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` |
 
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@@ -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)