CVE-2026-84375

High

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.

SeverityHigh

CVSS7.5

Exploit probability—

ProductJs-yaml

PublishedSep 1, 2026

First seenSep 1, 2026

Weakness typeCWE-400Uncontrolled resource consumption CWE-407CWE-407

How to fix

Update

js-yaml4.3.2, 3.15.2 or later

Affected products

Product	Affected versions	Fixed in
js-yamlnpm	4.0.0 ≤ v < 4.3.2, 3.0.0 ≤ v < 3.15.2	4.3.2, 3.15.2

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