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Security: count each WebSocket fragment toward the message limit #582

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Merged
SandObserver merged 1 commit into from
Oct 27, 2026
Merged

Security: count each WebSocket fragment toward the message limit #582

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8 changes: 7 additions & 1 deletion api/src/websocket.js
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,9 @@ const crypto = require('crypto');
const GUID = '258EAFA5-E914-47DA-95CA-C5AB0DC85B11';
const MESSAGE_LIMIT = 4 * 1024 * 1024;
const WRITE_BACKLOG_LIMIT = 64 * 1024;
/* Each kept fragment costs heap whatever its length. Count at least this much
per fragment, or empty fragments grow the heap past the limit. */
const FRAGMENT_FLOOR = 1024;

const OP = { CONTINUATION: 0, TEXT: 1, BINARY: 2, CLOSE: 8, PING: 9, PONG: 10 };

Expand Down Expand Up @@ -103,6 +106,7 @@ function wrapSocket(socket, head, opts = {}) {
/** @type {Buffer[]} */
let parts = [];
let partsSize = 0;
let partsCost = 0;
let inMessage = false;

const finish = (/** @type {Error|null} */ err) => {
Expand Down Expand Up @@ -137,12 +141,14 @@ function wrapSocket(socket, head, opts = {}) {
inMessage = true;
} else throw protocolError('The server sent an unknown frame type.');
partsSize += payload.length;
if (partsSize > limit) throw tooLarge();
partsCost += Math.max(payload.length, FRAGMENT_FLOOR);
if (partsCost > limit) throw tooLarge();
parts.push(payload);
if (!fin) return;
const text = Buffer.concat(parts, partsSize).toString('utf8');
parts = [];
partsSize = 0;
partsCost = 0;
inMessage = false;
if (onMessage) onMessage(text);
else backlog.push(text);
Expand Down
13 changes: 13 additions & 0 deletions api/test/websocket.test.js
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Expand Up @@ -161,6 +161,19 @@ test('a message over the size limit ends the session', async t => {
);
});

test('empty continuation frames count toward the size limit', async t => {
const url = await wsServer(t, {
onMessage: (_, socket) => {
socket.write(serverFrame(OP.TEXT, '', false));
socket.write(Buffer.concat(Array.from({ length: 2000 }, () => serverFrame(OP.CONTINUATION, '', false))));
},
});
await assert.rejects(
() => jsonRpcUnchecked(url, { timeout: 3000, maxBytes: 1024 * 1024 }, s => s.call('x')),
e => e.responseTooLarge === true,
);
});

test('a masked frame from the server ends the session', async t => {
const url = await wsServer(t, { onMessage: (_, socket) => socket.write(encodeFrame(OP.TEXT, Buffer.from('{}'))) });
await assert.rejects(
Expand Down
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