filePaths.length === 0) {
+    console.error(`::error::No CVE JSON report files found for path: ${arg}`);
+    process.exit(1);
+  }
+
+  let loadedFiles = 0;
   for (const filePath of filePaths) {
     let stat;
     try {
       stat = fs.statSync(filePath);
-    } catch {
-      continue;
+    } catch (e) {
+      console.error(`::error::Failed to access report file ${filePath}: ${e.message}`);
+      process.exit(1);
+    }
+    if (stat.size === 0) {
+      console.error(`::error::CVE report file is empty (0 bytes): ${filePath}`);
+      process.exit(1);
     }
-    if (stat.size === 0) continue;
 
     const baseName = path.basename(filePath).toLowerCase();
     let arch = 'all';
@@ -62,14 +72,18 @@ function loadReportsForArg(arg) {
     try {
       const content = fs.readFileSync(filePath, 'utf8');
       data = JSON.parse(content);
-    } catch {
-      continue;
+    } catch (e) {
+      console.error(`::error::Invalid JSON in CVE report file ${filePath}: ${e.message}`);
+      process.exit(1);
     }
 
     if (!data || typeof data !== 'object' || !Array.isArray(data.Results)) {
-      continue;
+      console.error(`::error::Malformed Trivy report structure (missing Results array) in: ${filePath}`);
+      process.exit(1);
     }
 
+    loadedFiles++;
+
     for (const result of data.Results) {
       const target = result.Target || result.Type || 'App';
       const vulns = Array.isArray(result.Vulnerabilities) ? result.Vulnerabilities : [];
@@ -88,6 +102,11 @@ function loadReportsForArg(arg) {
     }
   }
 
+  if (loadedFiles === 0) {
+    console.error(`::error::No valid CVE reports could be loaded for path: ${arg}`);
+    process.exit(1);
+  }
+
   return cves;
 }
 
@@ -130,19 +149,17 @@ function formatVulnRow(v) {
  * @returns {string}
  */
 function renderMarkdownDiff(baseCves, branchCves, baseName = 'main') {
-  if (baseCves.size === 0) {
-    return [
-      `### 🔄 Vulnerability Delta (vs \`${baseName}\`)`,
-      '',
-      '> [!NOTE]',
-      `> No baseline scan from \`${baseName}\` was available to compare against.`,
-      '',
-    ].join('\n');
-  }
-
   const baseKeys = new Set(baseCves.keys());
   const branchKeys = new Set(branchCves.keys());
 
+  if (branchCves.size === 0) {
+    console.error('::warning::Head image contains 0 reported vulnerabilities. Verify that packages and dependencies were recognized.');
+  }
+
+  if (baseCves.size === 0) {
+    console.error(`::warning::Baseline target (${baseName}) contains 0 reported vulnerabilities.`);
+  }
+
   const newKeys = [];
   let unchangedCount = 0;
   for (const key of branchKeys) {
@@ -177,15 +194,30 @@ function renderMarkdownDiff(baseCves, branchCves, baseName = 'main') {
   const lines = [
     `### 🔄 Vulnerability Delta (vs \`${baseName}\`)`,
     '',
-    '| Metric | Count |',
-    '| :--- | :--- |',
-    `| 🚨 **New Vulnerabilities** | ${newVulns.length > 0 ? `🚨 **${newVulns.length}**` : '0'} |`,
-    `| ✅ **Resolved Vulnerabilities** | ${resolvedVulns.length > 0 ? `✅ **${resolvedVulns.length}**` : '0'} |`,
-    `| ⚪ **Unchanged Vulnerabilities** | ${unchangedCount} |`,
-    '',
-    `#### 🚨 New Vulnerabilities Introduced (${newVulns.length})`,
   ];
 
+  if (branchCves.size === 0 && baseCves.size === 0) {
+    lines.push('> [!WARNING]');
+    lines.push('> Both the head image and baseline target have **0 reported vulnerabilities**.');
+    lines.push('');
+  } else if (branchCves.size === 0) {
+    lines.push('> [!WARNING]');
+    lines.push('> The head image has **0 reported vulnerabilities**. Verify that OS packages and language dependencies were properly indexed in the SBOM.');
+    lines.push('');
+  } else if (baseCves.size === 0) {
+    lines.push('> [!WARNING]');
+    lines.push(`> The baseline image (\`${baseName}\`) has **0 reported vulnerabilities**.`);
+    lines.push('');
+  }
+
+  lines.push('| Metric | Count |');
+  lines.push('| :--- | :--- |');
+  lines.push(`| 🚨 **New Vulnerabilities** | ${newVulns.length > 0 ? `🚨 **${newVulns.length}**` : '0'} |`);
+  lines.push(`| ✅ **Resolved Vulnerabilities** | ${resolvedVulns.length > 0 ? `✅ **${resolvedVulns.length}**` : '0'} |`);
+  lines.push(`| ⚪ **Unchanged Vulnerabilities** | ${unchangedCount} |`);
+  lines.push('');
+  lines.push(`#### 🚨 New Vulnerabilities Introduced (${newVulns.length})`);
+
   if (newVulns.length > 0) {
     lines.push('| Severity | CVE | Package | Arch | Installed | Fixed In | Title |');
     lines.push('| :--- | :--- | :--- | :--- | :--- | :--- | :--- |');