#Locale

Examples use Indian conventions: ₹ amounts, IST, dd/mm/yyyy, Aadhaar and DPDP Act where a standard mentions identity or privacy law. Keep them when you copy an example.

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#Purpose

Rules for validating input on the server. The principle is one line: anything that crosses a trust boundary is parsed into a known type before any code acts on it.

Client-side validation is a user-experience feature. It provides no security — the client is under the attacker's control. Every rule here is server-side.

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#Parse, do not validate

Validation that returns a boolean leaves you holding the same untyped value. Parsing returns a new, typed value that cannot be wrong further down.

ts
const CreateOrder = z.object({
  customerId: z.string().uuid(),
  items: z.array(z.object({
    sku: z.string().regex(/^[A-Z0-9-]{3,32}$/),
    qty: z.number().int().min(1).max(999),
  })).min(1).max(100),
  note: z.string().max(500).optional(),
  currency: z.enum(["EUR", "USD", "GBP"]),
}).strict();                                   // reject unknown keys

app.post("/v1/orders", (req, res) => {
  const parsed = CreateOrder.safeParse(req.body);
  if (!parsed.success) return res.status(422).json(toFieldErrors(parsed.error));
  return createOrder(parsed.data);             // typed from here down
});

After this line, parsed.data.qty is a bounded integer by construction. No downstream function needs to re-check it, and the type system enforces that.

Tooling: zod/valibot/typebox (TypeScript), pydantic (Python), go-playground/validator (Go), Bean Validation (Java), dry-schema (Ruby). Where you already publish an OpenAPI document, generate the validator from it so the spec and the check cannot disagree. → API/open-api

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#.strict() is the mass-assignment fix

ts
// Without .strict(): { "email": "…", "role": "admin", "credits": 999999 }
await db.user.update({ where: { id }, data: req.body });   // ← privilege escalation

Unknown fields must be rejected, not ignored, and an object must never be spread straight into an ORM write. Build the update from explicitly named fields:

ts
const { email, displayName } = parsed.data;
await db.user.update({ where: { id }, data: { email, displayName } });

This is the same defect as params.permit misuse in Rails and @ModelAttribute binding in Spring. It is consistently in the OWASP top ten and it is one line to prevent.

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#Allowlist everything, and bound everything

Never write a denylist. if (input.includes("<script>")) is bypassed by <ScRiPt>, <img onerror>, and a hundred other encodings. Enumerate what is allowed.

Sanitising by stripping characters is worse than rejecting: it produces a value that passed no check and matches nothing you specified. Reject, and say why.

Compressed request bodies need a decompressed-size cap as well — a 1 KB gzip payload can expand to gigabytes.

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#Shape, then business rules

Two distinct layers, and they belong in different places:

A uniqueness check is not shape validation. Checking "is this email taken?" before inserting is a race — two concurrent requests both see "free". The database constraint is the guarantee; the pre-check is only a nicer error message. → Database/schema-design

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#Validation is not encoding

Validated input is still untrusted in a different context. A name that is perfectly valid input is still dangerous when concatenated into SQL, a shell command, a file path, or HTML.

• Parameterise SQL. → Security/sql-injection
• Never build shell commands from input. → Security/command-injection
• Resolve and confine file paths. → Security/path-traversal
• Escape on output, per context. → Security/xss
• Block private address ranges when fetching a supplied URL (SSRF).

Validation reduces the surface. Context-correct encoding is what actually prevents injection.

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#Error responses

json
{ "code": "validation_failed", "message": "Validation failed", "requestId": "req_01J8Z",
  "errors": [
    { "field": "items.0.qty", "code": "out_of_range", "message": "Must be between 1 and 999." },
    { "field": "currency",    "code": "invalid_enum", "message": "Must be one of EUR, USD, GBP." }
  ] }

• Return all failures at once, not the first. Otherwise the client fixes one field per round trip.
• Use a path (items.0.qty) that identifies the exact field.
• Stable machine code per error; the human message may change freely.
• Never echo the rejected value back if it might be a credential. → Backend/error-handling

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#Anti-patterns

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#Checklist

• Verify: Every request body, query and path parameter is parsed against a schema
• Verify: Parsing produces a typed value used downstream
• Verify: Unknown fields are rejected, not ignored
• Verify: No request object is spread into a database write
• Verify: Every string, array and number has explicit bounds
• Verify: Enumerations are closed sets
• Verify: Body size, decompressed size and upload size are capped
• Verify: Upload types are detected from content, not from the filename
• Verify: Allowlists are used throughout; no denylist filtering
• Verify: Shape validation is at the edge; business rules are in the domain layer
• Verify: Uniqueness and invariants are enforced by database constraints
• Verify: Output encoding is applied per context, independently of validation
• Verify: Client-supplied URLs are SSRF-guarded before any fetch
• Verify: Validation errors list every failure with a field path and a stable code
• Verify: Rejected values are not echoed when they may be sensitive