sbom-diff

Overview

SBOM diffing is the supply-chain primitive that turns two point-in-time inventory snapshots into an actionable change signal: which components are net-new in the target image, which were removed, and which changed version. Without a diff step, an SBOM is a compliance artifact; with it, the SBOM becomes a change-control gate.

Per github.com/CycloneDX/cyclonedx-cli, the CycloneDX CLI provides a first-class diff subcommand that accepts any two CycloneDX BOMs (XML, JSON, or Protobuf) and emits structured added/removed/modified output in text or JSON form.

Per github.com/anchore/syft, Syft generates the per-image CycloneDX or SPDX SBOMs that feed the diff step; the two tools compose naturally in CI.

Differentiation vs sibling skills:

• syft-generation generates SBOMs from a single image; it does not compare across versions.
• cyclonedx-format documents the schema; it does not run diffs.
• grype-scanning scans for vulnerabilities against a single SBOM; it does not detect drift between builds.

This skill covers the SBOM-to-SBOM comparison workflow end to end.

When to use

• A container image was rebuilt (base image update, dep update, CI pipeline change) and the team needs to know what changed in the dependency inventory.
• A release candidate branch must pass a "no unexpected new transitive dependencies" gate before merge.
• A scheduled nightly diff monitors a production image tag for supply-chain drift vs the last known-good SBOM.
• Post-incident: an unexpected library appeared in a running container and the team needs to pinpoint when it was introduced.

Step 1 - Install cyclonedx-cli

Per cdx-cli-gh install options:

# Homebrew
brew install cyclonedx/cyclonedx/cyclonedx-cli

# Docker (no local install required)
docker run cyclonedx/cyclonedx-cli diff sbom-from.json sbom-to.json --component-versions

# Binary - download from releases page
# https://github.com/CycloneDX/cyclonedx-cli/releases
# Latest stable: 0.32.0 (2024-05-14)

Syft is also required to generate input SBOMs. Per sf-gh:

curl -sSfL https://get.anchore.io/syft | sudo sh -s -- -b /usr/local/bin
# or: brew install syft

Step 2 - Generate the two SBOMs

Per sf-gh, generate one SBOM per image version in CycloneDX JSON format (the format accepted by cyclonedx-cli diff):

# Baseline image (e.g., the last release)
syft myapp:1.0 -o cyclonedx-json=sbom-v1.0.json

# Target image (e.g., the release candidate)
syft myapp:1.1 -o cyclonedx-json=sbom-v1.1.json

For local directory scans (non-container builds):

syft dir:./release-1.0 -o cyclonedx-json=sbom-v1.0.json
syft dir:./release-1.1 -o cyclonedx-json=sbom-v1.1.json

Use consistent format and source type across both runs so the diff is comparing equivalent inventories.

Step 3 - Run the diff

Per cdx-cli-gh, the diff subcommand syntax is:

cyclonedx diff <from-file> <to-file> [options]

Key flags per cdx-cli-gh:

Human-readable diff (text output, default):

cyclonedx diff sbom-v1.0.json sbom-v1.1.json --component-versions

Machine-readable diff (JSON output, for CI gate logic):

cyclonedx diff sbom-v1.0.json sbom-v1.1.json \
  --component-versions \
  --output-format json \
  > diff-result.json

The --component-versions flag is the signal flag: without it the diff reports structural BOM changes; with it it reports the dependency inventory delta (added, removed, modified components).

Step 4 - Interpret the output

With --component-versions --output-format json, the JSON output contains three categories of change per cdx-cli-gh:

• Added - components present in the target BOM but not the baseline. These are net-new dependencies introduced in this image version.
• Removed - components present in the baseline but absent from the target. Removals may indicate intentional cleanup or an unexpected packaging change.
• Modified - components present in both, but with a version change. A modified entry contains the old and new version values.

Any non-empty "added" list requires review. A component may be:

• An expected transitive dep of a package the team intentionally upgraded
• A net-new package pulled in by a base image update
• An unexpected addition (supply-chain risk, malicious dep injection, misconfigured build)

Removals and version changes warrant the same review, especially in regulated or security-sensitive contexts.

