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Response snapshot Oct 27, 2026, 01:51 UTC

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VulnerabilityCVE-2026-114129

stream-json: FlexAssembler string/RegExp path filters join the full key stack per container start, giving O(depth^2) CPU denial of service

`FlexAssembler`, the custom-container assembler shipped in the `stream-json` package (`src/core/utils/flex-assembler.js`, re-exported at `stream-json/utils/flex-assembler.js` and `stream-json/web/utils/flex-assembler.js`), compiles a string or RegExp path `filter` into a matcher that runs `keyStack.

Published Updated

At a glance

Exploited
Not known
Severity
6.2 Medium (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: stream-json: 3.7.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-jj87-hc4r-jwfw (opens a new tab)

What happened

## Summary

`FlexAssembler`, the custom-container assembler shipped in the `stream-json` package (`src/core/utils/flex-assembler.js`, re-exported at `stream-json/utils/flex-assembler.js` and `stream-json/web/utils/flex-assembler.js`), compiles a string or RegExp path `filter` into a matcher that runs `keyStack.join(separator)` on every container start.

Because the key stack grows one entry per open object or array with no depth bound, a JSON document nested to depth D drives D container-start tokens that each perform an O(D) join, for O(D^2) total CPU on the single-threaded event loop.

An unauthenticated party who can supply JSON bytes to a pipeline that uses a `FlexAssembler` with a string or RegExp rule filter can stall the process with a small input: roughly 96 KB (depth 16000) blocks the event loop for over a second, and the cost grows quadratically.

Continue reading

## Root Cause

`FlexAssembler` reconstructs JavaScript values from the parser token stream and supports custom containers selected by path-matching rules. When a rule `filter` is a string or RegExp, `compileFilter` returns a matcher that computes the joined path from the full key stack before making any match decision. That join is O(current depth), and it is invoked unconditionally for every rule on every container start.

The key stack that feeds the join has no depth cap. The package does have a depth guard (`maxDepth`), but it exists only in the unrelated filter base state machine and never executes on the `FlexAssembler` code path, so nested input is unbounded here.

```text src/core/utils/flex-assembler.js compileFilter (lines 5-21): string branch computes `path.join(separator)` (line 10) before the `=== filter || startsWith(...)` comparison; RegExp branch computes `filter.test(path.join(separator))` (line 17). The join runs first, unconditionally. FlexAssembler._matchRule (lines 160-166): calls `rule.filter(this.

keyStack)` for every rule; invoked from startObject (line 184) and startArray (line 197) on every container start. FlexAssembler._pushState (lines 168-176): pushes one entry onto `keyStack` per open container with no depth bound. ```

## Impact

An attacker who can deliver JSON to a pipeline whose `FlexAssembler` is configured with a string or RegExp rule filter (a documented, tested, first-class option: `Filter = string | RegExp | FilterFunction`) can cause quadratic CPU consumption. The join runs regardless of whether the rule ultimately matches, so no attacker knowledge of the schema is required beyond nesting depth, which the attacker fully controls.

The demonstrated capability is an availability impact: a small crafted document blocks the Node.js event loop, stalling all concurrent work on the process. There is no confidentiality or integrity impact. A consumer that uses only a function filter or no rules is not affected, which narrows the vulnerable population but does not raise attack complexity once such a configuration is in use.

## Proof of Concept

`Node.js and the published stream-json@3.6.0 package. Drive a FlexAssembler configured with a string rule filter, comparing against a function filter as a negative control.`

1. Install `stream-json@3.6.0`. 2. Construct a deeply nested object token stream (or equivalently parse a document like `'{"k":'` repeated D times, a `1`, then `'}'` repeated D times, through `parser`/`parserStream`). 3. Feed it to a `FlexAssembler` whose `objectRules` contains a rule with a string `filter`. 4. Observe wall-clock time scaling as roughly 4x per doubling of D (quadratic), while a function-filter configuration stays flat.

```js import FlexAssembler from 'stream-json/utils/flex-assembler.js';

