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High, CVSS 7.5amended

js-yaml: YAML merge-key chains can force quadratic CPU consumption

CVE-2026-59869GHSA-52cp-r559-cp3m

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (js-yaml 4.3.0, 3.15.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm js-yaml

    Affected
    >= 3.0.0, < 3.15.0>= 4.0.0, < 4.3.0
    Fixed in
    3.15.0, 4.3.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS
  • CVE-2026-59869: 0.58% estimated exploitation activity in the next 30 days; percentile 48.03 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Impact js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly. The issue is triggered by a chain of mappings where each mapping merges the previous one: For each new mapping, the loader has to enumerate the keys inherited from the previous mapping. With N chained mappings, this results in roughly 1 + 2 + ... + N merged-key visits, i.e., O(N^2) work for O(N) input size. PoC From N = 4000 delay become 1s (doc size < 100K) Patches Fix released. The most robust protection is to limit the total number of merged keys per parse call. This should close all past and future edge cases with merge. The default 10K-key limit should be okay in most cases.

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-52cp-r559-cp3m

  2. Revision 1 · created ·

    Published from osv record GHSA-52cp-r559-cp3m

Agent brief

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# Patch Next advisory brief: CVE-2026-59869

Advisory: js-yaml: YAML merge-key chains can force quadratic CPU consumption
IDs: CVE-2026-59869, GHSA-52cp-r559-cp3m
Record: https://patchnext.com/advisories/cve-2026-59869 (revision 2, source checked 26 Oct 2026, 12:57 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: js-yaml 4.3.0, 3.15.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm js-yaml: affected >= 3.0.0, < 3.15.0; >= 4.0.0, < 4.3.0; fixed in 3.15.0, 4.3.0

## Suggested inspection (read-only)
- `npm ls js-yaml --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on js-yaml.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-52cp-r559-cp3m (OSV record GHSA-52cp-r559-cp3m lists affected ranges: npm js-yaml >=3.0.0 =4.0.0 <4.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-52cp-r559-cp3m (GitHub-reviewed advisory GHSA-52cp-r559-cp3m lists affected ranges: npm js-yaml >=3.0.0 =4.0.0 <4.3.0)

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