CVE-2026-59868

A flaw was found in js-yaml, a JavaScript YAML parser and dumper. When merge keys are enabled, a remote attacker could exploit this vulnerability by crafting a YAML document where a chain of mappings uses merge keys, each merging the previous one. This can lead to a significant increase in CPU time required to parse the document, resulting in a Denial of Service (DoS) for the affected system.

js-yaml versions 5.0.0 through 5.1.x are vulnerable to a denial of service. When merge keys ('<<') are enabled during YAML parsing, js-yaml re-merges the accumulated mapping into each subsequent mapping in a chain. A document consisting of a linear chain of mappings that each merge the previous one causes quadratic (O(n^2)) CPU consumption relative to input size, even though the document size itself grows only linearly. This can be triggered via yaml.load() on untrusted YAML input that uses merge keys. Fixed in js-yaml 5.2.0.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Upgrade to js-yaml 5.2.0 or later. Where an immediate upgrade is not possible, avoid parsing untrusted YAML documents that use merge keys ('<<'), or apply size and time limits when parsing untrusted YAML input.