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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

No dated EPSS signal available.

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 When merge keys are enabled, an attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default maxTotalMergeKeys limit. Fix Count every merge source mapping as one budget unit in addition to counting its keys.

Published by the source

29 Sep 2026, 17:57 UTC

Last modified by the source

29 Sep 2026, 18:15 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-r3ph-w7gj-g6xm

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-r3ph-w7gj-g6xm

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-r3ph-w7gj-g6xm

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# Patch Next advisory brief: GHSA-r3ph-w7gj-g6xm

Advisory: js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources
IDs: GHSA-r3ph-w7gj-g6xm
Record: https://patchnext.com/advisories/ghsa-r3ph-w7gj-g6xm (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: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: js-yaml 5.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm js-yaml: affected >= 5.0.0, <= 5.4.0; fixed in 5.4.1

## 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-r3ph-w7gj-g6xm (GitHub-reviewed advisory GHSA-r3ph-w7gj-g6xm lists affected ranges: npm js-yaml >=5.0.0 <=5.4.0)
- Verified: https://osv.dev/vulnerability/GHSA-r3ph-w7gj-g6xm (OSV record GHSA-r3ph-w7gj-g6xm lists affected ranges: npm js-yaml >=5.0.0 <5.4.1 (fixed))

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