Step 5 - CI gate on net-new components

The JSON output can be parsed to fail the pipeline when net-new components appear. Example GitHub Actions step using jq:

- name: SBOM diff gate
  run: |
    ADDED=$(jq '.added | length' diff-result.json)
    echo "Net-new components: $ADDED"
    if [ "$ADDED" -gt "0" ]; then
      echo "::error::Net-new components detected. Review diff-result.json."
      jq '.added' diff-result.json
      exit 1
    fi

To allow a pre-approved set of additions (e.g., a known intentional dependency upgrade), maintain an allowlist and subtract matches before the count check:

UNAPPROVED=$(jq --rawfile allow allowlist.txt \
  '[.added[] | select(.name as $n | $allow | test($n) | not)] | length' \
  diff-result.json)

Adjust the gate threshold and allowlist policy to the team's change-control requirements; the CI step above enforces zero-tolerance as the strictest form.

Step 6 - Alerting pattern (nightly drift detection)

For production image monitoring, run a nightly diff against the last known-good SBOM rather than comparing two build artifacts:

jobs:
  sbom-drift:
    runs-on: ubuntu-latest
    schedule:
      - cron: "0 2 * * *"
    steps:
      - name: Generate current SBOM
        run: syft myapp:production -o cyclonedx-json=sbom-current.json

      - name: Download last known-good SBOM
        run: |
          aws s3 cp s3://sbom-store/sbom-last-good.json sbom-baseline.json

      - name: Diff
        run: |
          cyclonedx diff sbom-baseline.json sbom-current.json \
            --component-versions --output-format json \
            > drift-result.json

      - name: Alert on drift
        run: |
          ADDED=$(jq '.added | length' drift-result.json)
          REMOVED=$(jq '.removed | length' drift-result.json)
          if [ "$ADDED" -gt "0" ] || [ "$REMOVED" -gt "0" ]; then
            echo "Supply-chain drift detected"
            cat drift-result.json
            # Pipe to Slack/PagerDuty/JIRA as needed
            exit 1
          fi

      - name: Rotate known-good on clean diff
        if: success()
        run: aws s3 cp sbom-current.json s3://sbom-store/sbom-last-good.json

The "rotate known-good on clean diff" step ensures the baseline advances only when the image passes the gate, catching regressions introduced in a later build.

Example

Comparing myapp:2.3.1 (baseline) to myapp:2.4.0 (candidate):

syft myapp:2.3.1 -o cyclonedx-json=sbom-2.3.1.json
syft myapp:2.4.0 -o cyclonedx-json=sbom-2.4.0.json
cyclonedx diff sbom-2.3.1.json sbom-2.4.0.json --component-versions --output-format json

Truncated example JSON output per cdx-cli-gh documented output shape (added/removed/modified keys):

{
  "added": [
    {"name": "protobuf", "version": "4.25.3", "type": "library"}
  ],
  "removed": [],
  "modified": [
    {"name": "openssl", "from-version": "3.1.4", "to-version": "3.3.0", "type": "library"}
  ]
}

The team reviews: protobuf is net-new. If it is a known transitive dep of a gRPC library the team deliberately upgraded, it is added to the allowlist. If unexplained, the release is blocked pending investigation.

Anti-patterns

Limitations

• cyclonedx-cli diff operates on CycloneDX format (XML, JSON, Protobuf) per cdx-cli-gh; SPDX-format SBOMs require conversion first via cyclonedx convert before diffing.
• The diff output reflects component-level inventory changes, not vulnerability changes. Pair with grype-scanning to assess whether net-new components carry known CVEs.
• Syft inventory accuracy affects diff signal quality: if Syft misses a component in one scan but not the other, the diff will misattribute the delta. See the accuracy-validation section in syft-generation.
• For multi-arch images, diff each platform separately using --platform=linux/amd64 on both Syft runs; a combined diff of mixed-arch SBOMs will conflate platform-specific inventory as change.

References

• cdx-cli-gh - cyclonedx-cli repository: install, subcommand reference, diff flags, output format documentation
• sf-gh - Syft repository: SBOM generation, output formats, source types
• syft-generation - generate the input SBOMs
• cyclonedx-format - CycloneDX schema reference
• grype-scanning - vuln-scan net-new components after the diff gate passes
• vuln-prioritizer - unifier agent