# 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 file has held exactly
one entry so far, and how it ended is the point.

A PyJWT advisory rated HIGH - algorithm confusion, forged HS256 tokens - needs a
verifier configured with both symmetric and asymmetric algorithms AND a raw JSON
JWK passed as the key. This application configures neither: there is no
`JWT_ALGORITHM` or `JWT_DECODE_ALGORITHMS` anywhere in the tree, so
flask-jwt-extended runs HS256-only against a string secret. That was written
down, with `invalidatedby` recording what would make it untrue - SSO issuing
tokens verified against a JWK.

Then PyJWT was upgraded, the advisory stopped matching, and the entry was
deleted. The allowlist is empty again, which is the state to aim for.

`invalidatedby` is the field 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 smallest upgrade that
clears every advisory against that component, and how many it clears.

"Smallest" is doing real work there. An advisory patched on several release
branches lists a fix per branch - nanoid's is fixed in both 3.3.18 and 5.1.16 -
and taking the highest recommends a major upgrade when a patch on the current
branch would do. Where the package is transitive that major may not even be
reachable: nanoid arrives under postcss, pinned `^3.3.11`, a range 5.x cannot
satisfy. So the target is the lowest published fix above the version in use.
Fixes on older branches are ignored rather than offered as a downgrade.

A target on a different major version is still labelled `MAJOR`, but now that
label means something: no fix exists on the current branch, so the upgrade is
genuinely a different job - it can be ESM-only, or drop an API.