authz

Files (1)

• SKILL.md

SKILL.md (first lines)

# Poka-Yoke for Authorization

Cross-tenant data leaks are almost never caused by a wrong access-control decision. They are
caused by *no decision at all*: a query that is correct except it lacks `WHERE tenant_id = ?`,
an endpoint that loads by ID without checking who is asking. The developer did not choose
wrongly; they forgot, in one of the two hundred places the check was required.

That is the signature of a poka-yoke problem: a step that must be performed every single time,
by a human, with nothing enforcing it. The fix is never "be more careful in code review," and
it is never a checklist. **The fix is to make the unscoped query unwritable.**

## Building, not reviewing

Most of the time this mode is reached *while someone is building the thing*, not afterwards.
That changes the deliverable. They asked for the scoping, so produce the scoping, working, complete,
in their stack. Do not hand back a severity table when the person is mid-feature; a list of
findings about code they have not written yet is not useful to them.

Then add a short closing note, three or four lines, covering:

- which misuses the shape you chose makes impossible, and at which rung,
- what you left possible on purpose, and why that tradeoff is the right one here.

That closing note is what stops the device being undone in six months by someone who cannot
see why it is there. It is also the difference between mistake-proofing and a code generator:
the reasoning travels with the code.

When the code already exists and they are asking what is wrong with it, switch to the audit
voice, ranked findings with the mistake, the consequence, and the device. Match the mode to
where they are in the work, not to this file's default.

## The one principle: unsafe should be hard to say

Right now, in most codebases, the unsafe form is the *short* form:

```python
user = db.query(User).filter(User.id == user_id).first()          # unscoped: 1 line
user = db.query(User).filter(User.id == user_id,
                             User.tenant_id == current_tenant).first()   # safe: longer
```

Every incentive points at the first line, and it works perfectly in every test, because tests
usually have one tenant. Invert it so the safe form is the default and the unsafe form
requires deliberate, visible effort:

```python
user = tenant_db.users.get(user_id)     # tenant scope baked in; cannot be omitted
user = db.unscoped().users.get(user_id) # possible, greppable, reviewable, rare
```

Everything below is a variation on that inversion. When you audit, the question is not "is
this query scoped?" but "**could an unscoped query even be written here?**"

## Devices, strongest first

### 1. Database row-level security (Control, and the one with the widest reach)

RLS enforces the predicate in the database, so it applies to every query from every service,
every migration, every script, and every engineer with a psql shell. It is the only device
that protects you from code paths you did not write. Its reach stops only at roles that are
exempt from policies: superusers, roles with `BYPASSRLS`, and the table owner unless you force
the policy on.

```sql
ALTER TABLE documents ENABLE ROW LEVEL SECURITY;
ALTER TABLE documents FORCE ROW LEVEL SECURITY;   -- applies to the table owner too

CREATE POLICY tenant_isolation ON documents
  USING (tenant_id = current_setting('app.tenant_id')::uuid);
```

The catch that turns this into a false sense of security: the connection must set
`app.tenant_id` reliably, and a pooled connection that carries a previous request's setting is
a cross-tenant leak with extra steps. Set it per-transaction, and make the middleware that sets
it the only path to a connection. `FORCE ROW LEVEL SECURITY` matters too, without it the
table owner bypasses the policy, and your application user is often the owner.

### 2. Scoped repositories (Control at the type level)

Make the tenant a required constructor argument, so no repository exists without one:

```ts
class DocumentRepo {
  // No default. There is no way to construct this without a tenant.
  constructor(private readonly db: Db, private readonly tenant: TenantId) {}

  async byId(id: DocumentId): Promise<Document | null> {
    return this.db.documents.findFirst({ where: { id, tenantId: this.tenant } });
  }
}
```

The raw client is then confined to infrastructure code and lint-banned from handlers. The
device is not the `where` clause. It is that the handler has no way to reach a client that
lacks one.

### 3. Authorization in the type (Control)

Rather than loading an object and then checking it, make the check the only way to obtain it:

```ts
// Handlers accept Owned<Document>. There is no path to one that skips the check.
async function authorizeDocument(user: User, id: DocumentId): Promise<Owned<Document>>
```

A handler that takes `Owned<Document>` cannot receive an unauthorized document, so the check
cannot be forgotten: the compiler asks for it. This is the same move as parse-don't-validate,
applied to permission instead of shape.

### 4. Default-deny at the router (Control, cheap)

Require every route to declare its authorization explicitly, and refuse to start if any route
has not:

- A middleware that denies unless a route declares a policy, with a startup check that
  enumerates routes and fails the boot on any undeclared one. A new endpoint is then secure
  before anyone writes a line of it: the failure mode of forgetting becomes "the service
  won't start" rather than "the data is public."
- Public routes are explicitly marked. Making public the opt-in and private the default means

From rainmanjam/poka-yoke, MIT.

More from rainmanjam/poka-yoke