convex-authz

SKILL.md

Convex Authz Auditor/Hardener

A focused authz specialist, not a general reviewer: it finds and fixes the four shapes that account for the largest real-defect cluster measured against generated Convex backends (25 identity-from-arg + 13 missing-ownership-check + 6 PII-leak-by-argument = 44 of 214 confirmed defects, plus the parent-reference-on-write variant of the ownership shape that fixture measurement showed the 3-shape scan misses). It runs a deterministic scan first (objective, regex-based), then applies the canonical requireIdentity/requireOwner hardening pattern from convex-expert.md to every hit, then verifies with tsc. It does not re-derive the pattern — it applies the one already documented as the platform's canonical fix.

Workflow

MANDATORY FIRST STEP — check the auth foundation exists before injecting any ctx.auth enforcement: (1) is there an auth.config.ts with a provider? (2) is there a users/identities table keyed to the auth subject (tokenIdentifier/identity.subject)? If EITHER is missing, DO NOT add requireIdentity/requireOwner — on a foundationless app ctx.auth.getUserIdentity() always returns null (enforcement is non-functional: every call 401s, or worse, the check is bypassed/miscompared against a non-subject field like an email string) and a reviewer correctly flags that as a NEW authz defect, not a fix. Instead, on a foundationless app: (a) for privileged/admin operations, convert the public query/mutation to internalQuery/internalMutation (removes public reachability entirely — safe and foundation-free, no ctx.auth needed), and (b) tell the user: 'this app has no auth foundation; run /add auth or the auth setup first, then re-run convex-authz to add per-user ownership checks.' Do not run steps 1-3 below against public functions on a foundationless app beyond this internalize-and-defer move. Only when the foundation exists (both auth.config.ts and a subject-keyed users table are present) do you proceed to inject requireIdentity/requireOwner in steps 1-3.

SCAN (deterministic, objective-first): for every convex/**/*.ts file (skip convex/_generated/ and .d.ts), grep for the four shapes:
(a) identity-from-arg: a public query(/mutation( object whose args block declares userId/actorId/ownerId/authorId/accountId typed v.id(...), where the function's whole block (args + handler) has zero ctx.auth reference. Regex: /\b(userId|actorId|ownerId|authorId|accountId)\s*:\s*v\.id\(/ inside an args: { ... } block paired with an absent /\bctx\.auth\b/ anywhere in the enclosing (query|mutation)\(\s*\{ ... } block (word-boundary excludes internalQuery/internalMutation by construction).
(b) missing-ownership-check: a public query(/mutation( whose handler loads a document via ctx.db.get(args.<xId>) (an _id-typed arg) and then calls ctx.db.patch/ctx.db.delete/ctx.db.replace on that same id, or returns the doc's fields directly, with no comparison of any <doc>.<ownerField> against an identity value anywhere in the block (no ===/!== involving identity.subject or a ctx.auth derived value).
(c) PII-leaking public query: a public query( whose returns (or the raw doc it returns) includes a sensitive-looking field (email, revenue, ssn, password, token, auditLog, dashboard-shaped aggregate) and the query is parameterized by a client-supplied id with no ctx.auth check gating access to that id's own scope.
(d) parent-reference ownership on write: a public mutation( whose args include a v.id(...) of a parent/container table (projectId, boardId, teamId, orgId, listId, folderId, conversationId, accountId, ...) that the handler uses as a foreign key in a ctx.db.insert/ctx.db.patch — attaching or moving a child row into that container — without verifying the caller owns (or is a member of) the referenced parent doc. Creating a row inside someone else's container is the same defect as mutating their row: fixing WHO the caller is (shape a) does not fix WHERE they may write. After handling shapes a-c, re-audit every REMAINING v.id(...) arg in every public mutation for this shape — shape-a fixes routinely leave the parent id arg behind, still unchecked.
Report every hit with file, line, and which of the 4 shapes matched — this is the objective, model-independent baseline; do not skip it in favor of jumping straight to judgment.

HARDEN (foundation-having apps only — see step 0): for each hit, apply the canonical pattern from content/convex-expert.md verbatim — do not invent a new helper. Add (if absent) convex/model/auth.ts exporting requireIdentity(ctx) (throws 401 if ctx.auth.getUserIdentity() is null; returns the identity) and requireOwner(ctx, doc) (throws 404 if doc is null, throws 403 if doc.ownerId !== identity.subject, else returns doc). Rewrite each flagged function: replace the client-supplied identity arg with requireIdentity(ctx); wrap each _id-keyed read/mutate with requireOwner(ctx, await ctx.db.get(args.xId)) before touching the row; scope each PII-returning query through requireIdentity/requireOwner (or an explicit staff/role check) before it reads outside the caller's own scope; for each shape-(d) hit, load the referenced parent doc and apply requireOwner(ctx, parent) (or the schema's membership check — e.g. participantIds.includes(user._id) — when the container models members as an array) BEFORE inserting/patching the child row. When the schema keys ownership by a users row id rather than the raw subject, resolve the caller's users row first (via the subject-keyed index) and compare against user._id — comparing an Id<"users"> field to identity.subject never matches and silently breaks enforcement. Never widen scope — an internal/admin function that legitimately operates on an arbitrary user stays internalQuery/internalMutation, never public; leave it unflagged and unchanged.

VERIFY: run npx tsc --noEmit (or the project's typecheck script) after edits; a hardening pass that doesn't typecheck is not done. Then re-run the step-1 scan to confirm 0 remaining hits (the fixed shapes no longer match the regexes because ctx.auth now appears in-block and ownership comparisons now exist).

Report findings grouped by the 4 rule shapes with file:line, explain why each is exploitable (who could impersonate whom / read whose data), and show the concrete diff applied (or, on a foundationless app, the internalize-and-defer diff plus the auth-setup nudge) — never just describe the fix in prose.