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High, CVSS 7.5amended

js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources

CVE-2026-84375GHSA-2883-xcg3-v3hh

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (js-yaml 4.3.2, 3.15.2), 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
    >= 4.0.0, < 4.3.2>= 3.0.0, < 3.15.2
    Fixed in
    4.3.2, 3.15.2

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-2026-84375: 0.54% estimated exploitation activity in the next 30 days; percentile 45.54 (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.

Summary maxTotalMergeKeys does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit. Example For every target, the loader iterates all N elements of arr. This results in O(N K) work while totalMergeKeys remains unchanged. PoC Observed results: | N | YAML size | Time | |---:|---:|---:| | 800 | ~13 KB | ~20 ms | | 3200 | ~50 KB | ~180 ms | | 20000 | ~500 KB | ~13 s | Impact An attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default maxTotalMergeKeys limit. Fix Count each merge-source mapping as one budget unit, in addition to counting its keys. Difference with v5 In v3 & v4, merge is enabled by default. So, the severity score is higher.

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-2883-xcg3-v3hh

  2. Revision 2 · amended ·

    Amended from osv record GHSA-2883-xcg3-v3hh

  3. Revision 1 · created ·

    Published from ghsa record GHSA-2883-xcg3-v3hh

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-84375

Advisory: js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources
IDs: CVE-2026-84375, GHSA-2883-xcg3-v3hh
Record: https://patchnext.com/advisories/cve-2026-84375 (revision 3, source checked 26 Oct 2026, 13:02 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 4.3.2, 3.15.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm js-yaml: affected >= 4.0.0, < 4.3.2; >= 3.0.0, < 3.15.2; fixed in 4.3.2, 3.15.2

## 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://github.com/advisories/GHSA-2883-xcg3-v3hh (GitHub-reviewed advisory GHSA-2883-xcg3-v3hh lists affected ranges: npm js-yaml >=4.0.0 =3.0.0 <3.15.2)
- Verified: https://osv.dev/vulnerability/GHSA-2883-xcg3-v3hh (OSV record GHSA-2883-xcg3-v3hh lists affected ranges: npm js-yaml >=4.0.0 =3.0.0 <3.15.2 (fixed))

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