CVE-2026-53550: nodeca js-yaml denial of service in merge key handling

CVE-2026-53550 · Severity: medium · CVSS 5.3 · Published 2026-06-22

js-yaml is a popular JavaScript library used to read and write YAML files, commonly used in web servers and configuration tools. A vulnerability in how it handles specific data structures allows an attacker to send a small, specially crafted file that forces the server to perform excessive calculations. This can cause the application to become unresponsive or crash, leading to a denial of service for legitimate users.

Technologies: js-yaml (npm). Vendors: npm.

Quick answers

Is CVE-2026-53550 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-53550 being exploited in the wild.
How severe is CVE-2026-53550?
Medium severity, with a CVSS score of 5.3, published 22 June 2026.
Which products and versions are affected?
js-yaml (npm) <= 4.1.1, SEMVER:introduced=4.0.0,fixed=4.2.0.
Is there a fix for CVE-2026-53550?
Yes. js-yaml (npm): upgrade to 4.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-53550 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-53550?
Check whether you run js-yaml (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • js-yaml (npm): upgrade to 4.2.0 or later
    npm install js-yaml@4.2.0

Technical details

A denial-of-service vulnerability exists in js-yaml's merge-key (<<) processing within lib/loader.js. The root cause is inefficient algorithmic complexity (CWE-407) where the storeMappingPair function fails to deduplicate aliases in a merge sequence. When a YAML document repeats the same anchored alias multiple times, the parser redundantly executes mergeMappings, performing repeated Object.keys() allocations and hasOwnProperty checks. This results in O(K * M) complexity (where K is the number of keys and M is the number of merges) instead of linear scaling. An attacker can exploit this by submitting a small payload (tens of KB) to block the Node.js event loop for several seconds. The issue is resolved in version 4.2.0 by implementing reference-based deduplication during merge handling.

Affected products

  • nodeca js-yaml < 4.2.0

Identifiers

  • CVE-2026-53550
  • GHSA-h67p-54hq-rp68

Timeline

  • 2026-05-31: advisory: GitHub Security Advisory published by maintainers
  • 2026-06-22: disclosed: NVD publication date

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.