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CVE-2026-59869: js-yaml: YAML merge-key chains can force quadratic CPU consumption

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-07-08 · updated 2026-10-25
• Weakness: CWE-407

Description

js-yaml is a JavaScript YAML parser and dumper. From 3.0.0 before 3.15.0 and from 4.0.0 before 4.3.0, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly when a chain of mappings uses merge keys where each mapping merges the previous one. This issue is fixed in versions 3.15.0 and 4.3.0.

Affected products

• nodeca js-yaml: from 3.0.0 before 3.15.0; from 4.0.0 before 4.3.0 — fixed in 3.15.0, 4.3.0
• Npm Js-Yaml: from 3.0.0 before 3.15.0; from 4.0.0 before 4.3.0 — fixed in 3.15.0, 4.3.0

How to fix

Upgrade to a fixed version: nodeca js-yaml 3.15.0, 4.3.0; Npm Js-Yaml 3.15.0, 4.3.0. Check the vendor advisory for mitigations.

References

• https://github.com/nodeca/js-yaml/commit/24f13e79ee1343a7e30bd6f6c9d9cdbf0ac9b2b7
• https://github.com/nodeca/js-yaml/commit/59423c6f8cdc78742ac00e25a4dd39ef16b702e4
• https://github.com/nodeca/js-yaml/releases/tag/3.15.0
• https://github.com/nodeca/js-yaml/releases/tag/4.3.0
• https://github.com/nodeca/js-yaml/security/advisories/GHSA-52cp-r559-cp3m
• https://github.com/nodeca/js-yaml/security/advisories/GHSA-52cp-r559-cp3m

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