feat: validate values for cache-control and content-type headers in dev mode

.changeset/rich-pants-beam.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: validate values for `cache-control` and `content-type` headers in dev mode

packages/kit/src/runtime/server/respond.js

@@ -33,8 +33,10 @@ import { INVALIDATED_PARAM, TRAILING_SLASH_PARAM } from '../shared.js';
 import { get_public_env } from './env_module.js';
 import { load_page_nodes } from './page/load_page_nodes.js';
 import { get_page_config } from '../../utils/route_config.js';
+import { validateHeaders } from './validate-headers.js';
 
 /* global __SVELTEKIT_ADAPTER_NAME__ */
+/* global __SVELTEKIT_DEV__ */
 
 /** @type {import('types').RequiredResolveOptions['transformPageChunk']} */
 const default_transform = ({ html }) => html;
@@ -186,6 +188,10 @@ export async function respond(request, options, manifest, state) {
 		request,
 		route: { id: route?.id ?? null },
 		setHeaders: (new_headers) => {
+			if (__SVELTEKIT_DEV__) {
+				validateHeaders(new_headers);
+			}
+
 			for (const key in new_headers) {
 				const lower = key.toLowerCase();
 				const value = new_headers[key];

packages/kit/src/runtime/server/validate-headers.js

@@ -0,0 +1,64 @@
+/** @type {Set<string>} */
+const VALID_CACHE_CONTROL_DIRECTIVES = new Set([
+	'max-age',
+	'public',
+	'private',
+	'no-cache',
+	'no-store',
+	'must-revalidate',
+	'proxy-revalidate',
+	's-maxage',
+	'immutable',
+	'stale-while-revalidate',
+	'stale-if-error',
+	'no-transform',
+	'only-if-cached',
+	'max-stale',
+	'min-fresh'
+]);
+
+const CONTENT_TYPE_PATTERN =
+	/^(application|audio|example|font|haptics|image|message|model|multipart|text|video|x-[a-z]+)\/[-+.\w]+$/i;
+
+/** @type {Record<string, (value: string) => void>} */
+const HEADER_VALIDATORS = {
+	'cache-control': (value) => {
+		const error_suffix = `(While parsing "${value}".)`;
+		const parts = value.split(',').map((part) => part.trim());
+		if (parts.some((part) => !part)) {
+			throw new Error(`\`cache-control\` header contains empty directives. ${error_suffix}`);
+		}
+
+		const directives = parts.map((part) => part.split('=')[0].toLowerCase());
+		const invalid = directives.find((directive) => !VALID_CACHE_CONTROL_DIRECTIVES.has(directive));
+		if (invalid) {
+			throw new Error(
+				`Invalid cache-control directive "${invalid}". Did you mean one of: ${[...VALID_CACHE_CONTROL_DIRECTIVES].join(', ')}? ${error_suffix}`
+			);
+		}
+	},
+
+	'content-type': (value) => {
+		const type = value.split(';')[0].trim();
+		const error_suffix = `(While parsing "${value}".)`;
+		if (!CONTENT_TYPE_PATTERN.test(type)) {
+			throw new Error(`Invalid content-type value "${type}". ${error_suffix}`);
+		}
+	}
+};
+
+/**
+ * @param {Record<string, string>} headers
+ */
+export function validateHeaders(headers) {
+	for (const [key, value] of Object.entries(headers)) {
+		const validator = HEADER_VALIDATORS[key.toLowerCase()];
+		try {
+			validator?.(value);
+		} catch (error) {
+			if (error instanceof Error) {
+				console.warn(`[SvelteKit] ${error.message}`);
+			}
+		}
+	}
+}

packages/kit/src/runtime/server/validate-headers.spec.js

@@ -0,0 +1,99 @@
+import { describe, test, expect, beforeEach, vi } from 'vitest';
+import { validateHeaders } from './validate-headers.js';
+
+describe('validateHeaders', () => {
+	const console_warn_spy = vi.spyOn(console, 'warn');
+
+	beforeEach(() => {
+		vi.resetAllMocks();
+	});
+
+	describe('cache-control header', () => {
+		test('accepts valid directives', () => {
+			validateHeaders({ 'cache-control': 'public, max-age=3600' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+
+		test('rejects invalid directives', () => {
+			validateHeaders({ 'cache-control': 'public, maxage=3600' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('Invalid cache-control directive "maxage"')
+			);
+		});
+
+		test('rejects empty directives', () => {
+			validateHeaders({ 'cache-control': 'public,, max-age=3600' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('`cache-control` header contains empty directives')
+			);
+
+			validateHeaders({ 'cache-control': 'public, , max-age=3600' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('`cache-control` header contains empty directives')
+			);
+		});
+
+		test('accepts multiple cache-control values', () => {
+			validateHeaders({ 'cache-control': 'max-age=3600, s-maxage=7200' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+	});
+
+	describe('content-type header', () => {
+		test('accepts standard content types', () => {
+			validateHeaders({ 'content-type': 'application/json' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+
+		test('accepts content types with parameters', () => {
+			validateHeaders({ 'content-type': 'text/html; charset=utf-8' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+
+			validateHeaders({ 'content-type': 'application/javascript; charset=utf-8' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+
+		test('accepts vendor-specific content types', () => {
+			validateHeaders({ 'content-type': 'x-custom/whatever' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+
+		test('rejects malformed content types', () => {
+			validateHeaders({ 'content-type': 'invalid-content-type' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('Invalid content-type value "invalid-content-type"')
+			);
+		});
+
+		test('rejects invalid content type categories', () => {
+			validateHeaders({ 'content-type': 'invalid/type; invalid=param' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('Invalid content-type value "invalid/type"')
+			);
+
+			validateHeaders({ 'content-type': 'bad/type; charset=utf-8' });
+			expect(console_warn_spy).toHaveBeenCalledWith(
+				expect.stringContaining('Invalid content-type value "bad/type"')
+			);
+		});
+
+		test('handles case-insensitive content-types', () => {
+			validateHeaders({ 'content-type': 'TEXT/HTML; charset=utf-8' });
+			expect(console_warn_spy).not.toHaveBeenCalled();
+		});
+	});
+
+	test('allows unknown headers', () => {
+		validateHeaders({ 'x-custom-header': 'some-value' });
+		expect(console_warn_spy).not.toHaveBeenCalled();
+	});
+
+	test('handles multiple headers simultaneously', () => {
+		validateHeaders({
+			'cache-control': 'max-age=3600',
+			'content-type': 'text/html',
+			'x-custom': 'value'
+		});
+		expect(console_warn_spy).not.toHaveBeenCalled();
+	});
+});

packages/kit/test/apps/dev-only/src/routes/headers/invalid/+server.js

@@ -0,0 +1,9 @@
+/** @type {import("@sveltejs/kit").RequestHandler} */
+export function GET({ setHeaders }) {
+	setHeaders({
+		'cache-control': 'totally-invalid',
+		'content-type': 'not-a-real-type'
+	});
+
+	return new Response('Testing invalid headers');
+}