Update dependency js-yaml to v4.3.0 [SECURITY]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
js-yaml	4.2.0 → 4.3.0

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JS-YAML: Quadratic-complexity DoS in merge key handling via repeated aliases

CVE-2026-53550 / GHSA-h67p-54hq-rp68

More information

Details

Summary

A crafted YAML document can trigger algorithmic CPU exhaustion in js-yaml merge-key processing (<<) by repeating the same alias many times in a merge sequence.
This causes quadratic parse-time behavior relative to input size and can block a Node.js worker/event loop for seconds with a relatively small payload (tens of KB), resulting in denial of service.

Details

The issue is in merge handling inside lib/loader.js:

• storeMappingPair(...) iterates every element of a merge sequence when key tag is tag:yaml.org,2002:merge.
• For each element, it calls mergeMappings(...).
• mergeMappings(...) computes Object.keys(source) and performs _hasOwnProperty.call(destination, key) checks for each key.

When input is of the form:

a: &a {k0:0, k1:0, ..., kK:0}
b: {<<: [*a, *a, *a, ... repeated M times ...]}
all *a entries refer to the same anchored object. After the first merge, subsequent merges are semantically no-ops, but the parser still reprocesses all keys each time.
Resulting work is O(K * M), while input size is O(K + M), giving quadratic scaling as payload grows.
Relevant code path:
lib/loader.js in storeMappingPair(...) merge branch (keyTag === 'tag:yaml.org,2002:merge')
lib/loader.js mergeMappings(...)

Root cause

File: lib/loader.js
Function: storeMappingPair(state, _result, overridableKeys, keyTag, keyNode,
valueNode, startLine, startLineStart, startPos)
Lines: ~359-366

if (keyTag === 'tag:yaml.org,2002:merge') {
  if (Array.isArray(valueNode)) {
    for (index = 0, quantity = valueNode.length; index < quantity; index += 1) {
      mergeMappings(state, _result, valueNode[index], overridableKeys);
    }
  } else {
    mergeMappings(state, _result, valueNode, overridableKeys);
  }
}

When the merge value is a sequence (YAML 1.1 <<: [ *a, *a, ... ]), each element
is handed to mergeMappings() without deduplication. mergeMappings() then does

sourceKeys = Object.keys(source);
for (index = 0; index < sourceKeys.length; index += 1) {
  key = sourceKeys[index];
  if (!_hasOwnProperty.call(destination, key)) {
    setProperty(destination, key, source[key]);
    overridableKeys[key] = true;
  }
}

Every alias reference in the sequence resolves (by design) to the SAME object
via state.anchorMap. After the first merge, every subsequent merge of that same
reference is a pure no-op semantically, but still performs:

• one Object.keys(source) call (O(K))
• K _hasOwnProperty.call checks on the destination

Total: M * K hasOwnProperty checks + M Object.keys allocations, while the final
object and all observable side effects are identical to a single merge.

YAML semantics for <<: are idempotent and commutative over duplicate sources,
so collapsing duplicates preserves behavior exactly; this isn't a spec trade-off.

PoC

Environment:
js-yaml version: 4.1.1
Node.js: v24.5.0
Platform: arm64 macOS (reproduced consistently)
Reproduction script:
Create many keys in one anchored map (&a).
Merge that same alias repeatedly via <<: [*a, *a, ...].
Measure parse time and compare with control payload using single merge (<<: *a).
Observed repeated runs (same machine):
K=M=1000, input 9,909 bytes: ~33–36 ms
K=M=2000, input 20,909 bytes: ~121–123 ms
K=M=4000, input 42,909 bytes: ~524–537 ms
K=M=6000, input 64,909 bytes: ~1,608–1,829 ms
K=M=8000, input 86,909 bytes: ~3,395–3,565 ms
Control (single merge, similar key counts):
K=2000: ~1–2 ms
K=4000: ~3 ms
K=8000: ~5 ms
Also verified: repeated-merge output equals single-merge output (same key count and same JSON), confirming excess time is redundant computation.

Impact

This is a denial-of-service vulnerability (CPU exhaustion / algorithmic complexity).
Any service parsing untrusted YAML with js-yaml can be impacted, including API backends, CI tools, config processors, and automation services. An attacker can submit crafted YAML to significantly increase CPU time and reduce availability.

Suggested fix:

Dedupe the merge source list by reference before invoking mergeMappings. Any of
the following are minimal and preserve YAML 1.1 merge semantics:

dedupe in storeMappingPair:

if (keyTag === 'tag:yaml.org,2002:merge') {
  if (Array.isArray(valueNode)) {
    var seen = new Set();
    for (index = 0, quantity = valueNode.length; index < quantity; index += 1) {
      var src = valueNode[index];
      if (seen.has(src)) continue;   // idempotent; skip redundant alias
      seen.add(src);
      mergeMappings(state, _result, src, overridableKeys);
    }
  } else {
    mergeMappings(state, _result, valueNode, overridableKeys);
  }
}

Severity

• CVSS Score: 5.3 / 10 (Medium)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References

• https://github.com/nodeca/js-yaml/security/advisories/GHSA-h67p-54hq-rp68
• https://nvd.nist.gov/vuln/detail/CVE-2026-53550
• https://github.com/advisories/GHSA-h67p-54hq-rp68

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

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js-yaml: YAML merge-key chains can force quadratic CPU consumption

CVE-2026-59869 / GHSA-52cp-r559-cp3m

More information

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).

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Release Notes

nodeca/js-yaml (js-yaml)

v4.3.0

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