# Dependency security

Two scripts, one question each.

| question | script | output |
|---|---|---|
| what are we running | `scripts/generate_sbom.py` | `sbom.cdx.json`, CycloneDX 1.6, shipped with every release |
| are we exposed | `scripts/scan_sbom.py` | a report, and an exit code |

The SBOM was already generated and installed onto every server. Nothing read
it, so "are we exposed to this advisory" was still a person going and looking.
The scanner closes that.

## Running it

```bash
venv/bin/python scripts/scan_sbom.py                  # report, exit 0
venv/bin/python scripts/scan_sbom.py --gate           # exit 1 on a shipped finding
venv/bin/python scripts/scan_sbom.py --json out.json  # machine-readable too
```

With no `--sbom`, it generates one first, so there is never a stale input.

It needs outbound HTTPS to `api.osv.dev`. **Run it where the repository lives,
not on a customer's server** - a deployed instance is frequently air-gapped and
has nothing to query. That is also why the SBOM ships as a file: it is the
artefact a security team scans on their own terms.

## Why OSV

It takes a purl and returns the advisories that apply. No account, no API key,
no agent installed anywhere, no vulnerability database to keep current. The
SBOM is already the input it wants, so this is a lookup against what shipped
rather than a second scan of the tree that could disagree with it.

## Shipped and build-only are not the same finding

Two thirds of the components in the SBOM are build tooling - bundlers, test
runners, their transitive dependencies. They never reach a server, and the SBOM
already marks them `scope: excluded`.

Both are reported. **Only shipped components can fail a build.**

This is deliberate. At the time of writing, the highest-severity match in the
whole tree is a CRITICAL against the test runner, which is not installed
anywhere a release reaches. A gate that fails on that teaches everybody to pass
`--no-verify` within a month, and then it catches nothing at all.

## Severity is taken, not computed

Where an advisory records a plain word - GHSA gives LOW, MODERATE, HIGH,
CRITICAL - that word is used. Where there is only a CVSS vector, the report says
UNKNOWN rather than parsing the vector into a score it might get wrong.

UNKNOWN is shown and never gates. A guessed score is worse than an honest gap,
because somebody would act on it.

## The allowlist is a record, not a mute button

`scripts/vulnerability-allowlist.json` holds advisories that were reviewed and
accepted. An entry carries the advisory id, the reason, who accepted it, the
date, and what would invalidate the reasoning.

A good reason names the precondition that is absent. The first entry is the
worked example: a PyJWT algorithm-confusion advisory that needs a verifier
configured with both symmetric and asymmetric algorithms AND a raw JSON JWK
passed as the key. Neither is true here, and `invalidatedby` records the change
that would make it true.

That last field is the one that earns its keep. A reason stops being true when
the code that made it true changes, and nobody re-reads an allowlist looking for
that. Write down what would break it.

Upgrading is almost always cheaper than defending a decision to skip it. Do not
add an entry to make a build pass.

## What the report tells you to do

The report ends with a per-component upgrade rollup: the highest fix version
across that component's advisories, and how many it clears. Upgrading once to
the top of the list clears the ones below it.

A target on a different major version is labelled `MAJOR`. It is a different
job from a patch bump - it can be ESM-only, or drop an API - and it is marked
rather than listed as though the two cost the same.