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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-01 · updated 2026-09-28
Weakness: CWE-400

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.

Affected products

Npm Js-Yaml: from 4.0.0 before 4.3.2; from 3.0.0 before 3.15.2 — fixed in 4.3.2, 3.15.2
nodeca js-yaml: from 3.0.0, < 3.15.2; from 4.0.0, < 4.3.2; from 5.0.0, < 5.4.1

How to fix

Upgrade to a fixed version: Npm Js-Yaml 4.3.2, 3.15.2. Check the vendor advisory for mitigations.

References

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