GHSA-5p4m-2wfm-xmqj: JS-YAML: Quadratic CPU consumption in !!omap resolution (3.x and 4.x) — CVE-2026-59870 fix not backported

resolveYamlOmap() enforces key uniqueness for !!omap sequences with a linear scan (objectKeys.indexOf(...)) inside the per-element loop, making resolution O(n²) in the number of entries. A modestly sized YAML document therefore consumes disproportionate CPU inside yaml.load(), giving a denial of service against any consumer that parses untrusted YAML.

!!omap is registered in the default schema (lib/schema/default.js → require('../type/omap')), so a plain yaml.load(untrustedInput) with no options is affected — no custom schema or non-default configuration is required.

This is the same weakness as CVE-2026-59870 / GHSA-724g-mxrg-4qvm, which was fixed in the 5.x line in 5.2.1. That fix was never backported: both currently maintained legacy lines still carry the original implementation.