fix: reject malformed streamed data encoding

.changeset/common-wings-doubt.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: reject malformed streamed data encoding

packages/kit/src/runtime/client/ndjson.js

@@ -6,7 +6,7 @@ import { read_stream } from './stream.js';
  * @param {ReadableStreamDefaultReader<Uint8Array>} reader
  */
 export async function* read_ndjson(reader) {
-	for await (const block of read_stream(reader, '\n')) {
+	for await (const block of read_stream(reader, '\n', { fatal: true })) {
 		const line = block.trim();
 		if (line) {
 			yield JSON.parse(line);

packages/kit/src/runtime/client/ndjson.spec.js

@@ -0,0 +1,48 @@
+import { expect, test } from 'vitest';
+import { read_ndjson } from './ndjson.js';
+
+const encoder = new TextEncoder();
+
+/**
+ * @param {Uint8Array[]} chunks
+ */
+function read(chunks) {
+	return read_ndjson(
+		new ReadableStream({
+			start(controller) {
+				for (const chunk of chunks) {
+					controller.enqueue(chunk);
+				}
+				controller.close();
+			}
+		}).getReader()
+	);
+}
+
+/**
+ * @param {Uint8Array[]} chunks
+ */
+async function collect(chunks) {
+	const values = [];
+	for await (const value of read(chunks)) {
+		values.push(value);
+	}
+	return values;
+}
+
+test('parses streamed data when UTF-8 code points are split across chunks', async () => {
+	const bytes = encoder.encode(`${JSON.stringify({ type: 'data', value: '\u00e9' })}\n`);
+	const chunks = Array.from(bytes, (byte) => new Uint8Array([byte]));
+
+	await expect(collect(chunks)).resolves.toEqual([{ type: 'data', value: '\u00e9' }]);
+});
+
+test('rejects malformed UTF-8 before parsing streamed data', async () => {
+	const chunks = [
+		encoder.encode('{"type":"data","nodes":[{"id":239,'),
+		new Uint8Array([0xff]),
+		encoder.encode('"url":240}]}\n')
+	];
+
+	await expect(collect(chunks)).rejects.toThrow(TypeError);
+});

packages/kit/src/runtime/client/stream.js

@@ -4,11 +4,12 @@
  * stream closes.
  * @param {ReadableStreamDefaultReader<Uint8Array>} reader
  * @param {string} delimiter
+ * @param {TextDecoderOptions} [options]
  */
-export async function* read_stream(reader, delimiter) {
+export async function* read_stream(reader, delimiter, options) {
 	let done = false;
 	let buffer = '';
-	const decoder = new TextDecoder();
+	const decoder = new TextDecoder(undefined, options);
 
 	while (true) {
 		let split = buffer.indexOf(delimiter);