# CVE Differential

A self-contained Gitea and GitHub Actions composite action that extracts multi-platform SBOM attestations from container images, runs vulnerability scans with [Trivy](https://github.com/aquasecurity/trivy), and produces a set-based CVE delta report between a head image and a target baseline image.

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## ✨ Features

- **Dynamic Multi-Platform Support**: Automatically discovers and scans all architectures present in container images, or iterates over an explicit list of target architectures.
- **Fast Attestation Extraction**: Uses `docker buildx imagetools inspect` to extract platform-specific SBOMs directly from registry manifest indices in seconds without downloading layer blobs.
- **Graceful Fallbacks**: If SBOM attestations are missing for any platform, automatically falls back to generating SBOMs directly with Trivy per architecture.
- **Set-Based Differential Analysis**: Categorizes vulnerabilities into:
  - 🔴 **Introduced Vulnerabilities** (newly added in this branch / commit)
  - 🟢 **Fixed Vulnerabilities** (present in target baseline, resolved in this branch)
  - ⚪ **Unchanged Vulnerabilities** (pre-existing in target baseline)
- **Secure by Default**: Verified Trivy binary downloads against static SHA256 checksum lockfiles and single-step unified caching (binary + vulnerability database).

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## 📥 Inputs

| Input | Description | Required | Default |
| :--- | :--- | :--- | :--- |
| `head-image` | Head container image reference to scan (e.g. `registry.example.com/repo:git-sha`) | **Yes** | — |
| `target-image` | Target baseline container image reference to compare against (e.g. `registry.example.com/repo:latest`) | **Yes** | — |
| `target-branch` | Target branch name for report headings | No | `main` |
| `architectures` | Comma-separated architectures/platforms to scan (e.g. `amd64,arm64`). If omitted, scans all architectures present in the image. | No | _(all)_ |
| `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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## 📤 Outputs

| Output | Description |
| :--- | :--- |
| `delta-report` | Path to the generated CVE delta Markdown report |
| `summary-report` | Path to the full vulnerability Markdown summary report |

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## 🔍 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.

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## 🚀 Usage Example

```yaml
jobs:
  build-and-scan:
    runs-on: ubuntu-latest
    steps:
      
      - name: Build and Push Head Container Image
        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
          tags: registry.example.com/my-app:git-${{ github.sha }}

      - name: Compare Vulnerabilities (Head vs Target)
        uses: https://git.horstenkamp.eu/Philipp/cve-differential.git@main
        with:
          head-image: registry.example.com/my-app:git-${{ github.sha }}
          target-image: registry.example.com/my-app:latest
          target-branch: ${{ github.base_ref || 'main' }}
```

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## 📄 License

MIT