IDOR Fix

IDOR occurs when an ID in a URL/body maps directly to a DB record with no ownership check. An attacker increments ?invoice_id=1042 to 1043 and gets someone else's data.

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Two Independent Fixes (Both Required)

1. Opaque IDs     → make guessing futile
2. Ownership check → deny access even if ID is known

Neither alone is sufficient. Sequential IDs with ownership checks work. UUIDs without ownership checks don't.

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Fix 1: Use Non-Guessable IDs

ID type	Guessable?	Recommendation
Auto-increment 1, 2, 3	✅ trivially	Replace
Short hash / base62	⚠️ low entropy	Replace
UUIDv4	❌ 122 bits entropy	Use this
ULID / UUIDv7	❌ + sortable	Use for time-ordered records
CUID2	❌ + collision-resistant	Good alternative

-- Postgres: use gen_random_uuid() as default
CREATE TABLE invoices (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  owner_id UUID NOT NULL REFERENCES users(id),
  ...
);

Keep numeric PKs internally if needed for joins/indexes — expose only the UUID externally.

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Fix 2: Ownership Check in Every Query

Never fetch by ID alone. Always scope to the authenticated caller.

// ❌ Authenticated but not authorized
const invoice = await db.invoices.findById(req.params.id);

// ✅ Ownership enforced at the DB layer
const invoice = await db.invoices.findOne({
  id: req.params.id,
  owner_id: req.user.id,   // from JWT — never from request body
});
if (!invoice) return res.status(404).send(); // 404, not 403

Return 404, not 403. Returning 403 confirms the resource exists — itself an information leak.

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Non-Obvious Patterns to Check

Indirect references (not just /:id)

IDOR isn't only in URL params — check every place user input selects a record:

// Email/username lookup — same risk
const user = await db.users.findOne({ email: req.body.email });
// Fix: scope to org or confirm intent is public lookup

// File download by filename
res.sendFile(`/uploads/${req.query.filename}`); // path traversal + IDOR
// Fix: lookup file record by name scoped to owner, serve from that path

Write paths (often missed)

// ❌ User can update any invoice by ID
await db.invoices.update({ id: req.body.invoice_id }, data);

// ✅
await db.invoices.update(
  { id: req.body.invoice_id, owner_id: req.user.id },
  data
);
// If 0 rows updated → 404

Nested resources — check the parent too

// ❌ Checks comment owner but not post owner
const comment = await db.comments.findOne({ id: req.params.cid, author_id: req.user.id });

// ✅ Verify the parent belongs to caller (if that's the model)
const post = await db.posts.findOne({ id: req.params.pid, owner_id: req.user.id });
if (!post) return res.status(404).send();
const comment = await db.comments.findOne({ id: req.params.cid, post_id: post.id });

Batch endpoints

// ❌ Deletes any IDs passed — no ownership check
await db.invoices.deleteMany({ id: { in: req.body.ids } });

// ✅ Scope batch operation to owner
await db.invoices.deleteMany({
  id: { in: req.body.ids },
  owner_id: req.user.id,
});

Multi-tenant: user-scoped isn't enough

// ❌ Scoped to user but crosses tenant boundary
const records = await db.records.findMany({ owner_id: req.user.id });

// ✅ Always include tenant scope
const records = await db.records.findMany({
  owner_id: req.user.id,
  tenant_id: req.user.tenant_id, // from token, never from request
});

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Migrating Sequential IDs to UUIDs

-- 1. Add UUID column
ALTER TABLE invoices ADD COLUMN public_id UUID DEFAULT gen_random_uuid();
UPDATE invoices SET public_id = gen_random_uuid() WHERE public_id IS NULL;
ALTER TABLE invoices ALTER COLUMN public_id SET NOT NULL;
CREATE UNIQUE INDEX ON invoices(public_id);

-- 2. Keep internal id for FK integrity
-- 3. Expose only public_id in API responses
-- 4. Accept only public_id in API inputs — look up internal id server-side

// API layer translates: public_id → internal id
const invoice = await db.invoices.findOne({ public_id: req.params.id, owner_id: req.user.id });

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Audit Checklist

• All externally exposed IDs are non-sequential (UUID/ULID/CUID2)
• Every findById / findOne / update / delete includes owner_id or tenant_id scope
• Write paths (POST/PUT/PATCH/DELETE) ownership-checked, not just reads
• Nested resource routes verify parent ownership before child access
• Batch operations scoped to owner — not just array of IDs
• File/asset endpoints look up record by name+owner, never serve by raw path
• 404 returned for unauthorized access (not 403)
• owner_id / tenant_id sourced from JWT — never from request body or query param