Denial of Service via crafted YAML merge keys

Summary

js-yaml is a JavaScript YAML parser and dumper. Prior to 4.2.0 and 3.15.0, a crafted YAML document can trigger algorithmic CPU exhaustion in js-yaml merge-key processing (<<) by repeating the same alias many times in a merge sequence. This causes quadratic parse-time behavior relative to input size and can block a Node.js worker/event loop for seconds with a relatively small payload (tens of KB), resulting in denial of service. The issue is in merge handling inside lib/loader.js. This vulnerability is fixed in 4.2.0 and 3.15.0. A flaw was found in js-yaml, a JavaScript YAML parser and dumper. A remote attacker can exploit this vulnerability by providing a specially crafted YAML document that repeatedly uses the same alias in a merge sequence. This can lead to algorithmic CPU exhaustion, causing the Node.js worker or event loop to be blocked for an extended period, resulting in a denial of service (DoS) for the affected system. Red Hat rates this flaw as Moderate impact, consistent with the upstream CVEORG assessment. The vulnerability causes CPU exhaustion via quadratic merge-key processing in js-yaml's YAML parser. In Red Hat products, all confirmed vulnerable code paths run client-side in the browser (OpenShift Console plugins, Kiali frontend, monitoring dashboard editors). A denial of service is limited to freezing the user's own browser tab when processing crafted YAML input, not server-side resource exhaustion. The user can recover by closing the tab. Red Hat severity: Moderate — CVSS 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L). Weakness: CWE-1333. Fixed by RHSA-2026:33866 — update the affected packages (`sudo dnf update`). Affected Red Hat products: Red Hat Hardened Images.