Is CVE-2026-59870 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-59870 being exploited in the wild.

How severe is CVE-2026-59870?

Medium severity, with a CVSS score of 5.3, published 8 July 2026.

Which products and versions are affected?

js-yaml (npm) >= 5.0.0, <= 5.2.0, SEMVER:introduced=5.0.0,fixed=5.2.1.

Is there a fix for CVE-2026-59870?

Yes. js-yaml (npm): upgrade to 5.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-59870 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-59870?

Check whether you run js-yaml (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-59870: nodeca js-yaml denial of service in YAML11_SCHEMA omap parsing

Q: Is there a fix for CVE-2026-59870?

Yes. js-yaml (npm): upgrade to 5.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

Q: How is CVE-2026-59870 attacked?

It is attacked over the network, in a software library.

Q: Am I affected by CVE-2026-59870?

Check whether you run js-yaml (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-59870 · Severity: medium · CVSS 5.3 · Published 2026-07-08

A performance vulnerability in the js-yaml library can allow an attacker to crash or slow down an application by sending a specially crafted YAML file. This library is commonly used to read configuration files and data; an exploit would block the application from processing other requests, leading to a denial of service.

Technologies: js-yaml (npm). Vendors: npm.

Quick answers

Is CVE-2026-59870 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-59870 being exploited in the wild.
How severe is CVE-2026-59870?: Medium severity, with a CVSS score of 5.3, published 8 July 2026.
Which products and versions are affected?: js-yaml (npm) >= 5.0.0, <= 5.2.0, SEMVER:introduced=5.0.0,fixed=5.2.1.
Is there a fix for CVE-2026-59870?: Yes. js-yaml (npm): upgrade to 5.2.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-59870 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-59870?: Check whether you run js-yaml (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

js-yaml (npm): upgrade to 5.2.1 or later
```
npm install js-yaml@5.2.1
```

Technical details

The vulnerability exists in the `omapTag.addItem()` function within `src/tag/sequence/omap.ts`. When parsing a YAML document using `YAML11_SCHEMA`, the library performs a linear O(n) scan for duplicate keys upon every insertion into an ordered map, resulting in quadratic O(n^2) time complexity. A remote attacker can provide a crafted YAML payload (e.g., 50,000 entries in a 2MB file) to block the Node.js event loop for several seconds, causing a Denial of Service. This issue was introduced in version 5.0.0 and is fixed in version 5.2.1 by implementing a Set-based lookup for duplicate detection.

Affected products

nodeca js-yaml >= 5.0.0, <= 5.2.0

Identifiers

CVE-2026-59870
GHSA-724g-mxrg-4qvm

Timeline

2026-06-20: other: Vulnerable 5.x series first published
2026-07-02: patched: Fix released in version 5.2.1
2026-07-08: advisory: NVD publication date
2026-07-20: advisory: GitHub Advisory published

References

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