stream-json is a micro-library of stream components for processing JSON and JSONC with a minimal memory footprint. Prior to 3.5.0, the path filters pick, ignore, filter, and replace in src/core/filters/filter-base.js recompute the full path string from the nesting stack for every checkable token. Because the stack length equals the current nesting depth and a checkable token is emitted at every level, a depth D document costs O(D²) rather than O(D) to process. The issue is triggered by nesting depth rather than byte volume, including the documented pick({filter: 'data'}) traversal-until-match path, so an application that sends untrusted JSON through a string or RegExp filter can block the Node.js event loop and cause denial of service with a small deeply nested document. The streamArray, streamObject, and streamValues streamers are not affected because they use the constant-time asm.depth getter. This issue is fixed in version 3.5.0.

Affected: npm: stream-json

## 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 f…