CVE-2026-59870: js-yaml DoS — Exploit & Fix

Description

js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 before 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. This issue is fixed in version 5.2.1.

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CVSS v3.1 Score

5.3

MEDIUM

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

EPSS — Exploit Prediction

0.0041

Probability of exploitation

0.34%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-407 CWE-407

CWE-770 CWE-770

Affected Products

Vendor	Product	Versions
nodeca	js-yaml	5.0.0 — 5.2.1

References

Advisories & Patches

https://github.com/nodeca/js-yaml/commit/39f3211a2f01b3c6982710cf21434ab7060acefe https://github.com/nodeca/js-yaml/security/advisories/GHSA-724g-mxrg-4qvm https://github.com/nodeca/js-yaml/security/advisories/GHSA-724g-mxrg-4qvm

Exploits

https://github.com/nodeca/js-yaml/security/advisories/GHSA-724g-mxrg-4qvm https://github.com/nodeca/js-yaml/security/advisories/GHSA-724g-mxrg-4qvm

Other References

https://github.com/nodeca/js-yaml/releases/tag/5.2.1

Frequently Asked Questions

What is CVE-2026-59870? +

js-yaml is a JavaScript YAML parser and dumper. From 5.0.0 before 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. This issue is fixed in version 5.2.1. It has a CVSS v3.1 base score of 5.3 (MEDIUM).

How severe is CVE-2026-59870? +

CVE-2026-59870 has a CVSS v3.1 score of 5.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0041, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-59870? +

CVE-2026-59870 affects products from nodeca, specifically: js-yaml. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-59870? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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