function run(filter, D) { const asm = new FlexAssembler(filter ? {objectRules: [{filter, create: () => ({})}]} : undefined); const t = performance.now(); asm.startObject(); for (let i = 0; i p.length > 0, D).toFixed(1), 'ms'); ```

```text string filter (path.join per token): function filter (negative control, no join): D=4000 91.9 ms D=4000 1.6 ms D=8000 341.9 ms D=8000 0.9 ms D=16000 1359.2 ms (~4x per doubling) D=16000 1.7 ms (flat) D=32000 5430.0 ms (= O(D^2)) D=32000 2.6 ms ```

End to end through a real `parserStream`, roughly 96 KB (depth 16000) of attacker-supplied JSON stalls the event loop for over 1.3 seconds.

## Attack Chain

1. **Exposure:** The attacker supplies JSON bytes to a service pipeline that assembles values with a `FlexAssembler` configured with a string or RegExp rule filter; `FlexAssembler` is a documented submodule of the published package on both the Node and Web Streams surfaces. 2. **Control:** The attacker controls the JSON bytes and therefore the nesting depth D, which directly controls the key stack length. 3.

**Path:** Each `{` or `[` emits `startObject`/`startArray`, which push onto `keyStack` and call `_matchRule`, which calls the compiled filter with the current key stack. 4. **Guard:** There is no depth or size cap on this path; the only `maxDepth` guard in the package lives in the filter base state machine and never runs in `FlexAssembler`. 5.

**Primitive:** For each of the D container starts, the compiled string/RegExp filter runs an O(D) `join` before any match decision, for O(D^2) total CPU. 6. **Result:** The single-threaded event loop stalls, denying service to all concurrent work on the process; a small input scales quadratically to sustained unavailability.

## Bypass Evidence

This is a novel finding on a code path that no prior fix protects, so there is no guard to bypass on the `FlexAssembler` path. The relevant prior fix is the `maxDepth` cap published as part of the remediation for CVE-2026-71429 (GHSA-528h-pc64-c93x), which addressed the analogous O(depth^2) behavior in the `pick`/`ignore`/`filter`/`replace` filters. That fix was placed only in `src/core/filters/filter-base.

js`; `src/core/utils/flex-assembler.js` neither imports nor invokes it, so the guard never executes here. A search for `maxDepth` across `src/` returns on

Affected package ranges reported by GitHub: stream-json: >= 2.0.0, <= 3.6.0

Scores and how they were measured

CVSS base severity

6.2

Version 3.1As of

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.2%

Scored Percentile 8.1%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-jj87-hc4r-jwfw (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jj87-hc4r-jwfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-114129, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 22). stream-json: FlexAssembler string/RegExp path filters join the full key stack per container start, giving O(depth^2) CPU denial of service (CVE-2026-114129). https://kevwire.com/record/CVE-2026-114129

MLA

"stream-json: FlexAssembler string/RegExp path filters join the full key stack per container start, giving O(depth^2) CPU denial of service (CVE-2026-114129)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-114129.

Chicago

KEVWire. "stream-json: FlexAssembler string/RegExp path filters join the full key stack per container start, giving O(depth^2) CPU denial of service (CVE-2026-114129)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-114129.

Evidence summary (plain text)

stream-json: FlexAssembler string/RegExp path filters join the full key stack per container start, giving O(depth^2) CPU denial of service (Oct 22, 2026).
`FlexAssembler`, the custom-container assembler shipped in the `stream-json` package (`src/core/utils/flex-assembler.js`, re-exported at `stream-json/utils/flex-assembler.js` and `stream-json/web/utils/flex-assembler.js`), compiles a string or RegExp path `filter` into a matcher that runs `keyStack.join(separator)` on every container start. Because the key stack grows one entry per open object or array with no depth bound, a JSON document nested to depth D drives D container-start tokens that each perform an O(D) join, for O(D^2) total CPU on the single-threaded event loop. An unauthenticated party who can supply JSON bytes to a pipeline that uses a `FlexAssembler` with a string or RegExp rule filter can stall the process with a small input: roughly 96 KB (depth 16000) blocks the event loop for over a second, and the cost grows quadratically.

`FlexAssembler` reconstructs JavaScript values from the parser token stream and supports custom containers selected by path-matching rules. When a rule `filter` is a string or RegExp, `compileFilter` returns a matcher that computes the joined path from the full key stack before making any match decision. That join is O(current depth), and it is invoked unconditionally for every rule on every container start. The key stack that feeds the join has no depth cap. The package does have a depth guard (`maxDepth`), but it exists only in the unrelated filter base state machine and never executes on the `FlexAssembler` code path, so nested input is unbounded here.
CVSS 6.2 (version 3.1; evidence as of Oct 22, 2026; https://github.com/advisories/GHSA-jj87-hc4r-jwfw).
EPSS 0.2% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-114129&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-jj87-hc4r-jwfw: https://github.com/advisories/GHSA-jj87-hc4r-jwfw
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNTo1Mjo1NlrOAAfxiA%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTozNlrOAAf0Jw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTowNVrOAAf0JQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0Mzo0NVrOAAf0Iw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0MzozMVrOAAf0Ig%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozNTowMVrOAAZRcw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozMDoyN1rOAAY_5Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODoyOTowNVrOAAfz8g%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-114129&date=2026-10-26
Record ID: CVE-2026-114129.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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